Research published today in the journal Science serves to further calibrate the ticking clock of genetic mutation. By tediously analyzing the genomes from multiple generations of the plant Arabidopsis thaliana, scientists from the Max Planck Institute have shown that spontaneous mutations can arise rather frequently in the genome. Despite commonly held assumptions of minimally effectual rarity, the study demonstrates that the rate at which novel mutations occur have more than ample potency to drive evolution. Moreover, when considering the total population size of Arabidopsis, the tempo of mutation indicates that evolution likely proceeds much faster than previously suspected.
Although a single seed of Arabidopsis thaliana may only posses a single point mutation (i.e. a single base or ‘letter’ of DNA) relative to its parents, when multiplied by millions of individual plants over multiple generations, that single mutation quickly compounds to produce tens of new base pairs in only a few years time. The rate of genetic change revealed by the Max Planck study not only aids our understanding of how observed resistance to chemical herbicides rapidly evolves within the weedy plants combated by agriculture, its findings also point to possibly underestimated speeds of mutation in other organisms – like those of humans.
For example, back in September a team headed by Sanger scientists published work in which the rate of mutation in humans was estimated at around 1 mutation per 15 to 30 million nucleotides. Comparatively, this estimate translates to about 150 point mutations in every individual as measured against their parents; that’s a mutation tempo 100 times fold that of Arabidopsis thaliana.
Ossowski, S., Schneeberger, K., Lucas-Lledo, J., Warthmann, N., Clark, R., Shaw, R., Weigel, D., & Lynch, M. (2009). The Rate and Molecular Spectrum of Spontaneous Mutations in Arabidopsis thaliana Science, 327 (5961), 92-94 DOI: 10.1126/science.1180677
Xue, Y., Wang, Q., Long, Q., Ng, B., Swerdlow, H., Burton, J., Skuce, C., Taylor, R., Abdellah, Z., & Zhao, Y. (2009). Human Y Chromosome Base-Substitution Mutation Rate Measured by Direct Sequencing in a Deep-Rooting Pedigree Current Biology, 19 (17), 1453-1457 DOI: 10.1016/j.cub.2009.07.032
Showing posts with label Adaptation. Show all posts
Showing posts with label Adaptation. Show all posts
Friday, January 1, 2010
Wednesday, December 30, 2009
The Poetry of the Convergent Lizards
Convergence at White Sands
Whiptails and fencers scurry and make haste,
cross White Sands their paths converge and pace.
Tho' differing by both origin and type,
their causal genotypes follow trace.
Independent mutations had aligned,
and were by adaptation refined.
Genes and the surroundings got a hitch,
when their ecosystems intertwined.
Nature reduced the melanin of two,
with methods mechanically askew.
They evolved cryptic gypsum like match,
avoiding predatory subdue.
Even when some common genes play actor,
trait expression is a big factor.
So says Rosenblum quite sincere
in a research paper found HERE.
In case that's not quite enough info here's the abstract:
There are many striking examples of phenotypic convergence in nature, in some cases associated with changes in the same genes. But even mutations in the same gene may have different biochemical properties and thus different evolutionary consequences. Here we dissect the molecular mechanism of convergent evolution in three lizard species with blanched coloration on the gypsum dunes of White Sands, New Mexico. These White Sands forms have rapidly evolved cryptic coloration in the last few thousand years, presumably to avoid predation. We use cell-based assays to demonstrate that independent mutations in the same gene underlie the convergent blanched phenotypes in two of the three species. Although the same gene contributes to light phenotypes in these White Sands populations, the specific molecular mechanisms leading to reduced melanin production are different. In one case, mutations affect receptor signaling and in the other, the ability of the receptor to integrate into the melanocyte membrane. These functional differences have important ramifications at the organismal level. Derived alleles in the two species show opposite dominance patterns, which in turn affect their visibility to selection and the spatial distribution of alleles across habitats. Our results demonstrate that even when the same gene is responsible for phenotypic convergence, differences in molecular mechanism can have dramatic consequences on trait expression and ultimately the adaptive trajectory.
Rosenblum, E., Rompler, H., Schoneberg, T., & Hoekstra, H. (2009). Molecular and functional basis of phenotypic convergence in white lizards at White Sands Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0911042107
Whiptails and fencers scurry and make haste,
cross White Sands their paths converge and pace.
Tho' differing by both origin and type,
their causal genotypes follow trace.
Independent mutations had aligned,
and were by adaptation refined.
Genes and the surroundings got a hitch,
when their ecosystems intertwined.
Nature reduced the melanin of two,
with methods mechanically askew.
They evolved cryptic gypsum like match,
avoiding predatory subdue.
Even when some common genes play actor,
trait expression is a big factor.
So says Rosenblum quite sincere
in a research paper found HERE.
In case that's not quite enough info here's the abstract:
There are many striking examples of phenotypic convergence in nature, in some cases associated with changes in the same genes. But even mutations in the same gene may have different biochemical properties and thus different evolutionary consequences. Here we dissect the molecular mechanism of convergent evolution in three lizard species with blanched coloration on the gypsum dunes of White Sands, New Mexico. These White Sands forms have rapidly evolved cryptic coloration in the last few thousand years, presumably to avoid predation. We use cell-based assays to demonstrate that independent mutations in the same gene underlie the convergent blanched phenotypes in two of the three species. Although the same gene contributes to light phenotypes in these White Sands populations, the specific molecular mechanisms leading to reduced melanin production are different. In one case, mutations affect receptor signaling and in the other, the ability of the receptor to integrate into the melanocyte membrane. These functional differences have important ramifications at the organismal level. Derived alleles in the two species show opposite dominance patterns, which in turn affect their visibility to selection and the spatial distribution of alleles across habitats. Our results demonstrate that even when the same gene is responsible for phenotypic convergence, differences in molecular mechanism can have dramatic consequences on trait expression and ultimately the adaptive trajectory.
Rosenblum, E., Rompler, H., Schoneberg, T., & Hoekstra, H. (2009). Molecular and functional basis of phenotypic convergence in white lizards at White Sands Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0911042107
Wednesday, December 23, 2009
Twisted Penises, Convoluted Vaginas and Really Ducked-up Sex
Wow, I really had a tough time coming-up with a title for this post! There were so many possibilities…
In the December 23rd edition of the journal of the Proceedings of the Royal Society B, several scientists from Yale University detailed their recent experiments into the coevolution of male and female duck genitalia. Specifically, the researchers wanted to evaluate whether or not the physical morphologies exhibited by duck penises and duck vaginas provided evidence of ducks being actively engaged in a sexually antagonistic ‘arms race.’ In hopes of resolving this reproductive riddle, the folks from Yale traveled to a commercial duck farm in California, while equipped with artificial duck vaginas, mineral oil and a high speed video camera.
Sexual conflict arises when the strategies used to optimize reproductive fitness differ between the males and females of a species. In the case of muscovy ducks, like those used in the Yale research, the strategy of the male is to mate as frequently as possible, and with as many different females as he can find – even if sexual coercion is the only means available to achieve these goals. Conversely, the strategy of the female is to select only the healthiest of males as reproductive partners and to outright reject all those deemed unworthy. Because the male and female strategies are in conflict – quantity vs. quality - both sexes have undergone adaptations to out-maneuver the opposite gender. In this back-and-forth battle for reproductive victory the process of sexual selection has discovered an affective tactic - to change the physical shape of the duck’s genitalia.
In addition to being one of the few birds with a penis at all, the phallus of the male muscovy duck is physiologically and morphologically very unique. Unlike the penises of most mammals, which when erect are hydraulically rigid and supported by axial-orthogonal layers of inextensible collagen fibers, the penis of the muscovy has an arrangement of supportive fibers that allow for complete flexibility while erect. Besides being highly limber, the shape of the muscovy’s twenty centimeter long penis is ‘twisted’ forming a distinctive counter-clockwise spiral. These functional traits allow the male to very quickly insert into the female during copulation.
The speed at which copulation occurs is of vital importance to the male, it gives him the ability to inseminate even those females that actively defy his advance. Physical resistance is minimized because the male can insert his penis at a speed of about 1.6 meters per second and complete insemination in just 0.36 seconds – that doesn’t leave much time for fighting. Since the male’s seemingly invincible swiftness eliminates combat as a pragmatic brood planning option, the female line has employed an anatomical counter adaptation to ensure its continued mate choice.
As a defensive escalation in the muscovy’s inter-sexual arms race, the female has undergone adaptation for a counter armament – she’s changed the shape of her vagina. The female duck’s vagina can be found to exist in one of two possible shapes, either with a 135-degree bend, or with a clockwise ‘twist.’ During forced copulations, the morphology of the vagina functionally prevents the male’s penis, or his sperm, from entering sufficiently far into the reproductive tract to result in fertilization. However, when the female muscovy is receptive to a male suitor – when SHE chooses a mate – she actively facilitates the sexual congress by taking on a body posture with an elevated tail. The elevated tail, in combination with muscular contractions, permits the male duck to penetrate further into the reproductive tract and thus improves the likelihood of successful fertilization.
To uncover the fascinating story of co-evolving duck genitalia, the researchers from Yale presented aroused males with replicated duck vaginas. In conducting the experiment, female ducks were placed in a cage with a male, once the male initiated a mounting behavior, the female was quickly removed and the male was filmed with a high speed video camera as he inserted his penis into one of four different shaped molded test-tubes. The mineral oil covered test-tubes either had a straight shape, mimicked the male’s penis with a counterclockwise twist, or they had a female-like clockwise twist or 135-degree bend. After analyzing the film, the scientists discovered that the replicated vaginas that matched the shape and dimensions of real-life female muscovies effectively blocked the male’s penis from reaching very far into the test-tube.
Patricia L. R. Brennan, Christopher J. Clark and Richard O. Prum (2009). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proc. R. Soc. B
In the December 23rd edition of the journal of the Proceedings of the Royal Society B, several scientists from Yale University detailed their recent experiments into the coevolution of male and female duck genitalia. Specifically, the researchers wanted to evaluate whether or not the physical morphologies exhibited by duck penises and duck vaginas provided evidence of ducks being actively engaged in a sexually antagonistic ‘arms race.’ In hopes of resolving this reproductive riddle, the folks from Yale traveled to a commercial duck farm in California, while equipped with artificial duck vaginas, mineral oil and a high speed video camera.
Sexual conflict arises when the strategies used to optimize reproductive fitness differ between the males and females of a species. In the case of muscovy ducks, like those used in the Yale research, the strategy of the male is to mate as frequently as possible, and with as many different females as he can find – even if sexual coercion is the only means available to achieve these goals. Conversely, the strategy of the female is to select only the healthiest of males as reproductive partners and to outright reject all those deemed unworthy. Because the male and female strategies are in conflict – quantity vs. quality - both sexes have undergone adaptations to out-maneuver the opposite gender. In this back-and-forth battle for reproductive victory the process of sexual selection has discovered an affective tactic - to change the physical shape of the duck’s genitalia.
In addition to being one of the few birds with a penis at all, the phallus of the male muscovy duck is physiologically and morphologically very unique. Unlike the penises of most mammals, which when erect are hydraulically rigid and supported by axial-orthogonal layers of inextensible collagen fibers, the penis of the muscovy has an arrangement of supportive fibers that allow for complete flexibility while erect. Besides being highly limber, the shape of the muscovy’s twenty centimeter long penis is ‘twisted’ forming a distinctive counter-clockwise spiral. These functional traits allow the male to very quickly insert into the female during copulation.
The speed at which copulation occurs is of vital importance to the male, it gives him the ability to inseminate even those females that actively defy his advance. Physical resistance is minimized because the male can insert his penis at a speed of about 1.6 meters per second and complete insemination in just 0.36 seconds – that doesn’t leave much time for fighting. Since the male’s seemingly invincible swiftness eliminates combat as a pragmatic brood planning option, the female line has employed an anatomical counter adaptation to ensure its continued mate choice.
As a defensive escalation in the muscovy’s inter-sexual arms race, the female has undergone adaptation for a counter armament – she’s changed the shape of her vagina. The female duck’s vagina can be found to exist in one of two possible shapes, either with a 135-degree bend, or with a clockwise ‘twist.’ During forced copulations, the morphology of the vagina functionally prevents the male’s penis, or his sperm, from entering sufficiently far into the reproductive tract to result in fertilization. However, when the female muscovy is receptive to a male suitor – when SHE chooses a mate – she actively facilitates the sexual congress by taking on a body posture with an elevated tail. The elevated tail, in combination with muscular contractions, permits the male duck to penetrate further into the reproductive tract and thus improves the likelihood of successful fertilization.
To uncover the fascinating story of co-evolving duck genitalia, the researchers from Yale presented aroused males with replicated duck vaginas. In conducting the experiment, female ducks were placed in a cage with a male, once the male initiated a mounting behavior, the female was quickly removed and the male was filmed with a high speed video camera as he inserted his penis into one of four different shaped molded test-tubes. The mineral oil covered test-tubes either had a straight shape, mimicked the male’s penis with a counterclockwise twist, or they had a female-like clockwise twist or 135-degree bend. After analyzing the film, the scientists discovered that the replicated vaginas that matched the shape and dimensions of real-life female muscovies effectively blocked the male’s penis from reaching very far into the test-tube.
Patricia L. R. Brennan, Christopher J. Clark and Richard O. Prum (2009). Explosive eversion and functional morphology of the duck penis supports sexual conflict in waterfowl genitalia Proc. R. Soc. B
Friday, December 11, 2009
The Convergent Brains of Humans and Elephants
In recent evolution news, a research article published this month in the Proceedings of the National Academy of Sciences has demonstrated that the brains of elephants and humans have followed similar adaptive paths.
The collaborative study, undertaken by scientists from several U.S. universities, analyzed the phylogenomic patterns displayed by genes linked to brain metabolism in fifteen different vertebrate groups; including eleven placental mammals, a marsupial, a monotreme, a bird and an amphibian. Of specific interest to the researchers was the evolutionary history of the elephant brain, which has many physical and functional similarities to that possessed by humans.
Although about four times larger, the brains of elephants are like human brains in that they boast extensive regions of neocortex. The neocortex is the neurological structure responsible for an animal’s sensory perception, motor commands and higher thought processes. Elephants were of particular interest in the study because they - like humans - are known for displaying intelligence, complex social behavior and empathy.
In conducting the research, the scientists examined the occurrence of both synonymous and non-synonymous nucleotide substitutions within each brain-linked gene. Synonymous substitutions can be thought of as ‘silent’ because these changes don’t illicit a change in the resulting amino acid. By contrast, non-synonymous substitutions do result in the incorporation of a different amino acid, and therefore advent a potential for novel risks or benefits on which natural selection can act.
In the case of the brain genes evaluated in this study, the scientists found that when compared to the other vertebrate groups the genomes of elephants and humans show increased frequencies of non-synonymous substitutions in areas responsible for the brain’s aerobic metabolism. So, not only are brains of both elephants and humans relatively large, but they’ve also followed a similar adaptive path in obtaining their current state.
Goodman, M., Sterner, K., Islam, M., Uddin, M., Sherwood, C., Hof, P., Hou, Z., Lipovich, L., Jia, H., Grossman, L., & Wildman, D. (2009). Phylogenomic analyses reveal convergent patterns of adaptive evolution in elephant and human ancestries Proceedings of the National Academy of Sciences, 106 (49), 20824-20829 DOI: 10.1073/pnas.0911239106
The collaborative study, undertaken by scientists from several U.S. universities, analyzed the phylogenomic patterns displayed by genes linked to brain metabolism in fifteen different vertebrate groups; including eleven placental mammals, a marsupial, a monotreme, a bird and an amphibian. Of specific interest to the researchers was the evolutionary history of the elephant brain, which has many physical and functional similarities to that possessed by humans.
Although about four times larger, the brains of elephants are like human brains in that they boast extensive regions of neocortex. The neocortex is the neurological structure responsible for an animal’s sensory perception, motor commands and higher thought processes. Elephants were of particular interest in the study because they - like humans - are known for displaying intelligence, complex social behavior and empathy.
In conducting the research, the scientists examined the occurrence of both synonymous and non-synonymous nucleotide substitutions within each brain-linked gene. Synonymous substitutions can be thought of as ‘silent’ because these changes don’t illicit a change in the resulting amino acid. By contrast, non-synonymous substitutions do result in the incorporation of a different amino acid, and therefore advent a potential for novel risks or benefits on which natural selection can act.
In the case of the brain genes evaluated in this study, the scientists found that when compared to the other vertebrate groups the genomes of elephants and humans show increased frequencies of non-synonymous substitutions in areas responsible for the brain’s aerobic metabolism. So, not only are brains of both elephants and humans relatively large, but they’ve also followed a similar adaptive path in obtaining their current state.
Goodman, M., Sterner, K., Islam, M., Uddin, M., Sherwood, C., Hof, P., Hou, Z., Lipovich, L., Jia, H., Grossman, L., & Wildman, D. (2009). Phylogenomic analyses reveal convergent patterns of adaptive evolution in elephant and human ancestries Proceedings of the National Academy of Sciences, 106 (49), 20824-20829 DOI: 10.1073/pnas.0911239106
Monday, November 23, 2009
The Mythical Adaptationist and the Pretend Pluralist’s Aimless Plea
Wow… I generally tend to stick to narrative posts discussing natural history, but in light of a short commentary that I just read from The Society for the Study of Evolution’s journal, I feel obligated to throw a couple of reckless comments onto the web…
The article in point, ADAPTIONISM—30 YEARS AFTER GOULD AND LEWONTIN, was written by Rasmus Nielsen of the University of Copenhagen and to the best of my interpretative ability seems to be making a plea to so called ‘adaptationists’ to reconsider their errant ways..?
Apparently, the author is under the impression that the world’s evolutionary biologists can be dichotomously classified as either ‘adaptationists’ or ‘pluarlists.’ And further, that those classified in the former category should seek reincarnation as enlightened members of the latter.
The commentary’s argument begins (predictably) with Stephen J. Gould and Richard C. Lewontin’s ‘The Spandrels of San Marco,’ an article originally published by the Royal Society in 1979. The broad point of the original Gould piece was to encourage scientists to look beyond natural selection as the sole process of change and to instead consider organisms as complex entities influenced by a myriad of evolutionary forces. In short, to think of the organism’s evolutionary history as being an emergent property derived from its whole, not one measured through the summing of its individual traits. This cautionary imperative is certainly as valid today as it was back in ’79 and it should be heeded; however, it isn’t by any means novel, nor does it say anything in regards to the reality of research – it is simply a warning to be cautious of personal and professional bias.
The Nielsen article moves from The Spandrels to contemporary times in order to assess what valuable lessons have been gleaned from that momentous (?) work of 30-years past. Unfortunately for the field, in conducting this assessment it becomes blatantly apparent that the Gouldian forewarning has fallen on deaf ears…
“…although Gould and Lewontin’s paper did not spell the end to adaptationist storytelling, it radically increased the awareness among evolutionary biologists about the pitfalls of adaptationism.”
Whew... What a relief; but what does that mean exactly?
“Evolutionary biologists are today, arguably, much more reluctant to invent adaptive stories without direct evidence for natural selection acting on the traits in question. We still regularly encounter very naive adaptive stories, particularly about human behavior, but rarely in journals such as Evolution or other related journals with high standards…”
‘Rare’ in this case is good – I think? I’m so glad we have The Society for the Study of Evolution’s journal to guide our path!
What should we do to remain of wholesome purity; what should we do to keep the path?
“…we must rely on inferences regarding past events by observing scant fossil evidence and the current pattern of genetic and phenotypic variation. We may be able to detect selection, but we may never be able to directly determine which traits selection acted on.”
So, we can see selection, but our ignorance blinds us to the characteristics driving that selection…
“Although the presence of selection acting on genes underlying a phenotypic trait of interest does help support adaptive stories, it does not establish that selection acted directly on the specific trait of interest.”
Because,
“[m]ost genes have pleiotropic effects and establishing the direct cause of selection in an organism such as humans might in most cases be difficult or impossible.”
That’s a reasonable statement, but couldn’t the before-mentioned inferences guide the ‘adaptationist’s’ filthy lust for storytelling? What prophylactics are available in the event that an adaptationist fails to maintain self-control?
“…speculation… must be done acknowledging that no simple experiment or functional data can falsify or “validate” historical adaptive hypotheses.”
Geee, thanks for the heads-up, I’m going to spread the good-word!
“And in communicating with our peers, and with the popular press in particular, we may individually, and as a scientific field, benefit from understanding the societal impact of the statements we make.”
Or, on the other hand, maybe not…
In closing, I have difficulty believing that radical adaptationists are running rampant in evolutionary biology. I can’t think of a single practicing biologist, in academia or otherwise, that doesn’t consider the impact of drifting allelic frequencies and other possible influences outside the scope of natural selection. As far as the relative importance, or weight, granted to such alternative processes in determining an organism’s evolutionary path, that isn’t a question of individual preference. Rather it’s something that is assessed quantitatively through experimentation and study.
I can’t help but wonder if the entire “adaptationist Vs pluralist” debate is an artificial construct intentionally designed for generating publicity. Judging by this post and similar arguments had at Sandwalk it seems to work…
Nielsen, R. (2009). ADAPTIONISM-30 YEARS AFTER GOULD AND LEWONTIN Evolution, 63 (10), 2487-2490 DOI: 10.1111/j.1558-5646.2009.00799.x
The article in point, ADAPTIONISM—30 YEARS AFTER GOULD AND LEWONTIN, was written by Rasmus Nielsen of the University of Copenhagen and to the best of my interpretative ability seems to be making a plea to so called ‘adaptationists’ to reconsider their errant ways..?
Apparently, the author is under the impression that the world’s evolutionary biologists can be dichotomously classified as either ‘adaptationists’ or ‘pluarlists.’ And further, that those classified in the former category should seek reincarnation as enlightened members of the latter.
The commentary’s argument begins (predictably) with Stephen J. Gould and Richard C. Lewontin’s ‘The Spandrels of San Marco,’ an article originally published by the Royal Society in 1979. The broad point of the original Gould piece was to encourage scientists to look beyond natural selection as the sole process of change and to instead consider organisms as complex entities influenced by a myriad of evolutionary forces. In short, to think of the organism’s evolutionary history as being an emergent property derived from its whole, not one measured through the summing of its individual traits. This cautionary imperative is certainly as valid today as it was back in ’79 and it should be heeded; however, it isn’t by any means novel, nor does it say anything in regards to the reality of research – it is simply a warning to be cautious of personal and professional bias.
The Nielsen article moves from The Spandrels to contemporary times in order to assess what valuable lessons have been gleaned from that momentous (?) work of 30-years past. Unfortunately for the field, in conducting this assessment it becomes blatantly apparent that the Gouldian forewarning has fallen on deaf ears…
“…although Gould and Lewontin’s paper did not spell the end to adaptationist storytelling, it radically increased the awareness among evolutionary biologists about the pitfalls of adaptationism.”
Whew... What a relief; but what does that mean exactly?
“Evolutionary biologists are today, arguably, much more reluctant to invent adaptive stories without direct evidence for natural selection acting on the traits in question. We still regularly encounter very naive adaptive stories, particularly about human behavior, but rarely in journals such as Evolution or other related journals with high standards…”
‘Rare’ in this case is good – I think? I’m so glad we have The Society for the Study of Evolution’s journal to guide our path!
What should we do to remain of wholesome purity; what should we do to keep the path?
“…we must rely on inferences regarding past events by observing scant fossil evidence and the current pattern of genetic and phenotypic variation. We may be able to detect selection, but we may never be able to directly determine which traits selection acted on.”
So, we can see selection, but our ignorance blinds us to the characteristics driving that selection…
“Although the presence of selection acting on genes underlying a phenotypic trait of interest does help support adaptive stories, it does not establish that selection acted directly on the specific trait of interest.”
Because,
“[m]ost genes have pleiotropic effects and establishing the direct cause of selection in an organism such as humans might in most cases be difficult or impossible.”
That’s a reasonable statement, but couldn’t the before-mentioned inferences guide the ‘adaptationist’s’ filthy lust for storytelling? What prophylactics are available in the event that an adaptationist fails to maintain self-control?
“…speculation… must be done acknowledging that no simple experiment or functional data can falsify or “validate” historical adaptive hypotheses.”
Geee, thanks for the heads-up, I’m going to spread the good-word!
“And in communicating with our peers, and with the popular press in particular, we may individually, and as a scientific field, benefit from understanding the societal impact of the statements we make.”
Or, on the other hand, maybe not…
In closing, I have difficulty believing that radical adaptationists are running rampant in evolutionary biology. I can’t think of a single practicing biologist, in academia or otherwise, that doesn’t consider the impact of drifting allelic frequencies and other possible influences outside the scope of natural selection. As far as the relative importance, or weight, granted to such alternative processes in determining an organism’s evolutionary path, that isn’t a question of individual preference. Rather it’s something that is assessed quantitatively through experimentation and study.
I can’t help but wonder if the entire “adaptationist Vs pluralist” debate is an artificial construct intentionally designed for generating publicity. Judging by this post and similar arguments had at Sandwalk it seems to work…
Nielsen, R. (2009). ADAPTIONISM-30 YEARS AFTER GOULD AND LEWONTIN Evolution, 63 (10), 2487-2490 DOI: 10.1111/j.1558-5646.2009.00799.x
Wednesday, October 28, 2009
Venomous Fables and Phenotypic Variations at the Molecular Level
Remembered for both his lucid writing ability and his tedious nature, the Greek historian Herodotus has often been criticized for the habit of adding unnecessary embellishment to his otherwise candid historical accounts. Focused primarily on the Greco-Persian Wars and personal travels around the Mediterranean, Herodotus’ works also included – on occasion – particulars that many of his 5th Century B.C. contemporaries considered questionable.
One such questionable account from Herodotus centered on the life of a certain Phrygian slave with a knack for composing short prose. Not quite Shakespearian in character, the prose rendered by this slave usually involved references to anthropomorphized wildlife and included a take-home moral message. Now typically, writing prose or verse is a rather benign enterprise, particularly when writing stories about personified animals. Moralizing on the other hand… Moralizing can sometimes get you in trouble, and regrettably for the Phrygian slave, trouble was precisely the result of his high-minded allegories - he was reportedly thrown from a precipice in Delphi for being ugly and “offensive.”
Modern historians raise doubt as to the veracity of Herodotus’ accounts of the Phrygian slave. In fact, many current scholars suspect that the slave never really existed, and further, that the voluminous writings credited to the fictional Phrygian are actually plagiarized collections of Indian folklore. Despite the questionable origins of the parables, the Phrygian slave’s name remains nearly synonymous with “allegory” to this day, and as far as I’m concerned some lessons can still be gleaned from Aesop’s Fables.
One fable in particular came to mind last Wednesday as I was traversing an ecotone between a cypress swamp and a small area of pine flatwoods near Goethe State Forest in central Florida. My eyes being drawn to the rank-and-file procession of hydrophytic plants marching from the wetlands, my ears neglected to notice the slight hum of a rattling creature underfoot. Well, the creature wasn’t quite underfoot; it was in reality about a foot’s distance from being underfoot, but none-the-less the range was sufficiently narrow to take me by surprise. After a couple of second’s pause to regain my bearings, I realized that the insect-like hum that I had heard was in actuality the warning emitted from the shaking tail of the beast – a pygmy rattlesnake!
Pygmy rattlesnakes (Sistrurus miliarius) are members of the Subfamily Crotalinae - the pitviper group - and like their larger Crotalus cousins (true rattlesnakes) the venom of the pygmy contains a toxic cocktail of tissue degrading molecules. Not exactly the kind of critter you want to accidently step on. The potency of the pygmy’s venom is uncontested and is comparable to that produced by other pit-vipers; however the quantity produced by S. miliarius is unlikely to cause death in a human – at least not the death of an adult human. As opposed to human prey, snakes belonging to the Genus Sistrurus have undergone adaptation to capture lizards, small mammals, insects and other snakes as foodstuffs, therefore striking a heavy-footed ecologist would be biting-off more than the snake could chew.
Don’t be misled however, even though the venom of the pygmy is unlikely to cause death, it would certainly pack enough punch to ruin your day; proteins “designed” to enzymaticaly induce hemorrhaging couldn’t be a good thing to have pulsing through your veins… I put the word “designed” in quotes in the previous sentence to emphasize that the protein concoction injected by Sistrurus is very much a product of adaptation - it’s a functional trait shaped by the process of natural selection. But before getting into that story, which will be forthcoming, let me first show you a snapshot of the snake encountered last week and share the Aesop fable that was alluded to above.
One such questionable account from Herodotus centered on the life of a certain Phrygian slave with a knack for composing short prose. Not quite Shakespearian in character, the prose rendered by this slave usually involved references to anthropomorphized wildlife and included a take-home moral message. Now typically, writing prose or verse is a rather benign enterprise, particularly when writing stories about personified animals. Moralizing on the other hand… Moralizing can sometimes get you in trouble, and regrettably for the Phrygian slave, trouble was precisely the result of his high-minded allegories - he was reportedly thrown from a precipice in Delphi for being ugly and “offensive.”
Modern historians raise doubt as to the veracity of Herodotus’ accounts of the Phrygian slave. In fact, many current scholars suspect that the slave never really existed, and further, that the voluminous writings credited to the fictional Phrygian are actually plagiarized collections of Indian folklore. Despite the questionable origins of the parables, the Phrygian slave’s name remains nearly synonymous with “allegory” to this day, and as far as I’m concerned some lessons can still be gleaned from Aesop’s Fables.
One fable in particular came to mind last Wednesday as I was traversing an ecotone between a cypress swamp and a small area of pine flatwoods near Goethe State Forest in central Florida. My eyes being drawn to the rank-and-file procession of hydrophytic plants marching from the wetlands, my ears neglected to notice the slight hum of a rattling creature underfoot. Well, the creature wasn’t quite underfoot; it was in reality about a foot’s distance from being underfoot, but none-the-less the range was sufficiently narrow to take me by surprise. After a couple of second’s pause to regain my bearings, I realized that the insect-like hum that I had heard was in actuality the warning emitted from the shaking tail of the beast – a pygmy rattlesnake!
Pygmy rattlesnakes (Sistrurus miliarius) are members of the Subfamily Crotalinae - the pitviper group - and like their larger Crotalus cousins (true rattlesnakes) the venom of the pygmy contains a toxic cocktail of tissue degrading molecules. Not exactly the kind of critter you want to accidently step on. The potency of the pygmy’s venom is uncontested and is comparable to that produced by other pit-vipers; however the quantity produced by S. miliarius is unlikely to cause death in a human – at least not the death of an adult human. As opposed to human prey, snakes belonging to the Genus Sistrurus have undergone adaptation to capture lizards, small mammals, insects and other snakes as foodstuffs, therefore striking a heavy-footed ecologist would be biting-off more than the snake could chew.
Don’t be misled however, even though the venom of the pygmy is unlikely to cause death, it would certainly pack enough punch to ruin your day; proteins “designed” to enzymaticaly induce hemorrhaging couldn’t be a good thing to have pulsing through your veins… I put the word “designed” in quotes in the previous sentence to emphasize that the protein concoction injected by Sistrurus is very much a product of adaptation - it’s a functional trait shaped by the process of natural selection. But before getting into that story, which will be forthcoming, let me first show you a snapshot of the snake encountered last week and share the Aesop fable that was alluded to above.
The Crow and the Snake
A hungry Crow spied a Snake lying asleep in a sunny spot, and, picking it up in his claws, he was carrying it off to a place where he could make a meal of it without being disturbed, when the Snake reared its head and bit him. It was a poisonous Snake, and the bite was fatal, and the dying Crow said, "What a cruel fate is mine! I thought I had made a lucky find, and it has cost me my life!" (Aesop’s Fables A New Translation by V. S. Vernon Jones, 1912 edition)
The pygmy is still considered a lucky find, though it would have been nice if the picture would have turned out a little better. I could have gotten a bit closer, but ‘as the crow teaches us’ using caution is important and un-scrupulous actions can turn a good scenario into a bad one very quickly… OK, enough with the corny fable, on with the science - before readers encouraged me to follow Aesop over the precipice!
The “designed” venom of Sistrurus… As stated above, the word “designed” refers to a natural process in which variations in phenotype have contributed to differential reproductive success between the individual organisms exhibiting the traits. Specific to this case, the proteins that make-up the hemorrhagic venom are coded for by genes contained within the snake’s genome. Variations in the venom’s genotype from individual to individual cause the overall “potency” of venom to also vary from individual to individual. So, reductively stated, variations in genes coding for venom lead to variations in the “strength” of that venom. The strength of the venom in turn affects the number and quality of prey killed by the snake.
Risking redundancy and stated a third time slightly differently, molecular alterations within the modified saliva of a snake change how those molecules fold and biosynthesize to form the constituent proteins of venom. These phenotypic variations at the molecular level translate to a modified functional trait in the snake’s predator-prey dynamic. Better venom chemistry equals more food, increased survivability and increased fecundity for the snake. Extending the gene’s reach even further, it is conceivable that the molecularly induced change in such a functional trait could even allow the snake to specialize on a certain type of prey, thereby changing its ecology all together.
Interestingly enough, earlier this year H. Lisle Gibbs of Ohio State University, and Stephen P. Mackessy from University of Northern Colorado published work in the journal Toxicon discussing the venom of several Sistrurus species. Their work centered on prey specific effects, and they hypothesized that a “high level of variation in venom at the inter- or intraspecific level allows snakes to specialize on different prey.” They tested the effects of venom on mice, lizards and frogs (typical prey items for Sistrurus) and determined that “toxicity to mammals [was] a major axis along which venom evolution has occurred among Sistrurus rattlesnakes, with little evidence for evolutionary changes in toxicity towards the other prey tested.”
So, the research published in Toxicon demonstrated that not only is the phenotypic functionality of Sistrurus’ venom significant, but also that the response to that venom by prey species may open the door to an evolutionary arms race between predator and prey - an epic battle between toxicity and immunity. Changes in venom chemistry could allow new prey items to be added to the snake’s menu, or conversely, the changes could, in time, ultimately remove current prey species from the carte du jour. Such shifts in dietary preference could easily serve as focal points for selective pressures.
I don’t know if the epic battle between snake venom and prey immunity would stir Herodotus’ interest in war writings, but the story behind the evolutionary dynamic is adequately fascinating to not require very much embellishment. As for me, I’ll take Aesop’s lesson about haste to heart and give greater attention to what’s underfoot.
Gibbs, H., & Mackessy, S. (2009). Functional basis of a molecular adaptation: Prey-specific toxic effects of venom from Sistrurus rattlesnakes Toxicon, 53 (6), 672-679 DOI: 10.1016/j.toxicon.2009.01.034
Sunday, April 26, 2009
How Discriminating Ants Choose
In addition to merging sci-fi art with the reality of science ('tagged ant' image below), researchers at the School of Biological Sciences within the University of Bristol have demonstrated that the ‘irrationality’ associated with contextual decision making is avoided in the ant Temnothorax albipennis as it chooses between alternative nesting sites.
Tagged Ant!
As the article "Do ants make direct comparisons?" explains, Temnothorax albipennis makes a collective decision when a colony emigrates to a new nest. Scouting ants that discover new nests assess them on the basis of multiple attributes. Some of these scouts subsequently recruit nest-mates to the new nest using tandem-running, where an informed ant leads a second ant to her destination. When the number of ants in the new nest reaches a quorum, scouts begin rapid transport of the rest of the colony by carrying nest-mates and brood. Colonies are able to choose the best of several nests. Two individual-level mechanisms for this collective choosiness have been identified. Some ants visit both nest sites, and subsequently recruit only to the better site, which has been taken as evidence for direct comparison. However, ants that visit only one site still contribute to the colony decision, by starting to recruit earlier (i.e. using a shorter recruitment latency) when a nest is of higher quality.
According to Elva Robinson, "Each ant appears to have its own 'threshold of acceptability' against which to judge a nest individually. Ants finding the poor nest were likely to switch and find the good nest, whereas ants finding the good nest were more likely to stay committed to that nest. When ants switched quickly between the two nests, colonies ended up in the good nest. Individual ants did not need to comparatively evaluate both nests in order for the entire colony to make the correct decision.
On the other hand, animals – including humans – who use comparative evaluation frequently make 'irrational' decisions, due to the context in which options are compared or by inconsistently ranking pairs of options, (for example option A preferred to B, B preferred to C but C preferred to A).
The ants' threshold rule makes an absolute assessment of nest quality that is not subject to these risks, and circumvents the necessity for memorization and comparison of every site visited. Thus, simple individual behavior substitutes for direct comparison, facilitating effective choice between nest sites for the colony as a whole."
Specimen: CASENT0173192Species: Temnothorax albipennis
Photographer: April NobileDate Uploaded: 08/09/2007
Copyright: California Academy of Sciences, 2000-2007
Specimen: CASENT0173192Species: Temnothorax albipennis
Photographer: April NobileDate Uploaded: 08/09/2007
Copyright: California Academy of Sciences, 2000-2007
Robinson, E., Smith, F., Sullivan, K., & Franks, N. (2009). Do ants make direct comparisons? Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2009.0350
Sunday, April 19, 2009
The Fire Gene Described
A lengthy introduction during the initial post on this topic (available Here) contrasted a harmonious view of nature with the perspective of nature as a series of oppositional organisms struggling to gain a competitive edge over rivals. As a model of this outlook, the ecotone boundaries between various sets of differing plant communities were offered as a case study. More specifically, the prairie, savanna and hardwood hammock ecosystems of the Big Cypress Preserve were forwarded along with the proposition that members of these communities actively challenged each other for limited resources. In staging this proposition the question was asked, “Why don’t trees invade - and take over – the prairie communities currently occupied by grasses?” After eliminating the likelihood that densely growing stands of grass crowded-out young saplings by denying them access to sunlight, cyclic wildfire were explained and presented as an alternative explanation. Moving forward with this production, a profile of one of the previously introduced characters is in order – the conifer tree Pinus elliotti.
Slash Pine Growing from a Log
Slash Pine is a species of the genus Pinus (pine tree) which branched from genus Picea (spruce tree) during the Cretaceous Period, somewhere between 87 and 193 million years ago. There are two distinct varieties of slash pine, variety elliotti and variety densa, although there are several important distinctions, for purposes here both varieties can be considered one and the same. Pine and spruce trees are grouped together with cycads, gnetophytes and ginkgo as gymnosperms, which had a start back in the Pennsylvanian Period of the Carboniferous more than 300 million years ago. The long history of the pine trees, and the slash pine in particular, is significant because these trees have one of the largest and most complex genomes of any organism on the planet today – a result of varied evolutionary forces. Of specific interest in regards to evolutionary history is that gymnosperms arose from the Carboniferous swamps during a period of rapid plant adaptation. In addition to the advent of the bark fiber “lignin,” plants during that period underwent a multitude of morphological changes - many of these changes were adaptations to wildfire. Unlike the 21% atmospheric oxygen present today, the carboniferous boasted 35% oxygen content, this in conjunction with an abundance of herbaceous material (remember Carboniferous = “coal age”) resulted in frequent – and intense – wildfires.
Wildfires…
So, do wildfires prevent trees, such as slash pine, from invading prairie strongholds held by grasses? Not really, some especially intense (“intensity” being a measure of a fire’s maximum temperature and duration) wildfires may destroy slash pine, but fires capable of doing so are relatively rare. The typical “fire seasons,” as described in the first post, may have sufficient intensity to kill some young saplings, but remember - slash pines also have “initial rapid growth genes” which provide a solid head start in defending themselves. Essentially, any sapling greater than two years old has a good chance of getting through the “average” wildfire. As for the periodic “non-average” wildfire, one that is of an unusually high intensity, slash pines may need to rely on evolutionary adaptations other than “initial rapid growth genes” – they may need to lean on morphological phenotypes resulting from a “fire gene.”
A fire gene is a genetic compliment possessed by an organism that is expressed in such a manner that the presence of fire improves the likelihood of that genotype being passed on to future generations. In other words, if a population of trees exists in which some members have a genotype that provides phenotypical resistance to fire, and that population is then exposed to fire, killing a certain percentage of the population, those trees with fire gene advantage will have higher survivability and greater measures of fitness than will those not possessing a fire gene. Through this process of “selection by fire,” the fire gene would become more prevalent in the population, eventually becoming so common as to be called characteristic.
This is precisely what has occurred with the pine trees of the Big Cypress. Through millennia of “trial by fire,” only those trees expressing the most fire tolerant phenotypes have survived. Morphological features such as thickly armored plates of bark shield the trunk from heat, scale plated meristems guard against flames and the pine’s reproductive strategies take into account spring fires by germinating in the fall and producing periodic mast crops. However, these products of natural selection are merely defenses, what is truly remarkable is that another aspect of the fire gene contributes to offensive maneuvers.
As a thought experiment only, image being a tree with the cognitive function of a human and the knowledge that you have an inherit resistance to fire; a resistance that many of your competitors do not posses. If locked in a battle for survival, and you had a match in hand, (or rather, a match in “branch”) would you start a fire?
Of course, matches are of little use to trees outside of thought experiments, but what if there was an adaptation that would provide not only defense, but also allow trees to harness naturally occurring fires to their advantage? Genes don’t exist in isolation; frequently they form partnerships to gain mutual advantage. Epistasis, the interaction between genes, has occurred in pine trees to accomplish the same goal. Not only do the trees have defensive morphologies, they have also adapted the chemistry of their leaves (i.e. pine needles) such that while on the tree the leaves produce flame resistant chemicals, but when wildfires are absent for extended periods of time leaf chemistry changes. In the absence of wildfires leaves are randomly shed, accumulate in the area around the tree and - as opposed to being flame retardant - they become easily ignited at low temperatures and burn at an intensity that, well… An intensity that only a slash pine would love…
Some fires do adversely affect slash pine, but the presence of a “fire gene” provides both defensive and offensive adaptations that can –and have been – utilized to survive. So, why don’t trees such as slash pines invade prairies? It’s a “one-two punch.” Through heat stressing the trees, fires slow down advancing slash pines; however it is what happens after the fire season that stops them cold in their tracks – flooding. Summer rains pile on additional stress to what has already accumulated due to fire defense investment. Grasses are in the same boat, but due to a better water tolerance they can bounce back more readily. The slash pine can survive fires or flood, but taken together these two modes of environmental disturbance overwhelm the trees and limit their prairie-ward charge. This is, however, a function of seasonality, climate and cyclic wildfires; with climate change and alteration of these natural processes all bets are off. (But that’s a topic for another time…)
Beckage, B., Gross, L., & Platt, W. (2006). Modelling responses of pine savannas to climate change and large-scale disturbance Applied Vegetation Science, 9 (1) DOI: 10.1658/1402-2001(2006)9[75:MROPST]2.0.CO;2
Nordlund, D., & Lewis, W. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions Journal of Chemical Ecology, 2 (2), 211-220 DOI: 10.1007/BF00987744
Morse, A., Peterson, D., Islam-Faridi, M., Smith, K., Magbanua, Z., Garcia, S., Kubisiak, T., Amerson, H., Carlson, J., Nelson, C., & Davis, J. (2009). Evolution of Genome Size and Complexity in Pinus PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004332
Platt, W. J., J. M. Huffman, M. G. Slocum, and B. Beckage. In press. Fire regimes and trees in Florida dry prairie landscapes. In: Noss, R. & Singh, S. (eds.) Land of fire and water: The Florida dry prairie ecosystem. Avon Park Air Force Range and Department of Defense, Avon Park, FL,
US.Kabrick, John M.; Dey, Daniel C.; Gwaze, David, eds. Shortleaf pine restoration and ecology in the Ozarks: proceedings of a symposium; 2006 November 7-9; Springfield, MO. Gen. Tech. Rep. NRS-P-15. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 28-32.
Slash Pine Growing from a Log
Slash Pine is a species of the genus Pinus (pine tree) which branched from genus Picea (spruce tree) during the Cretaceous Period, somewhere between 87 and 193 million years ago. There are two distinct varieties of slash pine, variety elliotti and variety densa, although there are several important distinctions, for purposes here both varieties can be considered one and the same. Pine and spruce trees are grouped together with cycads, gnetophytes and ginkgo as gymnosperms, which had a start back in the Pennsylvanian Period of the Carboniferous more than 300 million years ago. The long history of the pine trees, and the slash pine in particular, is significant because these trees have one of the largest and most complex genomes of any organism on the planet today – a result of varied evolutionary forces. Of specific interest in regards to evolutionary history is that gymnosperms arose from the Carboniferous swamps during a period of rapid plant adaptation. In addition to the advent of the bark fiber “lignin,” plants during that period underwent a multitude of morphological changes - many of these changes were adaptations to wildfire. Unlike the 21% atmospheric oxygen present today, the carboniferous boasted 35% oxygen content, this in conjunction with an abundance of herbaceous material (remember Carboniferous = “coal age”) resulted in frequent – and intense – wildfires.
Wildfires…
So, do wildfires prevent trees, such as slash pine, from invading prairie strongholds held by grasses? Not really, some especially intense (“intensity” being a measure of a fire’s maximum temperature and duration) wildfires may destroy slash pine, but fires capable of doing so are relatively rare. The typical “fire seasons,” as described in the first post, may have sufficient intensity to kill some young saplings, but remember - slash pines also have “initial rapid growth genes” which provide a solid head start in defending themselves. Essentially, any sapling greater than two years old has a good chance of getting through the “average” wildfire. As for the periodic “non-average” wildfire, one that is of an unusually high intensity, slash pines may need to rely on evolutionary adaptations other than “initial rapid growth genes” – they may need to lean on morphological phenotypes resulting from a “fire gene.”
A fire gene is a genetic compliment possessed by an organism that is expressed in such a manner that the presence of fire improves the likelihood of that genotype being passed on to future generations. In other words, if a population of trees exists in which some members have a genotype that provides phenotypical resistance to fire, and that population is then exposed to fire, killing a certain percentage of the population, those trees with fire gene advantage will have higher survivability and greater measures of fitness than will those not possessing a fire gene. Through this process of “selection by fire,” the fire gene would become more prevalent in the population, eventually becoming so common as to be called characteristic.
This is precisely what has occurred with the pine trees of the Big Cypress. Through millennia of “trial by fire,” only those trees expressing the most fire tolerant phenotypes have survived. Morphological features such as thickly armored plates of bark shield the trunk from heat, scale plated meristems guard against flames and the pine’s reproductive strategies take into account spring fires by germinating in the fall and producing periodic mast crops. However, these products of natural selection are merely defenses, what is truly remarkable is that another aspect of the fire gene contributes to offensive maneuvers.
As a thought experiment only, image being a tree with the cognitive function of a human and the knowledge that you have an inherit resistance to fire; a resistance that many of your competitors do not posses. If locked in a battle for survival, and you had a match in hand, (or rather, a match in “branch”) would you start a fire?
Of course, matches are of little use to trees outside of thought experiments, but what if there was an adaptation that would provide not only defense, but also allow trees to harness naturally occurring fires to their advantage? Genes don’t exist in isolation; frequently they form partnerships to gain mutual advantage. Epistasis, the interaction between genes, has occurred in pine trees to accomplish the same goal. Not only do the trees have defensive morphologies, they have also adapted the chemistry of their leaves (i.e. pine needles) such that while on the tree the leaves produce flame resistant chemicals, but when wildfires are absent for extended periods of time leaf chemistry changes. In the absence of wildfires leaves are randomly shed, accumulate in the area around the tree and - as opposed to being flame retardant - they become easily ignited at low temperatures and burn at an intensity that, well… An intensity that only a slash pine would love…
Some fires do adversely affect slash pine, but the presence of a “fire gene” provides both defensive and offensive adaptations that can –and have been – utilized to survive. So, why don’t trees such as slash pines invade prairies? It’s a “one-two punch.” Through heat stressing the trees, fires slow down advancing slash pines; however it is what happens after the fire season that stops them cold in their tracks – flooding. Summer rains pile on additional stress to what has already accumulated due to fire defense investment. Grasses are in the same boat, but due to a better water tolerance they can bounce back more readily. The slash pine can survive fires or flood, but taken together these two modes of environmental disturbance overwhelm the trees and limit their prairie-ward charge. This is, however, a function of seasonality, climate and cyclic wildfires; with climate change and alteration of these natural processes all bets are off. (But that’s a topic for another time…)
Beckage, B., Gross, L., & Platt, W. (2006). Modelling responses of pine savannas to climate change and large-scale disturbance Applied Vegetation Science, 9 (1) DOI: 10.1658/1402-2001(2006)9[75:MROPST]2.0.CO;2
Nordlund, D., & Lewis, W. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions Journal of Chemical Ecology, 2 (2), 211-220 DOI: 10.1007/BF00987744
Morse, A., Peterson, D., Islam-Faridi, M., Smith, K., Magbanua, Z., Garcia, S., Kubisiak, T., Amerson, H., Carlson, J., Nelson, C., & Davis, J. (2009). Evolution of Genome Size and Complexity in Pinus PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004332
Platt, W. J., J. M. Huffman, M. G. Slocum, and B. Beckage. In press. Fire regimes and trees in Florida dry prairie landscapes. In: Noss, R. & Singh, S. (eds.) Land of fire and water: The Florida dry prairie ecosystem. Avon Park Air Force Range and Department of Defense, Avon Park, FL,
US.Kabrick, John M.; Dey, Daniel C.; Gwaze, David, eds. Shortleaf pine restoration and ecology in the Ozarks: proceedings of a symposium; 2006 November 7-9; Springfield, MO. Gen. Tech. Rep. NRS-P-15. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 28-32.
Saturday, April 18, 2009
The Fire Gene: One Organism’s Ability to Exploit Fire
Gazing across the tranquil landscape of the Big Cypress Preserve, nature seems to be in balance, unchanging and at peace - picturesque beyond any poetic description. Here, anthropogenic throngs of sharply angled concrete and glass edifices suspend their battle for roadside dominance and yield themselves to a sea of sparsely treed savanna, rolling prairies of grass, and randomly scattered islands of thickly vegetated hammocks; the perfect environment for a relaxing stroll, a picnic, or even a quick nap. All may appear calm within this enchanting panorama; however, the perceived tranquility is but a chimera. A mere illusion of serenity resulting from shortfalls in the ability of Homo sapiens’ photoreceptors to see beyond the narrow range of the electromagnetic spectrum called visible light, an inability to hear sound outside of 22000 Hertz, and the failure of the human olfactory system to nose its way into the vast chemo-landscape of pheromones and other volatile chemicals in which it is continuously assailed. If the sensory apparatus of Homo sapiens was keener - more finely calibrated – the landscape of the Big Cypress would appear very different.
Very different indeed, imagine the ecological interplay that could be interpreted if humans could see ultraviolet light through the eyes of a bee, smell pheromones from six-miles’ distance like a moth, or interpret chemical stimuli through soil like a plant… Far from serene, if viewed through time, adaptive maneuvers, survival strategies and arms races would be manifest in every action undertaken by the immense diversity of organisms on Earth. If these actions could be viewed more directly, the landscape would appear saturated with war. Even the plant community boundaries which demarcate prairie from savanna from hammock in the above described landscape are maintained by way of fierce battles waged over evolutionary time. These ecosystems, which appear stable and so pleasingly haphazardly scattered, are in fact tightly ordered armies of competing plants struggling for resources and existence. In these recurrent ecotonal conflicts one species has honed a new weapon – it has adapted to exploit the power of fire.
Naturally existing plant communities exist in a continuum of ecosystems which through evolution have adapted to almost every available habitat on the planet; from “box thorns” (Lycium pallidum) in Death Valley to fully aquatic hyacinths (Eichhornia paniculata) floating around the lakes of Brazil, genetic plasticity in plants is the product of natural selection. Although diverse habitats represent a surmountable challenge, a multitude of both biotic and abiotic factors determine the overall abundance (density), composition (diversity) and ultimate success of plant communities at any given location.
For example, looking across the landscape of the Big Cypress, densely concentrated hardwood trees form hammocks which, due to the broad area of their collective canopies, limit the amount of sunlight available to underlying herbaceous groundcover. This is a straight forward relationship - no sun reaching the ground means fewer plants on the ground. Following this rationale, if the tree canopy should be opened (by a storm, hurricane or by the death of older trees) and sunlight is able to temporarily penetrate to the floor, a rapid emergence of both herbaceous plants and new saplings would be predicted. This is precisely what happens; sunlight is the limiting resource, once made available, those plants best able to take advantage of the situation (through rapid growth) will be able to literally overshadow their competitors; plants with genetic compliments favoring a period of “initial rapid growth” are at an advantage and will be positively selected.
Extrapolating this scenario to the prairies of Big Cypress begs the question – why aren’t there any trees in the prairie?
Prairies, typically found on relatively low topographical gradients in Florida, have an abundance of soil nutrients and water; at least enough to support the enormous quantity of grasses and herbaceous plants currently found there. Additionally, from the perspective of a tree, grasses present little competition for sunlight. So, what is it that prevents trees from invading the prairie?
One often suggested possibility is that because prairies are occupied by dense populations of grasses - some of which more than six feet in height - young trees are prevented from taking hold; sort of like a reverse hammock scenario in which the grasses overshadow the young trees thereby starving them of sunlight instead of vice versa… This is plausible, but why wouldn’t trees take hold after wildfires? Wildfires have been historically inevitable in Florida and have the effect of clearing grasses long enough for those trees possessing an “initial rapid growth gene” in their arsenals to stake a claim.
Slash Pine (Pinus elliotti), for example, has just such an initial rapid growth gene. This permits the tree to take advantage of any opportunity to seize real estate, whether it is in a forest or a prairie. Slash Pine is even capable of expressing secondary needles in less than six month’s time – seedlings grow rapidly. Once present, this conifer could easily out-compete grasses for sunlight.
A quick word about wildfires: Florida’s climate cycle is punctuated by alternating dry and wet periods. November to February is the dry season, with relatively little precipitation, and is followed by heavy rains and thunderstorms (particularly near the coasts) during the months of June, July and August. The spring season, February through May, represents a transition from dry to wet, but during this period lightening strikes often cause wildfires due to the parched conditions of plants – parched, having just come out of the dry season. Regularity of climate has resulted in a cyclic “fire season” arriving during the early spring.
Speaking of wildfires… Being a regular occurrence, they are often offered as another explanation for limiting the advance of trees into prairies. Although this suggestion is partially correct, it isn’t the whole story - in some instances trees have even wielded fire as a weapon to destroy its grass competitors.
4-19-09 UPDATE: The Second Half of this Post can be Viewed HERE.
Beckage, B., Gross, L., & Platt, W. (2006). Modelling responses of pine savannas to climate change and large-scale disturbance Applied Vegetation Science, 9 (1) DOI: 10.1658/1402-2001(2006)9[75:MROPST]2.0.CO;2
Very different indeed, imagine the ecological interplay that could be interpreted if humans could see ultraviolet light through the eyes of a bee, smell pheromones from six-miles’ distance like a moth, or interpret chemical stimuli through soil like a plant… Far from serene, if viewed through time, adaptive maneuvers, survival strategies and arms races would be manifest in every action undertaken by the immense diversity of organisms on Earth. If these actions could be viewed more directly, the landscape would appear saturated with war. Even the plant community boundaries which demarcate prairie from savanna from hammock in the above described landscape are maintained by way of fierce battles waged over evolutionary time. These ecosystems, which appear stable and so pleasingly haphazardly scattered, are in fact tightly ordered armies of competing plants struggling for resources and existence. In these recurrent ecotonal conflicts one species has honed a new weapon – it has adapted to exploit the power of fire.
Naturally existing plant communities exist in a continuum of ecosystems which through evolution have adapted to almost every available habitat on the planet; from “box thorns” (Lycium pallidum) in Death Valley to fully aquatic hyacinths (Eichhornia paniculata) floating around the lakes of Brazil, genetic plasticity in plants is the product of natural selection. Although diverse habitats represent a surmountable challenge, a multitude of both biotic and abiotic factors determine the overall abundance (density), composition (diversity) and ultimate success of plant communities at any given location.
For example, looking across the landscape of the Big Cypress, densely concentrated hardwood trees form hammocks which, due to the broad area of their collective canopies, limit the amount of sunlight available to underlying herbaceous groundcover. This is a straight forward relationship - no sun reaching the ground means fewer plants on the ground. Following this rationale, if the tree canopy should be opened (by a storm, hurricane or by the death of older trees) and sunlight is able to temporarily penetrate to the floor, a rapid emergence of both herbaceous plants and new saplings would be predicted. This is precisely what happens; sunlight is the limiting resource, once made available, those plants best able to take advantage of the situation (through rapid growth) will be able to literally overshadow their competitors; plants with genetic compliments favoring a period of “initial rapid growth” are at an advantage and will be positively selected.
Extrapolating this scenario to the prairies of Big Cypress begs the question – why aren’t there any trees in the prairie?
Prairies, typically found on relatively low topographical gradients in Florida, have an abundance of soil nutrients and water; at least enough to support the enormous quantity of grasses and herbaceous plants currently found there. Additionally, from the perspective of a tree, grasses present little competition for sunlight. So, what is it that prevents trees from invading the prairie?
One often suggested possibility is that because prairies are occupied by dense populations of grasses - some of which more than six feet in height - young trees are prevented from taking hold; sort of like a reverse hammock scenario in which the grasses overshadow the young trees thereby starving them of sunlight instead of vice versa… This is plausible, but why wouldn’t trees take hold after wildfires? Wildfires have been historically inevitable in Florida and have the effect of clearing grasses long enough for those trees possessing an “initial rapid growth gene” in their arsenals to stake a claim.
Slash Pine (Pinus elliotti), for example, has just such an initial rapid growth gene. This permits the tree to take advantage of any opportunity to seize real estate, whether it is in a forest or a prairie. Slash Pine is even capable of expressing secondary needles in less than six month’s time – seedlings grow rapidly. Once present, this conifer could easily out-compete grasses for sunlight.
A quick word about wildfires: Florida’s climate cycle is punctuated by alternating dry and wet periods. November to February is the dry season, with relatively little precipitation, and is followed by heavy rains and thunderstorms (particularly near the coasts) during the months of June, July and August. The spring season, February through May, represents a transition from dry to wet, but during this period lightening strikes often cause wildfires due to the parched conditions of plants – parched, having just come out of the dry season. Regularity of climate has resulted in a cyclic “fire season” arriving during the early spring.
Speaking of wildfires… Being a regular occurrence, they are often offered as another explanation for limiting the advance of trees into prairies. Although this suggestion is partially correct, it isn’t the whole story - in some instances trees have even wielded fire as a weapon to destroy its grass competitors.
4-19-09 UPDATE: The Second Half of this Post can be Viewed HERE.
Beckage, B., Gross, L., & Platt, W. (2006). Modelling responses of pine savannas to climate change and large-scale disturbance Applied Vegetation Science, 9 (1) DOI: 10.1658/1402-2001(2006)9[75:MROPST]2.0.CO;2
Nordlund, D., & Lewis, W. (1976). Terminology of chemical releasing stimuli in intraspecific and interspecific interactions Journal of Chemical Ecology, 2 (2), 211-220 DOI: 10.1007/BF00987744
Morse, A., Peterson, D., Islam-Faridi, M., Smith, K., Magbanua, Z., Garcia, S., Kubisiak, T., Amerson, H., Carlson, J., Nelson, C., & Davis, J. (2009). Evolution of Genome Size and Complexity in Pinus PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004332
Platt, W. J., J. M. Huffman, M. G. Slocum, and B. Beckage. In press. Fire regimes and trees in Florida dry prairie landscapes. In: Noss, R. & Singh, S. (eds.) Land of fire and water: The Florida dry prairie ecosystem. Avon Park Air Force Range and Department of Defense, Avon Park, FL,
US.Kabrick, John M.; Dey, Daniel C.; Gwaze, David, eds. Shortleaf pine restoration and ecology in the Ozarks: proceedings of a symposium; 2006 November 7-9; Springfield, MO. Gen. Tech. Rep. NRS-P-15. Newtown Square, PA: U.S. Department of Agriculture, Forest Service, Northern Research Station: 28-32.
Sunday, March 29, 2009
The Hunting Modes of a Wolf Spider
Taking a break from stalking prey amid the jungle of leaf litter in my front yard, this spider sprinted across my patio last Monday night. I managed to drop my compass next to him (the edge of which has a two inch ruler) and approximate his Cephalothorax-to-spinneret length at about 7/8 of an inch.
My best guess is Lycosa gulosa, though I’m somewhat doubtful due to the size of this beast… (If anyone else has an alternative ID be sure to let me know.)
Wolf Spiders are ferocious predators, and in captivity have been shown to scavenge as well as hunt. They hunt mostly by cruising; moving slowly and deliberately through high prey areas, taking precaution as to not raise the notice of local cockroaches, moths and other quarry. They also occasionally practice an ambush mode of hunting during which they remain motionless – or even actively conceal themselves – waiting for unwary meals to stumble on them.
The cruising and ambush styles of hunting have evolved specifically as counters to the sensory systems of prey species – to either avoid detection while on approach, or, to completely overwhelm the capabilities of the prey’s sense organs. As detailed in a recent article by Jérôme Casas, Thomas Steinmann and Olivier Dangles, “crickets, cockroaches and other orthropteroid insects are equipped with air-flow sensors (filiform hairs) at the rear end of their abdomen. They possess many short hairs, serving as acceleration sensors, and fewer long hairs (velocity sensors) on their cerci. These mechanosensors are among the most sensitive sensors in the animal kingdom, with action potentials triggered by less than one tenth the energy of a photon; indeed, the orthropteroid escape system, and in particular fluid flow sensing using filiform hairs, has maintained textbook-example status over many years. Thus, we hypothesised that spiders use the two different hunting strategies to cope with optimal air-flow detection by crickets.”
If you have an interest in spiders, or arthropods at all, check out The Aerodynamic Signature of Running Spiders to see the results of using Digital particle image velocity (DPIV) measurements of a running spider in an air tunnel to estimate prey response…
Casas, J., Steinmann, T., & Dangles, O. (2008). The Aerodynamic Signature of Running Spiders PLoS ONE, 3 (5) DOI: 10.1371/journal.pone.0002116
My best guess is Lycosa gulosa, though I’m somewhat doubtful due to the size of this beast… (If anyone else has an alternative ID be sure to let me know.)
Wolf Spiders are ferocious predators, and in captivity have been shown to scavenge as well as hunt. They hunt mostly by cruising; moving slowly and deliberately through high prey areas, taking precaution as to not raise the notice of local cockroaches, moths and other quarry. They also occasionally practice an ambush mode of hunting during which they remain motionless – or even actively conceal themselves – waiting for unwary meals to stumble on them.
The cruising and ambush styles of hunting have evolved specifically as counters to the sensory systems of prey species – to either avoid detection while on approach, or, to completely overwhelm the capabilities of the prey’s sense organs. As detailed in a recent article by Jérôme Casas, Thomas Steinmann and Olivier Dangles, “crickets, cockroaches and other orthropteroid insects are equipped with air-flow sensors (filiform hairs) at the rear end of their abdomen. They possess many short hairs, serving as acceleration sensors, and fewer long hairs (velocity sensors) on their cerci. These mechanosensors are among the most sensitive sensors in the animal kingdom, with action potentials triggered by less than one tenth the energy of a photon; indeed, the orthropteroid escape system, and in particular fluid flow sensing using filiform hairs, has maintained textbook-example status over many years. Thus, we hypothesised that spiders use the two different hunting strategies to cope with optimal air-flow detection by crickets.”
If you have an interest in spiders, or arthropods at all, check out The Aerodynamic Signature of Running Spiders to see the results of using Digital particle image velocity (DPIV) measurements of a running spider in an air tunnel to estimate prey response…
Casas, J., Steinmann, T., & Dangles, O. (2008). The Aerodynamic Signature of Running Spiders PLoS ONE, 3 (5) DOI: 10.1371/journal.pone.0002116
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