Wednesday, January 14, 2009

Improbable Mountains in the Landscape

Chris Nedin at Ediacaran posted an excellent blog discussing Richard Dawkins’ Mt. Improbable analogy. Below I have cut and pasted a copy with my thoughts interdispersed; for the purposes of commenting only. His text is in quotes.

"In the ongoing Adaptationist v. Pluralist debate, both sides agree on a surprising amount. Both sides agree that there is more to evolution than adaptation by natural selection. However, Adaptationist would argue that adaptation by natural selection is the most important, or even the overwhelming, evolutionary process, and that evolution can be described as climbing Mt. Improbable – with adaptation to environment similar to climbing a fitness landscape peak towards optimal fitness (but please note, never, ever, reaching the top!)"

This misses the mark somewhat, and I think there are a couple of flawed inferences that would benefit from a touch of clarification. Firstly, the mountain metaphor was (and is) intended as fortification against the argument from design; or contra spontaneous materialization of “fully formed” and/or “fully adapted” organisms. When referenced in debates such as that of the “Adaptationist v. Pluralist” it can also be interpreted as being oppositional to radical saltations and other major macromutations of behavior or morphology that would result in the instantaneous generation of novel adaptations, or even a new species – such as making the leap from an individual amphibian to a reptile in a single birthing episode. In terms of fitness landscapes- peaks and mountains are only informative with respect to local genotypic values; in this way the analogy can be seen as limiting to lateral jumps between said peaks. In either the design or anti-saltation interpretations, Mt. Improbable certainly isn’t proposed as a formal hypothesis or mathematical model; it’s proposed as a mental tool to assist the lay science reader and general population in understanding the fundamental idea of change through time.

Mt. Improbable’s “peak” isn’t equivalent to “optimal fitness” or “adaptive perfection.” Indeed, adaptive perfection doesn’t exist, nor would Dawkins propose that it does; natural selection is only capable of providing a “better” solution, not a “best” solution. The mountain’s peak represents the current state and condition (inclusive of fitness) of any given organism, which when viewed from afar gives the FALSE appearance of being optimal, designed or perfectly adapted. From this point of view Mt. Improbable doesn’t detail where an organism is going or to what environmental factors it’s trying to adapt; instead it explains how the organism reached it’s current circumstance (i.e. gradually, not in a single giant leap).

The steady incline located on the opposite side of Mt. Improbable is the antithesis of a sheer cliff or rock face that would necessitate assistance from an external source or some other highly statistically unlikely event (i.e. improbable) in order to surmount or obtain it (“it,” being the before mentioned current condition). In this sense, the analogy of “climbing” wasn’t meant to represent a movement towards increased complexity, optimum fitness or some ideal form, but rather it was intended as a model for step-by-step, gradual processes in which leaps, jumps and major saltations are avoided. [To be clear - “gradual” used in this way doesn’t imply “slow,” or imperceptibly minute as may be interpreted during common use, but instead it means “graduated” as in stepwise or measured – not “slow change” but “bit-by-bit” by degrees of change.]

Regarding the statement, “but please note, never, ever, reaching the top!” – Of course it can’t be reached; as described above, a “better” solution (or increased fitness) is the only option. That said, some highly specialized organisms do get really close to the “end point” of their available genetic variation, when this occurs either the environment changes and they move in a different direction (up or down whatever mountain on which they happen to reside), or they enter a period of stasis (during stasis allele frequencies do change, but selection pressure isn’t sufficient to drive novel adaptations).

"I disagree. I think genetic drift accounts for most of the evolution that occurs, and natural selection, while very important – especially in creating diversity – accounts for a smaller percentage. However, both have worked together to produce the diversity of life on Earth."

Genetic drift absolutely serves as a source of variation within populations, and it may lead to adaptations, BUT in order for that variation or adaptation to become fixed in any meaningful way we need natural selection.

"I also have a problem with the Mt Improbable analogy . . . well, actually I have two problems.1) It perpetuates the idea that evolution is an upward striving process, and that derived or adapted groups are higher, than the less derived or less adapted and, as a consequence, fitter, advanced . . .better. (the old Tree of Life analogy problem.)"
There’s a key concept being missed here. The Mt. Improbable analogy does not, and is not intended to, predict or delineate evolutionary drivers, pressures or goals. Nor does it create a continuum of higher or lesser organisms. Its perspective is from the rearview mirror, it looks backwards in time.

Question: How did this organism reach such a state of seemingly perfect environmental synchrony?

Mt. Improbable’s Answer: Through a gradual bit-by-bit process, not in a single saltation.

Furthermore, the analogy doesn’t attempt to rank or compare the vastness of the Earth’s biota; it doesn’t establish a scale in order to weigh relative fitness or adaptation between individuals or species. It’s one critter atop the mount looking backwards though ancestral lineages.

"OK, maybe it is applicable to a fitness landscape, but there is no reason that the landscape has to have the peaks pointing upwards, . . . is there? Surely it's the distance between where you are on the peak and the schmucks on the fitness plane that is important, not the direction of that distance?"

No, there isn’t any particular reason why an upward rising mountain needs to be called into play in order for this concept to be understood. However neutral language does not lend itself to memorable metaphors; simplicity, clarity and fit are far more important. If envisioning an organism gazing up from a pit is more appealing than is considering one peering from a peak, then by all means…

Unless our goal is to create an artificial value system to scale and rank levels of fitness between unrelated organisms, the inter-schmuck distances and movements (upwards or downwards) seem irrelevant.

"Plus, fitness landscapes, are not permanent, or even solid. They change with the environment. A population/species, or whatever, may be quite "high" (see how hard it is to use neutral language)on a fitness peak one minute, and find itself down on the plane, or even in a fitness trough, with hardly any change in allele frequency, but a significant change in environment. In other words the fitness landscape moved underneath it."

Unless someone is implying that landscapes, environmental conditions, allele frequencies, selective pressures or any other biologic variables are static and non-dynamic - this almost seems a non sequitur. Of course, everything is in a state of flux; does the mountain analogy suggest stagnation of ecology?

"2) The real problem with the Mt Improbable analogy though, is that it gives the impression that as hard as it is to ‘climb up’ (and it is), the analogy suggests that it is relatively easier to ‘climb down’ - and it isn’t because its actually harder. OK that might be pushing the analogy a bit far – but that’s the point, it doesn’t hold up to detailed scrutiny."

It’s an analogy used to paint a picture of a basic premise. “Hard” climbing is in the eye of the beholder; a movement “up” or “down” requires the same amount of expenditure and neither direction denotes relative superiority.

"The real problem is that adaptation, in the broader picture, is an evolutionary cul-de-sac."
Quite the contrary, adaptation is antithetical to a “dead end” – Show me a successful lineage of organisms and I’ll show you an adaptation!

"Adaptation means that certain alleles are being selected for because they confer an advantage in a particular environment. If the environment changes, then the alleles that conferred an advantage may no longer do so. Worse, the very process of selecting for certain alleles may well have stopped other alleles getting fixed through drift – alleles which might be beneficial in the new environment. Even worse, the alleles that were originally selected for may be costly to produce and maintain where they confer no advantage, and thus be deleterious."

Does this imply, or openly state, that adaptation prevents and/or hinders biological evolution?

"But the really bad thing is that, as hard as it is to gain the alleles that provided an advantage, it is even harder to loose them, as this would require specific mutations to affect those particular alleles (rather than the random process that produced them). You could reduce them to a vestigial level, provided you survived long enough to do so. Difficult though, if you are struggling to survive in a new environment where the competition does not have the adaptive dead weight (unless you have some other advantage.)"

Phenotypes are not pro-actively reduced to a vestigial state; rather they’re no longer actively maintained. There’s no longer a pressure for the original phenotype to be preserved.

Once again, this would occur in a gradual manner (up or down directionality is irrelevant): If a species of fish finds itself in a dark cave portions of its anatomy supporting eyesight may no longer be positively selected. Overtime, sub-portions of this vision complex may undergo mutation, or be selected for another biological function. This may result in a slow degeneration of the complex through time. The eye will not disappear in its entirety overnight – it will be phased out. This however says nothing about complexity or the direction of evolution; the fish isn’t “more evolved” it’s better adapted to a given environment.

"The more adapted a group becomes, the more imbedded it is in a particular environment, and the more sensitive it is to changes to that environment."

Yes! This is the keystone of resource conservation – Save the Environment!

"Eventually all strategies lead to extinction, but during environmental change, it’s the generalists that survive, not the specialists. Adaptation generally leads to extinction. Highly adapted groups/species and ecosystems delicately balanced on a web of interconnected adaptations, will crash once environments change."

Seems a bit redundant here, everything leads to extinction + adaptation leads to extinction. My understanding is that some strategies improve fitness; others do indeed lead to extinction. And, generalists sometimes go extinct; specialists sometimes survive an environmental change.

Ecosystem’s “crash” is relative to which organisms are becoming extinct and which are becoming more specialized (adapted) to use (or better use) what resources remain.

"Adaptation is not climbing up Mt Improbable, it’s climbing down Pit Improbable!The pits are hard to find, but once in, it’s easier to go down than it is to back out, and if you adapt too far, you are trapped in a cul-de-sac with no way out when the environment changes. The generalists that flirt with the rim of the pit, or on the fitness plane have a better chance of surviving to become the stem stock for new adaptations. "
"It may well be that some species or groups of species in a pit break through to new fitness landscapes and produce new groups (e.g. birds and mammals from reptiles) because fitness landscapes are not flat, but curved.But for most populations/species, adaptation is a pit of no return."
Speciation, radiation, variety and biodiversity represent the consequences of specialization regardless of the drivers that push them to existence. “Ultimate survival” is a test to which we are all doomed to fail; however local survival in a given habitat, microhabitat or niche can be achieved and, in fact, is achieved more frequently than one should expect mathematically. This, in its majority, is due to specialization. From hyperthermophiles to lemurs – it’s adaptation that pushes life to the extremes, providing opportunity and prospect to the full range of creatures on the planet today. As the world’s climates continue to change, adaptation to the Arctic environment may eventually remove polar bears from existence; however it’s that same specialization and adaptation that put them there in the first place.

PS- It has just been pointed out to me that the Mt Improbable analogy was blogged (back in July) by Larry Moran at Sandwalk; with a counter argument by Jason Rosenhouse from EvolutionBlog.


  1. Hi Johnny,

    Thanks for the plug, and the comments on my blog.

    I'll keep things brief and just make some comments.

    I understand that the Mt Improbable analogy was formulated to argue against the radical saltation/macromutation idea. In my view the pit analogy is a better one, and one which more closely reflects what populations are actually doing.

    For example, you say about the peak Regarding the statement, “but please note, never, ever, reaching the top!” – Of course it can’t be reached; as described above, a “better” solution (or increased fitness) is the only option. That said, some highly specialized organisms do get really close to the “end point”. This implies that the peak preceded the population that climbs it, and so is "guiding" the population (even if there is no conscious guidance).

    In the pit analogy, which I am in the process of expanding, the pits are created by the concentration on alleles. As the concentration increases, so the pit deepens. There is no pre-existing landscape. The features on the landscape are thus formed by the activities of the populations.

    I also understand that Mt Improbable is not predicated on creating a continuum of higher or lesser organisms, but it gives the impression but showing that more adapted forms occupy the higher slopes.

    I agree that we need clear, accessible, memorable metaphors, and that Mt Improbable is definitely that, but can we also make time for others that may be a more accurate(?) metaphors.

    In fact I am trying to avoid value systems (actual or implied), but the distance between the pit and the landscape or peak and the landscape is important because it provides information on evolution.

    you say, Of course, everything is in a state of flux; does the mountain analogy suggest stagnation of ecology?.

    Well, yes I think it does, since you have to have the peak prior to the population that climbs it. Pl, the further up the peak the longer the peak has been there.

    You also say, “Hard” climbing is in the eye of the beholder; a movement “up” or “down” requires the same amount of expenditure.

    Actually, I think it doesn't. One of the main inferences for the Mt. Improbable analogy is that youcannot go down and on to another peak. It is easier for populations to acquire alleles that confer an advantage, than it is for them to loose those alleles. Since to loose the alleles requires specific mutations rather than the random ones with provided them.

    This is a major point. Once climbed, a peak isolates the population. But the analogy suggests that going back (down) would be easier. I think the pit more accurately reflects the actual situation here.

    When I say adaptation is a dead end, I mean that it is a valid mechanism to exploit current environments, but it's success is short term as it is linked to the current environment. Once that environment changes all bets are off. Almost by definition, the most adapted forms (i.e. those with the most invested in the adaptations) are usually the most successful in a specific environment. When that environment changes and the adaptations are no longer useful, those forms are most at risk. Yes, they can survive, but it seems intuitive that those forms with less "dead allele weight" have a better chance of surviving.

    With your fish example, a better one would be, you have two species of fish, one with large, well developed eyes, and another with small poorly functioning eyes. Which one has the higher maintenance cost for no gain? Which one is more likely to adapt first to the new environment?

    I fully agree that adaptation is a powerful mechanism, I'm just saying that specialisation comes with some drawbacks for long term survival.

    Again thanks for the interest.

  2. Chris,

    Many thanks for the added clarification of your perspective. I have responded to your above comments in a new blog post (RE: Improbable Mountains...) - I wanted to add a graph depicting what I believe to be the relationship that you're describing and couldn't do that here.

    I enjoy your work and hope that you get a chance to have a look.

    Thanks again.