Wednesday, September 30, 2009

Demonstrating Speciation via Fisherian Runaway

The 1930 publication of Ronald A. Fisher’s eminent work The Genetical Theory of Natural Selection helped pave the way for development of the modern evolutionary synthesis.


Actually… If truth-be-told, I’d be inclined to give Fisher’s work a bit more credit for Neo-Darwinism than just “helped pave the way”. Not to shortchange the labors of Huxley, Haldane and the rest of modern biology’s forefathers, but in my assessment Fisher’s linkage of natural selection with genetics was the perhaps the greatest single contribution to the study of evolution since Charles Darwin.

Getting back to the reason for this blog post…

One of Fisher’s many contributions to biology was the idea that through a process of sexual selection, the favored phenotypic traits exhibited by individuals can – if consistently chosen during courtship – increase in frequency and become more numerous in a population, even if the traits offer no fitness advantage.

For example, if the females in a hypothetical bird species happen to find large and colorful feathers attractive, they may choose male birds who display brilliant feathers as their reproductive partners; at least more often than they choose males with dull feathers. Over multiple reproductive episodes, the hypothetical bird population may come to have more-and-more males with brilliant feathers. A greater abundance of colorful males means that competition between males for female access becomes more challenging. The reason for this increase in competition is that the colorful males are no longer competing with just dull feathered rivals, now they find themselves in contest with other brilliants! From the female’s perspective, choosing a mate becomes more tedious, now she must pick not only a male with brilliant feathers, now she must scrutinize each suitor for the brightest and boldest of feathers – the best of the best. This process of increased selection pressure for feather color continues – in a “runaway” process – that culminates with not only a population of birds in which males display brilliantly colored feathers, but even more, the end result is a population that displays larger, more ornate and more colorful feathers; the process compounds and undergoes a positive feedback. All of this due to a female’s mate preference, which she passes on to her female offspring and potentially has nothing to do with fitness (As a side note, this would be a good spot to pitch a previous post Sexual Selection, Good Genes and Condition-Dependent Handicaps).

OK, enough about birds-of-paradise, peacocks or whatever hypothetical bird population I was referring to, the point of all this was to say the following: although the tendency for preferred traits to compound through time has long been observed, it was Fisher that made the connection to genetics. He explained how secondary sexual characters and mating preferences are genotypically bound to mate choice itself. Further, because of the selection pressure levied against secondary sexual characteristics, the potential to sexually isolate a population is ever present; should a new phenotypic variation arise that falls outside the range of female preference, a new species could arise. And, new research published at BMC Biology affirms this Fisherian Runaway dynamic.

From the abstract: This study spotlighted SLa as a novel mate-choice gene in fish. In addition, these results are the first demonstration of a single gene that can pleiotropically and harmoniously change both secondary sexual characters and mating preferences. Although theoretical models have long suggested joint evolution of linked genes on a chromosome, a mutation on a gene-regulatory region (that is, switching on/off of a single gene) might be sufficient to trigger two 'runaway' processes in different directions to promote (sympatric) speciation.

Basically stated, a single gene alteration in a fish changes its color, and the preference for that color by the opposite sex. In turn, this leads to sexual isolation and ultimately a new species…

Fisherian Runaway!

Fukamachi, S., Kinoshita, M., Aizawa, K., Oda, S., Meyer, A., & Mitani, H. (2009). Dual control by a single gene of secondary sexual characters and mating preferences in medaka BMC Biology, 7 (1) DOI: 10.1186/1741-7007-7-64


  1. cool... less need for mountains, rivers and geographical separation.

  2. Daniel – right on!

    The origin of new species due to sexual isolation by physical barriers, such as mountains and rivers is called “allopatric speciation.” The type of speciation described in the Fukamachi article is referred to as “sympatric”. With sympatric speciation populations share the same range.

    Thanks for stopping by!

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