Blute Blog

Blute's blog about evolutionary theory: biological, sociocultural and gene-culture.

Posts Tagged ‘phylogenetics

Two evolutionary pathways meet phylogenetics

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In a previous post and elsewhere (e.g. here Biological Theory 2(1) 10-22 gated and here Spontaneous Generations ungated ) I suggested extending Van Valen’s definition of evolution to incorporate development and ecology and to acknowledge the fact that there are two distinct pathways to evolution by natural selection. New genes can reconstruct old environments or new environments can reinduce the expression of old genes or both, but even if both, one or the other is likely to lead and the other to follow initially. In an article this month (Trends in Ecology and Evolution V26#3 here), Schwander and Leimar call the first “genes as leaders” and the second “genes as followers”. P. Z. Myers once exquisitely analysed an experimental case on Pharyngula here (a case originally published by H. F. Nijhout in Science).

It is important that the possibility of “genes as followers” not be misunderstood as sneaking Lamarckian explanations for adaptation in by the back, or should I say by a side door! Environmental influences on phenotypes beyond the historically experienced range, just as novel genes expressing a norm of reaction beyond that historically expressed, are more likely to be maladaptive than adaptive – which is not to say that either cannot have a selective effect – albeit one usually negative, at least at first.

Now to the new development. Schwander and Leimar make the novel (but in retrospect obvious) proposal that which direction change has taken place in any particular case can be detected using phylogenetic methods. Recall that Darwin said that his theory of descent with modification was composed of two great principles – “the unity of types” (i.e. history) and the “conditions of existence” (i.e. natural selection). Since the work of Harvey and Pagel in the late 1980s and early 1990s, it has become common in evolutionary ecology, and even in some cases in anthropology, to control for the former in testing particular hypotheses about the latter. However such methods have never before to my knowledge been applied to this question.

Schwander and Leimar apply “ancestral state reconstruction methods” to the question of how commonly switches (and losses) have taken place between genetic polymorphisms and polyphenisms. They admit to many limitations of their study. They consider only discrete and not continuously varying characteristics. They use selected examples, i.e. the evidence is anecdotal. Nevertheless, by mapping genetic polymorphisms and polyphenisms (as well as losses) for the same phenotypic characteristics in the same larger group such as winged and winglessness in carabid beetle species and right and left handedness in Heteranthera plant species for example onto their phylogenies, they have begun to pave the way towards answering this fascinating question. Bravo!

From the series of examples considered, they conclude that there is “no clear tendency for genes to be followers or leaders overall”. However it is important to understand that an historical change to, does not necessarily mean a change by means of . An evolutionary change from a genetically polymorphic species to a polyphenic one could nevertheless have been initiated by a genetic mutation or recombination followed by selection for adaptive phenotypic plasticity under conditions of uncertainty but with reliable cues. However, given that virtually the identical alternative phenotype was present as a genetic alternative in the ancestral polymorphic species, it is more likely that the alternative phenotype was latent in the formation of the new species and came to be induced there at least initially by an altered environment. In other words, it was likely a change initiated by environmentally inductive means as well as a change to environmentally inductive control. Still, as the authors eventually make clear, “the frequency and direction of transitions between them depends not only on how often either system emerges but also on how often one system is more beneficial than the other. Polyphenism should be favoured when a phenotype-determining environmental cue accurately predicts the selective condition for which the corresponding morph is suited, whereas genetic polymorphism should be favoured when such environmental cues are lacking and there is local frequency dependence or spatial variation in conditions, combined with limited gene flow.”

Written by Marion Blute

March 21, 2011 at 8:03 pm

Sociocultural Phylogenetics

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The text of Darwinian Sociocultural Evolution: Solutions to Dilemmas in Cultural and Social Theory went in at the end of March 2009, so it was almost a year before it was published. I would not change the thesis of the whole nor of each chapter today, but new material continues to come out which I would like to comment on in an informal context, while also posting about more general evolutionary issues.

For example, remarkable empirical results in sociocultural phylogenetics continued to be produced in 2009. Gray Drummond and Greenhill, extending an earlier analysis, applied Bayesian phylogenetic methods [gated] to 210 lexical items of 120 languages to shed light on the peopling of the Pacific. Their analysis concluded that the evolution of Austronesian languages was closely coupled with the geographic expansion of people and proceeded in a series of pulses interrupted by pauses originating in modern day Taiwan around 5,230 years ago and ending eventually in New Zealand between 1200 and 1800 years ago. They hypothesize that the pulses were the result of technological inventions, originally the outrigger canoe.

In related work, Jordan, Grey, Greenhill and Mace matched ethnographic data for 135 of these societies and with the same 210 lexical items and were able to infer that matrilocal rather than patrilocal or ambilocal post-marital residence was the original ancestral state of these societies. Finally, Rogers, Feldman and Ehrlich developed a method for identifying historical sequences in cultural data even in the presence of horizontal exchange and applying it to a set of data on Polynesian canoe design traits, they were able to infer Fiji as the ancestral homeland of these traits, that cultural distance increased with geographic distance, and that there was a serial founder effect of declining diversity with distance culturally just as there normally is genetically.

It is remarkable that these three studies show that applications of methods originating in biology can be used to reconstruct ancestral states and infer details of human cultural history across 5,000 years – linguistic in the first case, social organizational in the second and technological in the third. Knowledge across five thousand years may not seems like much to a paleontologist, a paleoanthropologist or even an archaeologist but to cultural anthropologists and sociologists it is remarkable.

Written by Marion Blute

March 4, 2010 at 2:31 am