Epigenetic Factors as 'Creative' influence in Evolution

Hi PR! Welcome to EVCforum. It's great to see a gen-u-ine evo psych researcher join the forum. It'll be a super addition to the usual run-of-the-mill molecular biologists, geneticists and others of that ilk that haunt this place when they should be working. A word of caution: do NOT engage resident poster Syamasu. He's convinced that evo psych represents the short road to perdition, and is ultimately responsible for every societal ill from eugenics, the Holocaust and the rise of national socialism in 1930's Germany to premature male balding... Otherwise, enjoy your stay - this is a great forum.Back on topic. You stated: I assume you're talking about the apple maggot fly (Rhagoletis pomonella), probably one of the most studied examples of sympatric speciation ever (for obvious commercial reasons). The mechanism and evolution of the two host races - one on hawthorn and the other on imported apple trees - is fairly well understood (or at least has reasonable empirical support). The initial host shift is likely to be related to competition and the availability in the immediate vicinity of a new resource (historical records indicate that many early apple trees were planted in proximity to existing populations of hawthorn). In addition, Rhagoletis appears to be highly plastic (i.e., adaptable). Finally, the heritable component the sympatry is likely related to the evolution of host-specific mating preferences - possibly caused by different chemical composition of the food sources. Interestingly, it appears that the different host races will freely inter-breed when in contact, which means we're not looking at distinct species yet. However, there is evidence that the rate of hybridization is getting smaller - a good indicator that we have a case of incipient speciation happening.To make a long story short, I'm not seeing how the differentiation developing between the hawthorn and apple-specific host races of Rhagoletis pomonella is related to epigenetics. It appears pretty straight-forward neodarwinism. There also doesn't appear to be evidence of "pro-active adaptive behavior" here. Could you expand on how this would be differentiated from any other speciation mechanism?refs:Bush GL, 1992, "Host race formation and sympatric speciation in Rhagoletis fruit flies (Diptera: Tephritidae)", Psyche 99:335—357. Bush is about the most cited authority on Rhagoletis, so you've probably run across him before.Dres M, Mallet J, 2002 "Host races in plant-feeding insects and their importance in sympatric speciation", Phil. Trans. R. Soc. Lond. 357:471—492, provides a good overview of the causes and genetics of host shift in phytophagous insects, including 21 case studies. Although primarily an attempt to develop a new definition and valid criteria for determining sympatry, it gives some good background on the issue from a genetic/ecology perspective.Hawthorne DJ, Via S, 2001, "Genetic linkage of ecological specialization and reproductive isolation in pea aphids", Nature 412:904—907. Although not about Rhagoletus, the article shows much more clearly how resource competition provides the initial impetus to host shift leading to creation of distinct host races - again from a neodarwinian standpoint.Looking foward to your reply.(edited to fix formatting)[This message has been edited by Quetzal, 03-04-2004]

Hi PR,I don't think too many people are going to violently disagree with this part: At least, as far as the effects of environmental triggers on both development - which may or may not be directly linked to adult morphology - and behavior. Behavior especially is highly modifiable by environment. Examples of learned behaviors abound in organisms as disparate as birds and primates (one of my favorite examples is the natural history of the Cocos Island Finch, which is a heterospecific behavioral mimic). I also don't think anyone is going to be arguing with your contention that DNA plays only one part - albeit IMO a fairly major one - in the phenotype (including behavior) of organisms. Adaptation can take numerous forms for numerous causes. However, I would argue that the fundamental genetic basis for either morphology or behavior must be in place in the genotype for any of these modifications to be inheritable. Mutation is simply the major way in which new variations are created in populations of most organisms. Humans, OTOH, seem to be more influenced by culture than biology. As it is clear that "cultural evolution" works at a substantially faster pace than biological, this is unsurprising. However, it isn't clear that the same applies to other organisms.On a side note, I have to apologize in advance for once again questioning one of your examples. There have been a number of studies done on A. tigrinum that indicate a) the development of cannibal morphs is critically dependent on environmental cues (see, for example, Hoffman EA, Pfennig DW, 1999, "Proximate causes of cannibalistic polyphenism in larval tiger salamanders", Ecology 80:1076-1080) and b) the "ability" to switch is genetically determined (see, for example, Pfennig DW, 1990 "The adaptive significance of an environmentally-cued developmental switch in an anuran tadpole" Oecologia 85:101-107 and Pfennig DW, Collins JP, 1993 "Kinship affects morphogenesis in cannibalistic salamanders" Nature 362:836-838). Finally, the idea that the ability to switch is genetically determined is underlined by the fact that morphogenesis into the cannibal morph can be stopped when the environmental cue is removed (see, Reilly SM, Lauder GV, Collins JP, 1992 "Performance consequences of a trophic polymorphism: feeding behavior in typical and cannibal phenotypes of Ambystoma tigrinum" Copeia 1992:672-679). Facultative metamorphosis into the cannibal variant is, as you said, environmentally triggered. However, the ability to do so is coded in the DNA and inherited. Absent the environmental cue, the existing capability is never expressed, and is simply passed on to the next generation with all the rest of the genotype. With the right trigger, we have critters eating their conspecifics.It's entirely possible I'm still missing what you're arguing. Excuse me if that's the case.

Hi PR. Thanks for your reply. I don't really disagree with this, except to say that some genes are activated or not activated by non-genetic factors (i.e., environment). However, the folks that take a "genes-eye view" would include the other genes in the body as part of the environment. IOW, whereas the expression of a given gene or genetic cascade may be effected by control genes, they're also often effected by non-control genes or factors. It almost looks like you're trying to analyze what happens to a given gene in isolation from the other genes - even non-control types - and I'm not sure that's valid. I would say that the specific sequences on a strand of DNA that code for the expression of particular genes ARE the fundamental starting points - and that everything else simply modifies how the gene is expressed. Perhaps I'm still not seeing what you're trying to get at (no surprise, genetics is not my field). Umm, okay. I'll grant you the evolvability of the system is dependent on environmental factors - after all, that's what natural selection is all about. However, I think I disagree with your statement that the ability to achieve differentiation is not in the genes. There are several interesting current research hypotheses that tend to indicate otherwise. For example,

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Kirschner M, Gerhart J, 1998 "Evolvability", PNAS 95:8420-8427

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Evolvability is an organism’s capacity to generate heritable phenotypic variation. Metazoan evolution is marked by great morphological and physiological diversification, although the core genetic, cell biological, and developmental processes are largely conserved. Metazoan diversification has entailed the evolution of various regulatory processes controlling the time, place, and conditions of use of the conserved core processes. These regulatory processes, and certain of the core processes, have special properties relevant to evolutionary change. The properties of versatile protein elements, weak linkage, compartmentation, redundancy, and exploratory behavior reduce the interdependence of components and confer robustness and flexibility on processes during embryonic development and in adult physiology. They also confer evolvability on the organism by reducing constraints on change and allowing the accumulation of nonlethal variation. Evolvability may have been generally selected in the course of selection for robust, flexible processes suitable for complex development and physiology and specifically selected in lineages undergoing repeated radiations.

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Another, a bit more extended, treatment is given in Lynn Caporale's popsci book "Darwin in the Genome" (McGraw-Hill, 2002). For a non-specialist like me, this was a thought-provoking book. I agree with this. However, I think I need to clarify what I mean by culture: "Culture or civilization, taken in its wide ethnographic sense, is that complex whole which includes knowledge, belief, art, morals, law, custom, and any other capabilities and habits acquired by man as a member of society" (Edward Tylor, 1920, quoted in Ehrlich P, 2000 "Human Natures", Penguin Science, page 352). Of course, this definition doesn't permit the existence of non-human "cultures", a conclusion I'm not comfortable with. So to refine it a bit, the way I used "culture" refers to those behavioral attributes that are emergent properties of the interactions between members of social species. Change in human-level complex culture is influenced by mechanism(s) analagous to, but not equivalent to, those involved in natural (phenotype) selection. Although, as culture is based on behavior there are inheritable genetic underpinnings, one of the key differences is that culture can be transmitted laterally as well generationally. Culture - or aspects of it anyway - can ALSO be modified (evolve?) in a single generation, unlike behaviors that are more dependent on genetics, which can mostly only evolve over multiple generations. I'm not sure this necessarily follows. The pattern of cannibal morph expression appears close to what I would expect to see if the allele is maintained in the species at relatively low frequency. Not all, or even most, members of the population would necessarily posess the allele. In addition, the environmental triggers are complex - and can include even the increased probability of inducement of cannibal morphs in populations that already contain cannibal morphs compared with populations that don't. IF there's no trigger present, then there would be no reason for the allele to be under selection - and it would then "hitch-hike" down the generations effected by drift or other stochastic processes. I don't know what linkages there are - i.e., whether the gene complex that creates the cannibal morph is linked to some other set of genes that IS under selective constraint and hence conserved in the population. Tis a puzzlement...