There’s a movement afoot by money-hungry but misguided scientists to claim that the Modern Synthesis (MS) of evolutionary biology is fatally flawed. (Many of these researchers are funded by the John Templeton Foundation, which recently handed out $11 million dollars for work on this fruitless endeavor.)
According to the critics, the areas that supposedly have inflicted mortal wounds on the MS are niche construction, evolutionary developmental biology (“evo devo”), developmental plasticity, and, above all, epigenetics—the idea that evolutionary adaptation can be fueled by changes in the genome produced not by mutation altering the sequence of basis in the DNA, but alterations in the genome induced by the environment itself, like changes in the histone scaffolding of DNA or methylation of the DNA bases.
I’ve discussed these so-called fatal flaws before (e.g., here) and find them grossly overblown. They either limn phenomena that already fit comfortably within the MS, or propose scenarios that, while logically plausible, are supported by almost no evidence.
One of the latter is epigenetics, about which I hear incessantly. Does the environment actually induce changes in DNA methylation that can be the basis of adaptive change? The evidence for that is virtually nonexistent, for those changes usually disappear after one or a few generations, and thus cannot be the permanent heritable variation required for long-term adaptation.
This issue was recently discussed at length in a new paper in the Proceedings of the Royal Society, “The sources of adaptive evolution” (reference below; download not free but some judicious inquiry might yield a pdf). The paper was written by my good friends Deborah Charlesworth, Nick Barton, and Brian Charlesworth (the Charlesworths were colleagues in Chicago until deciding to return to the UK). The paper, which considers the evidence for epigenetic bases of adaptation, is written very clearly, a refreshing change from the postmodernist babble I’ve been inflicting on you lately.
The paper’s verdict: there’s little evidence for environmentally induced DNA or histone alterations that can fuel adaptive evolution, and the MS remains on solid footing. A few words from their conclusions (I’ve omitted the references, and the bolded bit is my doing):
Genetic studies of adaptive phenotypes have yielded several further important conclusions. First, there are now many examples of phenotypic differences within and between species whose genetic control maps to a small region, but with multiple nucleotide differences within the region being causally involved. This supports Darwin’s and Fisher’s view that adaptive phenotypes are usually built up by a series of relatively small changes, which has been challenged by proponents of the EES.
Second, phenotypes that show plastic responses to environmental conditions also often show considerable genetic variation in these responses, and DNA sequence variants associated with these heritable differences have been identified, supporting the view that plasticity has evolved in a neo-Darwinian fashion. For example, vernalization responses in flowering plants involve a period of exposure to cold that is required for seed germination. (This was the basis for the notorious Lamarckian theories of T. D. Lysenko, which seriously damaged Soviet agriculture.) Vernalization is under the control of a complex epigenetic regulatory system, which is reset each generation. Natural vernalization response differences are controlled by DNA sequence variation in cis-acting regulatory sequences.
In contrast with the rigorous empirical evidence for the role of DNA sequence variants in adaptive evolution that we have outlined, there is currently little evidence for effects of epigenetic changes, although more data are required. Recent claims for such effects have been based on evidence that changes affecting the methylome are more numerous than some types of sequence variants in evolving lineages of Darwin’s finches and darter fish. Such comparisons, however, provide no evidence that the epigenetic variants in question had any role in phenotypic evolution.
The paper also considers the notion of “directed mutation” or “adaptive mutation”, a truly non-MS concept, but finds virtually no evidence for such nonrandom changes in DNA. Other topics discussed (and critiqued) are “evolvability” and non-genetic “adaptive plasticity” as a basis for evolution.
It’s a good paper, even if it does buttress the status quo!
h/t: Bruce
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Charlesworth, D., N. H. Barton, and B. Charlesworth. 2017. The sources of adaptive variation. Proceedings of the Royal Society B: Biological Sciences 284.DOI: 10.1098/rspb.2016.2864