How does this happen? How often does this happen? might be a response to the earlier post citing Waddington’s experiments, in which variation that originated as an appropriate response to environmental circumstances became more or less fixed over time in a population. To answer the “how often” question, researchers have to be looking for examples, and to look for examples they not only have to be motivated but to have a model of what they are looking for, that is, an idea of how it happens. First, how it happens in my view, then in the view of Waddington and others.
Consider some acquired character, moreover one that is an appropriate response to the environmental stress. Likely there will be a range in responsiveness. Moreover, the developmental paths to the “same” response will typically differ among individuals. If there is any survival and reproductive advantage to greater responsiveness, the offspring of matings in the population will come to concentrate in a subsection of the space of developmental paths. We can expect that eventually some individuals in that subspace of responsive developmental paths will show the character in the absence of the environmental stress. That is not always a good thing, but, if it is advantageous, the subspace of responsive developmental paths will come to include ever more of those “pre-responsive” individuals. The critical part of this process—some offspring in the “concentrated” subspace of responsive developmental paths showing the character in the absence of the environmental stress—is not as a logical matter, but as a consequence of development. No reverse transcription in individual transmission is needed in this Darwinian, population-thinking version of Lamarck.
If this is how this happens, I concede that we do not have much evidence to infer that this phenomenon is widespread in nature. However, until more researchers can conceive how it can happen, they won´t be looking for it, and so evidence for it will remain slim even if it occurs often.
There are other accounts of how this happens. In Waddington´s own terms the previous environmental induction of the characters has been taken over by a genetic switch and become “genetically assimilated” (Waddington 1961). More conventional neo-Darwinists than Waddington have obscured the implications of his experiments by inventing, without further investigation, gene-centered explanations. GC Williams, in his influential book, Adaptation and Natural Selection, attributed the fixation of acquired characters in Waddington´s experiments to a change in the balance between a fixed number of suppressor alleles and a variable number of positive alleles accumulated under natural selection. (Similarly, see Mayr 1970, 361-5.) The attention is diverted away from the non-random origin of the responses and onto the postulated suppressor or positive alleles, which presumably arise randomly (that is, without reference to the environmental stress) and are then naturally selected.
The random origin of characters and the conceptual separation of origin from their change in frequency in a population makes it difficult for evolutionary theory to give significance to the structured activity of organisms during their lifetime, something that may influence the direction of change in a population. Addressing that blindspot is important whatever the answer turn out to be to the question of how often variation that originated as an appropriate response to environmental circumstances becomes more or less fixed over time in a population.
This post is adapted from P. J. Taylor, “Historical versus Selectionist Explanations in Evolutionary Theory” Cladistics 3: 1-13,1987.
Mayr, E. 1970. Population, Species, Evolution. Cambridge: Harvard University Press.
Waddington, C. H., 1961. Genetic assimilation. Advances in Genetics, 10: 257-290.
Williams, G. C. 1966. Adaptation and Natural Selection. Princeton: Princeton University Press.