PHENOTYPIC PLASTICITY AND GENETIC ASSIMILATION IN DEVELOPMENT AND EVOLUTION

Phenotypic plasticity is the ability of a single genotype to produce more than one alternative form of morphology, physiological state, and/or behaviour in response to environmental conditions. Environmentally induced non-genetic effects can alter the direction of selection affecting transmitted gene frequencies by shifting the range of phenotypes expressed. The fact that selection acts on exprssed phenotypes is basic to the analysis of the specific ways in which plasticity can affect evolution.

The process of genetic assimilation is one by which a phenotypic character, which initially is produced only in response to some environmental influence, becomes, through a process of selection, taken over by the genotype, so-that it is formed even in the absence of environmental influence which had at first been necessary.

Genotypes are represented by one-dimensional ordered arrays (strings) of genes, each of which can be a 'IO, a 'O', or an ‘x’. The expression of both 01 and O is fixed, whereas that of x is subjected to variation and capable of resembling either the '01" level or the O level. The element of phenotypic plasticity that this implies is realised by a process of random coin-tossing. A fitness value is assigned to each (genotype by matching its phenotype with a pre-assigned target phenotype. It is in the expression of fitnesses that our model differs fundamentally from that of Hinton. From the fitnesses and the constraint of a fixed propulation of genotypes the number of individuals reaching reproductive maturity is determinded. Reproduction involves random mating and a single recombinational event between parents; one of the two progeny that result becomes an embryonic genotype for the next generation. As pointed out by Hinton and Nowlan (1987), phenotypic plasticity alters the shape of the search space in which evolution operates and, as we show, accelerates the course of evolution.

We, also, show that an optimum level of plasticity exists in more realistic models.