Plasticity-Mediated Persistence in New and Changing Environments

Figure 1

Baldwin’s theory of orthoplasy contrasted with Neo-Darwinism and Neo-Lamarckism. Adapted from [6], pp. 187-188. LL′ is the line of evolution. PO is the phenotypic optimum. Primes (′) denote generations. cm is the congenital mean (population mean phenotype, not plastic), v is the genetically based change in population mean phenotype (due to selection on mutations), c is the congenital endowment, and a is the environmental modification of the phenotype. Under Neo-Darwinism, evolution is due solely to the contribution of genetic variation, which is passed from generation to generation; change only occurs through selection on genetic variants. Plasticity may exist, but environmental modifications are not heritable and are therefore of limited adaptive value. Note that evolution in this scenario is directional—mutations that take individuals back to their ancestral phenotype are selected against. Under Neo-Lamarckism, each generation improves its fitness through use and disuse. The initial phenotype is added to by environmental modifications, and this full phenotype is passed on to the next generation as a congenital endowment. As such, the congenital endowment gets closer and closer to the phenotypic optimum with each generation. Finally, under Baldwin’s theory of orthoplasy, the first generation has a mean phenotype due in part to heritable variation, but this phenotype is far from the phenotypic optimum. Plasticity adjusts the phenotype to this optimum. These modifications are not passed on from generation to generation, but plasticity itself is; survivors in each generation can thus produce the optimum phenotype in the absence of genetic change. Genetic change does happen, however; any change in the direction of the modified phenotype (and thus the optimum) is favoured, while changes in the opposite direction are selected against. Each generation becomes less plastic, as it falls under greater genetic control. Note that PO is not original to Baldwin’s diagram; it was added by the present author for ease of comparison but certainly has its difficulties, particularly regarding costs to plasticity.