Phenotype then Gene

The theory of evolution of novelty by genetic mutation and reproductive isolation is challenged by concepts such as environmentally triggered traits, genetic accommodation, and phenotypic adjustment. The evolution of novelty (and species) under the modern synthesis is thought to arise from the reproductive isolation of populations and their genes. Changes are proposed to occur incrementally by mutational changes at the later stages of development pathways. This consistent with von Baer’s biogenetic rule. Some evo-devo biologists, on the other hand, propose that macroevolution may involve radical changes in structural relations, which the organism ‘adjusts to’ through phenotypic accommodation. Their ideas have parallels to formalist notions of literary evolution. Discrete modules are semi-isolated, preventing disruption to the whole structure. Environmental triggers can switch modules on or off, and these can be combined into novel forms. Therefore, novelty does not rely on chance genetic mutations. Recurrence of atavistic forms and even homoplasy (convergence) might be explained by the activation of pre-existing modules. In these evo-devo models, genes follow the phenotype, rather than the other way around, but remain the repositories of hereditary information. Phenotypic (structural) transformations are subjected to Darwinian natural selection; only then do they become genetically accommodated (as the genes associated with the novel phenotype increase in frequency). Changes may become fixed by genetic assimilation or may retain environmental sensitivity (and polyphenism).