The power of evo-devo to explore evolutionary constraints: experiments with butterfly eyespots

Abstract

Examples of adaptive radiation by Darwinian evolution will always enthral, and the theory of natural selection binds the whole of biology. But is selection all-powerful? Evolutionary developmental biology is beginning to provide the background to understanding how the internal organisation of organisms can influence the tempo and direction of evolutionary change. Do the mechanisms that generate the phenotype channel, bias or limit morphological evolution? The formation of eyespots on the wings of the butterfly Bicyclus anynana is a process which is becoming comparatively well understood from the genetical, developmental and ecological perspectives. We have begun to combine this knowledge with artificial selection experiments to examine the rates at which morphological changes of a particular eyespot pattern in different directions can be achieved. Our initial selection experiment performed over twenty-five generations has shown that in contrast to predictions based on shared genetical and developmental properties, two butterfly eyespots on the same wing surface can be freely uncoupled from each other with respect to their pattern of relative sizes. This flexibility in development and in response to selection may occur because our base population is part of a lineage with a long legacy of natural selection building up genetic variation that enables independent behaviour for eyespot size, and thus it no longer reflects a tightly modular organisation of eyespots as may well have existed close to their evolutionary origin in basal Lepidoptera. Comparisons can be made between such descriptions of the potential for short-term changes in morphology in B. anynana and observed patterns of divergence among all extant species in this species-rich genus. Being able to map phenotypes onto genotypes via developmental processes provides a powerful basis for exploring genetical and developmental constraints. This will eventually lead to examples of evolutionary constraints that represent more than 'just-so' stories, and thus to a more balanced view of Darwinian evolution.