A discussion is brewing at Heredity (Nature Publishing Group) – two letters to the editor in response to a review which suggests that a great deal of adaptive evolution is the fixation of traits which were previously plastic responses to different environments:
Heredity advance online publication 2 November 2011; doi: 10.1038/hdy.2011.97
Evolution of adaptive phenotypic traits without positive Darwinian selection
– A L Hughes
Recent evidence suggests the frequent occurrence of a simple non-Darwinian (but non-Lamarckian) model for the evolution of adaptive phenotypic traits, here entitled the plasticity–relaxation–mutation (PRM) mechanism. This mechanism involves ancestral phenotypic plasticity followed by specialization in one alternative environment and thus the permanent expression of one alternative phenotype. Once this specialization occurs, purifying selection on the molecular basis of other phenotypes is relaxed. Finally, mutations that permanently eliminate the pathways leading to alternative phenotypes can be fixed by genetic drift. Although the generality of the PRM mechanism is at present unknown, I discuss evidence for its widespread occurrence, including the prevalence of exaptations in evolution, evidence that phenotypic plasticity has preceded adaptation in a number of taxa and evidence that adaptive traits have resulted from loss of alternative developmental pathways. The PRM mechanism can easily explain cases of explosive adaptive radiation, as well as recently reported cases of apparent adaptive evolution over ecological time.
From adaptation to molecular evolution – L-M Chevin and A P Beckerman
Dangers of ‘Adaptation’ – J F Y Brookfield