A Test of the “Flexible Stem” Model of Evolution: Ancestral Plasticity, Genetic Accommodation, and Morphological Divergence in the Threespine Stickleback Radiation

TLDR

In adaptive radiations, ancestral developmental plasticity can repeatedly generate similar phenotypes that, under parallel selection, lead to characteristic ecotypes, a pattern exemplified by the threespine stickleback marine form. The study tests whether phenotypic divergence in derived stickleback ecotypes mirrors ancestral developmental plasticity, as predicted by the flexible stem model. The authors assessed shape and trophic morphology plasticity in marine sticklebacks by rearing them on diets representative of benthic and limnetic ecotypes. Marine sticklebacks reared on benthic or limnetic diets showed head and mouth shape plasticity that mirrored ecotype divergence, supporting the flexible stem model, while benthic and limnetic populations displayed similar plasticity patterns with minor mean differences and subtle shape plasticity variations, suggesting genetic accommodation.

Abstract

If an ancestral stem group repeatedly colonizes similar environments, developmental plasticity specific to that group should consistently give rise to similar phenotypes. Parallel selection on those similar phenotypes could lead to the repeated evolution of characteristic ecotypes, a property common to many adaptive radiations. A key prediction of this "flexible stem" model of adaptive radiation is that patterns of phenotypic divergence in derived groups should mirror patterns of developmental plasticity in their common ancestor. The threespine stickleback radiation provides an excellent opportunity to test this prediction because the marine form is representative of the ancestral stem group, which has repeatedly given rise to several characteristic ecotypes. We examined plasticity of several aspects of shape and trophic morphology in response to diets characteristic of either the derived benthic ecotype or the limnetic ecotype. When marine fish were reared on alternative diets, plasticity of head and mouth shape paralleled phenotypic divergence between the derived ecotypes, supporting the flexible stem model. Benthic and limnetic fish exhibited patterns of plasticity similar to those of the marine population; however, some differences in population means were present, as well as subtle differences in shape plasticity in the benthic population, indicating a role for genetic accommodation in this system.