Data from: Ancestral plasticity and allometry in threespine stickleback reveal phenotypes associated with derived, freshwater ecotypes

For over a century, evolutionary biologists have debated whether and how
phenotypic plasticity impacts the processes of adaptation and
diversification. The empirical tests required to resolve these issues have
proven elusive, mainly because it requires documentation of ancestral
reaction norms, a difficult prospect where many ancestors are either
extinct or have evolved. The threespine stickleback radiation is not
limited in this regard, making it an ideal system in which to address
general questions regarding the role of plasticity in adaptive evolution.
As retreating ice sheets have exposed new habitats, oceanic stickleback
founded innumerable freshwater populations, many of which have evolved
parallel adaptations to their new environments. Because the founding
oceanic population is extant, we can directly evaluate whether specific
patterns of ancestral phenotypic expression in the context of novel
environments (plasticity), or over ontogeny, predisposed the repeated
evolution of “benthic” and “limnetic” ecotypes in shallow and deep lakes,
respectively. Consistent with this hypothesis, we found that oceanic
stickleback raised in a complex habitat and fed a macroinvertebrate diet
expressed traits resembling derived, benthic fish. Alternatively, when
reared in a simple environment on a diet of zooplankton, oceanic
stickleback developed phenotypes resembling derived, limnetic fish. As
fish in both treatments grew, their body depths increased allometrically,
as did the size of their mouths, while their eyes became relatively
smaller. Allometric trajectories were subtly but significantly impacted by
rearing environment. Thus, both environmental and allometric influences on
development, along with their interactive effects, produced variation in
phenotypes consistent with derived benthic and limnetic fish, which may
have predisposed the repeated genetic accommodation of this specific suite
of traits. We also found significant shape differences between marine and
anadromous stickleback, which has implications for evaluating the
ancestral state of stickleback traits.