Phenotypic plasticity drives local adaptation by disrupting a genetically integrated jaw apparatus in Trinidadian guppies

Consistently across clades researchers find a high degree of covariation
in the shape of disparate elements within the same individual, a pattern
known as morphological integration. Although integration can help to
maintain functionality of multi-component systems, it can also constrain
long-term evolution. On shorter time scales, morphology is often shaped by
plastic environmental responses, however, it remains an open question
whether plasticity is constrained by the patterns of morphological
integration that emerge over longer evolutionary time scales. Here we ask
whether an incipient adaptive trophic divergence among Trinidadian guppies
displays similar patterns of covariation between benthic and limnetic
populations that are either reared in a common lab environment or sampled
from wild populations. We perform geometric morphometric analysis (n=83)
on the premaxilla, dentary, anguloarticular, and lower pharyngeal jaw, and
find that morphology correlates with trophic niche in both lab-reared and
wild-caught fish. Furthermore, we find that integration among these bones
is significantly stronger in lab-reared fish (absence of environmental
differences), while their shapes vary independently in wild-caught
individuals. This suggests that phenotypic plasticity may allow organisms
to explore novel morphospace as they locally adapt, but does so at the
expense of maintaining a highly integrated feeding apparatus.