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.