Adaptive radiation despite conserved modularity patterns in San Salvador Island Cyprinodon pupfishes and their hybrids

Adaptive radiations are striking examples of rapid speciation along
ecological lines. In adaptive radiations, fast rates of lineage
diversification often pair with rapid rates of morphological
diversification. Such diversification has often been documented through
the lens of ecological drivers, overlooking the intrinsic structural
constraints that may also have a key role in configuring patterns of trait
diversification. Covariation within and between traits has been
hypothesized to govern the axes of trait evolution, either by increasing
the degree of covariation between traits (i.e., integration), which
promotes morphological coordination, or by strengthening the degree of
covariation within traits (i.e., modularity), which allows organisms to
explore novel trait combinations and different regions of morphospace.
Here, we study the modularity of the skull within an adaptive radiation of
pupfishes that is endemic to San Salvador Island, Bahamas. This radiation
exhibits divergent craniofacial morphologies, including generalist,
snail-eating specialist, and scale-eating specialist species. We assessed
morphological disparity, integration strength, and modularity patterns
across the sympatric San Salvador Island pupfish radiation, lab-reared
hybrids, and closely related outgroup species. Our findings revealed an
unexpected uniformity in the pattern of modularity across diverse species,
supporting a five-module functional hypothesis comprising the oral jaw,
pharyngeal jaw, neurocranium, hyoid apparatus, and hyomandibula. Despite
this conserved modularity pattern, all species exhibited weak but
significantly varying strengths of overall between-module integration and
significant disparity across all cranial regions. Our results suggest
rapid morphological diversification can occur even with conserved patterns
of modularity. We propose that broadscale patterns of modularity are more
conserved while between-module associations are more evolvable between
species.