Abstract
Sexes can differ in features associated with differential reproduction, which can be used during courtship or aggressive encounters. Some traits tend to evolve independently between sexes and emerge as sexually dimorphic within the organismal phenotype. We characterize such a relationship by estimating the phenotypic integration of the head morphology and modularity of the crest in the casque‐headed lizards (Corytophanidae). In this clade, some species show extreme sexual dimorphism (e.g., head crests in the genus Basiliscus) while in others, both sexes are monomorphic. To characterize these patterns, we define phenotypic integration at the interspecific level as a pattern or network of traits evidenced by phylogenetically adjusted correlations that persist among species. At this level, modularity is an increased connectedness (e.g., higher correlation) among sections of these networks that persist in a lineage during the evolution of complex phenotypes. To test both concepts, we used phylogenetic geomorphometrics to characterize the head structure of corytophanid lizards, based on a time‐calibrated phylogeny that includes candidate fossil ancestors. We found evidence of an older diversification of corytophanids than previously reported (~67 vs. ~23.5 MYA) and show that this clade includes two morphological head architectures: (1) Sexually dimorphic crests present in males that are evolving independently from the rest of the head structure, and (2) full integration of the head morphology in monomorphic species. We propose that both architectures are optimal evolutionary trajectories of the parietal crest bones in the head of these lizards. In sexually dimorphic species, these bones are elongated and thinner, and gave rise to the extended crest used in male courtship displays. In monomorphic species, the parietal crest grew thicker in both sexes to allow for a better insertion of muscles associated with a stronger bite.
Highlights
Sexual dimorphism is common and is usually evidenced by traits that are more developed or exaggerated in one particular sex (Shine, 1989)
We used geometric morphometric analyzes in combination with a time-adjusted phylogeny of the Corytophanidae to show that significant sexual dimorphism in head structure is present in Basiliscus
Earlier studies based only on molecular markers placed the crown of Corytophanidae at 23.5 MYA in the Oligocene-Miocene boundary
Summary
Sexual dimorphism is common and is usually evidenced by traits that are more developed or exaggerated (e.g., larger or ornamented) in one particular sex (Shine, 1989). Phenotypic integration uses multivariate studies of the phenotypic and genetic correlation/covariance assessed across members in a population (i.e., an observable pattern) and related discussions of linkage and pleiotropy as underlying mechanisms (Hallgrimsson et al, 2009) At this level, modularity as a concept is the division or grouping of traits observed among individuals in a population, usually through characterization such as developmental and genetic parcellation (i.e., differential gene expression). We characterized the head morphology of corytophanids in terms of individual traits (i.e., landmarks) that describe spatial features of the head, which can be tested for interspecific phenotypic integration and modularity as they are related to sexual dimorphism For this purpose, we estimated a time-calibrated phylogeny including fossil ancestors and traced the sexual differences in head morphology among all extant corytophanid species. Our objectives are to (1) re-estimate a family level phylogeny using both molecular and morphological characters from extant and fossil species (only morphological characters for the latter), (2) compare the differences between size and shape of the cranial features between sexes at the interspecific level, and (3) quantify geomorphometric differences of the cranial features as evidence of interspecific phenotypic integration and modularity, to explain sexual dimorphism in casque-headed lizards
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