Phenotypic covariation among traits significantly differs between SSI radiating and non-radiating groups.
The first aim of this study was to investigate if unique multivariate phenotypes and covariation between traits was associated with pupfish diversification on San Salvador Island, Bahamas. While pupfish have radiated in several lakes on SSI, specialist species are notably absent in other lakes on the same and nearby islands, despite their close proximity and environmental similarity. We initially predicted that the pattern of diversification would be explained by shifts in the mean trait values and patterns of covariation in the SSI radiating group. Our comparisons between mean trait values support this prediction, as SSI radiating groups had larger trait values than SSI generalist-only and Caribbean groups. This result was not surprising as the hallmark traits of the specialist species within the radiation are the expanded dorsal head of the maxillae and the larger jaw apparatus (Hernandez et al., 2018; Martin and Wainwright, 2011).
Comparisons of the relationship between traits, however, suggest that shifts in patterns of covariation primarily occur in the SSI generalist-only group, whereas the Caribbean and the SSI radiation have high levels of similarity between their P matrices (Figures 5 & 6). This could indicate that the P matrices of Caribbean populations of pupfish are “primed” for diversification but have not been exposed to the specific selective pressures that may induce such diversification. On the other hand, SSI generalist-only populations are exposed to similar environmental pressures, yet have not fully radiated, suggesting that the phenotypic matrix structure for these populations lacks an important quality for diversification. Indeed, we found that the P matrices of these non-radiating populations differed from those of SSI radiating and Caribbean populations (Figure 4), and our specific results indicate that diversification may be limited by stronger associations between traits, higher constraints, and lower flexibility (Figures 5 & 6).