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).