Differences in patterns of variation and covariation for specific traits
Our PCA investigating patterns across the P-matrices of Caribbean, SSI generalist-only, and SSI radiating groups indicates that jaw closing in-lever, maxilla length, and palatine height have strong negative correlations with PC1(~ -0.96 for all three traits), while nasal tissue protrusion has the only positive correlation value for PC1 (0.014; Figure 4; Appendix Table D3, Appendix Figure D2). For PC axis 2 we observed a strong positive correlation associated with maxillary head protrusion (0.91), moderate positive correlations for jaw opening in-lever (0.56) and head depth (0.51), and moderate negative correlations for lower jaw length (-0.59) and dentigerous arm width (-0.43; Figure 4; Appendix Table D3, Appendix Figure D2). Similar correlation values between traits along respective PC axes suggest that these traits share similar patterns of variation and covariation, while similar values with opposite signs indicate antagonist patterns. Further investigation shows that most of the variation along PC1 is contributed by the SSI generalist-only P matrix (52.5%), while most variation along PC2 is contributed by the SSI radiating P matrix (84.8%; Appendix Table D4).
Along PC1, we see that the major loadings of jaw closing in-lever, maxilla length, and palatine height have a strong relationship in the negative direction with SSI generalist-only traits, a variable relationship with SSI radiating traits, and strong relationship in the positive direction with Caribbean traits (Figure 4; Appendix Table D4). This suggests that as the covariation of jaw closing in-lever, maxilla length, and palatine height increases the covariation of other traits in SSI generalist-only groups also increase, while the covariation in Caribbean groups decreases. The relationship is more complicated in SSI radiating groups as the covariation of some traits, such as lower jaw length and dentigerous arm width, have a positive relationship with the covariation of jaw closing in-lever, maxilla length, and palatine height, while others such as nasal tissue protrusion and maxillary head protrusion have a negative relationship.
The major loadings for PC2 include maxillary head protrusion, lower jaw length, and jaw opening in-lever (Appendix Table D3). In general, the variation along PC2 corresponds to the SSI radiation P matrix (Figure 4; Appendix Table D4). For instance, for the SSI radiating group the covariation between maxillary head protrusion and jaw opening in-lever have positive relationship with one another but a negative relationship with lower jaw length, dentigerous arm width, and dentigerous arm base. These patterns are either significantly weaker, or opposite in the Caribbean and SSI generalist only matrices.