Are rates of species diversification correlated with rates of morphological evolution?

Abstract

Some major evolutionary theories predict a relationship between rates of proliferation of new species (species diversification) and rates of morphological divergence between them. However, this relationship has not been rigorously tested using phylogeny-based approaches. Here, we test this relationship with morphological and phylogenetic data from 190 species of plethodontid salamanders. Surprisingly, we find that rates of species diversification and morphological evolution are not significantly correlated, such that rapid diversification can occur with little morphological change, and vice versa. We also find that most clades have undergone remarkably similar patterns of morphological evolution (despite extensive sympatry) and that those relatively novel phenotypes are not associated with rapid diversification. Finally, we find a strong relationship between rates of size and shape evolution, which has not been previously tested.

Figure 1

Reduced phylogeny of 15 clades of plethodontid salamanders analysed in this study (species-groups of Plethodon follow Wiens et al. 2006a and previous authors). Branch lengths reflect estimated ages of clades, whereas widths of each clade block are proportional to the total number of described species. The complete phylogeny of 190 species is provided in the electronic supplementary material. Images of representative species for each clade are shown (see acknowledgments for photo credits).

Figure 2

Relationships between rates of morphological size evolution, morphological shape evolution and species diversification among 15 clades of plethodontid salamanders (clade numbers follow table 1). (a) Diversification rate versus rate of size evolution; (b) diversification rate versus rate of shape evolution; (c) diversification rate versus morphological disparity; (d) rate of size versus shape evolution; (e) rate of size evolution versus morphological disparity; and (f) rate of shape evolution versus morphological disparity. Only significant regressions are displayed. Full results are provided in table 2.

Figure 3

(a) Principal component plot of the morphometric data. Mean size and shape scores for each species are displayed. PC1 (size) describes 80.8% of the total variation, whereas PC2 (shape) explains 11.4% of the total variation. Species from major plethodontid clades are shown in similar colours; symbol and colour combinations signify 15 clades examined in this study. (b) Box and whiskers plot of PC1 for each of the 15 clades (medians and quartiles shown). Clades are: 1: Desmognathus, Phaeognathus (grey circle); 2: Aneides (grey square); 3: Western Plethodon (blue down triangle); 4: Plethodon cinereus group (blue circle); 5: Plethodon wehrlei-welleri clade (blue up triangle); 6: Plethodon glutinosus group (blue square); 7: Pseudotriton, Gyrinophilus, Stereochilus (Spelerpinae; red square); 8: Eurycea (Spelerpinae; red circle); 9: Nototriton (yellow square); 10: Oedipina (yellow circle); 11: Chiropterotriton (green circle); 12: Pseudoeurycea clade (Ixalotriton, Lineatriton, Parvimolge, Pseudoeurycea; white circle); 13: Bolitoglossa (subgenus Eladinea; black circle); 14: Bolitoglossa clade (subgenera Magnadigita, Oaxakia, and Pachymandra; black up triangle); 15: Bolitoglossa (subgenera Bolitoglossa, Mayamandra, and Nanotriton; black square). In (a), two species are shown to demonstrate typical body proportions of taxa found in different regions of the plot (arrows indicate placement of each species): above, Bolitoglossa colonnea from Bolitoglossa (subgenus Eladinea); below, Oedipina gracilis, representing Oedipina.