A new, fast method to search for morphological convergence with shape data.

Silvia Castiglione,Pasquale Raia,Antonio Profico,Marina Melchionna,Carmela Serio,Paolo Piras,Davide Tamagnini,Mirko Di Febbraro,Alessandro Mondanaro,Filippo Barattolo

doi:10.1371/journal.pone.0226949

Abstract

Morphological convergence is an intensely studied macroevolutionary phenomenon. It refers to the morphological resemblance between phylogenetically distant taxa. Currently available methods to explore evolutionary convergence either: rely on the analysis of the phenotypic resemblance between sister clades as compared to their ancestor, fit different evolutionary regimes to different parts of the tree to see whether the same regime explains phenotypic evolution in phylogenetically distant clades, or assess deviations from the congruence between phylogenetic and phenotypic distances. We introduce a new test for morphological convergence working directly with non-ultrametric (i.e. paleontological) as well as ultrametric phylogenies and multivariate data. The method (developed as the function search.conv within the R package RRphylo) tests whether unrelated clades are morphologically more similar to each other than expected by their phylogenetic distance. It additionally permits using known phenotypes as the most recent common ancestors of clades, taking full advantage of fossil information. We assessed the power of search.conv and the incidence of false positives by means of simulations, and then applied it to three well-known and long-discussed cases of (purported) morphological convergence: the evolution of grazing adaptation in the mandible of ungulates with high-crowned molars, the evolution of mandibular shape in sabertooth cats, and the evolution of discrete ecomorphs among anoles of Caribbean islands. The search.conv method was found to be powerful, correctly identifying simulated cases of convergent morphological evolution in 95% of the cases. Type I error rate is as low as 4–6%. We found search.conv is some three orders of magnitude faster than a competing method for testing convergence.

Highlights

A species’ phenotype depends on its ancestral state and the responses to selection or drift it experiences since that state
It is important to note that with geometric morphometric data (PC scores) the origin coincides with the consensus shape, so that, for instance, a large θ indicates the two species diverge from the consensus in opposite directions and the phenotypic vectors can be visualized in the PC space (S1 File)
We considered individual species as either grazing artiodactyls, grazing perissodactyls, or “others” depending on their molar morphology and tested whether grazing ungulates from different parts of the tree converged on similar mandibular morphologies by using the ‘state’ approach

Summary

Introduction

A species’ phenotype depends on its ancestral state and the responses to selection or drift it experiences since that state. Selection towards similar optima in different parts of a tree (which could be described by the Ornstein–Uhlenbeck (OU) mode of evolution [1,2]) generates a pattern of evolutionary convergence [3,4,5,6]. Examples of repeated convergence within a clade, known as iterative evolution, include the evolution of trenchant-heeled lower molar talonids in several canid lineages [12] and elongated and laterally-compressed upper canines within the cat family, Felidae [20]. This by no means exhaustive list just represents a brief account of a diffuse, widely occurring evolutionary pattern [21]

Methods

Results

Conclusion