Body Size Evolution in Extant Oryzomyini Rodents: Cope's Rule or Miniaturization?

At the macroevolutionary level, one of the first and most important hypotheses that proposes an evolutionary tendency in the evolution of body sizes is “Cope's rule". This rule has considerable empirical support in the fossil record and predicts that the size of species within a lineage increases over evolutionary time. Nevertheless, there is also a large amount of evidence indicating the opposite pattern of miniaturization over evolutionary time. A recent analysis using a single phylogenetic tree approach and a Bayesian based model of evolution found no evidence for Cope's rule in extant mammal species. Here we utilize a likelihood-based phylogenetic method, to test the evolutionary trend in body size, which considers phylogenetic uncertainty, to discern between Cope's rule and miniaturization, using extant Oryzomyini rodents as a study model. We evaluated body size trends using two principal predictions: (a) phylogenetically related species are more similar in their body size, than expected by chance; (b) body size increased (Cope's rule)/decreased (miniaturization) over time. Consequently the distribution of forces and/or constraints that affect the tendency are homogenous and generate this directional process from a small/large sized ancestor. Results showed that body size in the Oryzomyini tribe evolved according to phylogenetic relationships, with a positive trend, from a small sized ancestor. Our results support that the high diversity and specialization currently observed in the Oryzomyini tribe is a consequence of the evolutionary trend of increased body size, following and supporting Cope's rule

Bayes Factors used to test the observed versus expected values of the phylogenetic scaling parameter λ based on Random Walk model.

<p>The observed λ (mean; 95% HPD) were contrasted with values expected under the hypotheses of no phylogenetic signal (λ = 0) and the pure Random Walk model (λ = 1).</p><p>Bayes Factor (BF)≥3 indicates support for the observed λ parameter. When BF is ≤−3 the other model is chosen. Observed λ was contrasted versus λ = 0, and λ = 1.</p

Maximum likelihood parameter estimation and Akaike information criterion (AIC) values used to select the best model of speciation rate based on the Bayesian consensus tree.

<p><u>Df</u> = Degrees of freedom of each model; <u>lnLik</u> = Natural logarithm of Maximum Likelihood; <u>AIC</u> = Akaike information criterion; <u>ChiSq</u> = Chi Square value; <u>Drift</u> = tendency of body size evolution; and <u>Pr(>[Chi])</u> = Chi-square probability value.</p

Bayesian posterior probability distribution for the lambda (λ) parameter based on the ultrametric Bayesian consensus tree.

<p>Vertical blue bars indicate the 95% HPD.</p

Bayesian consensus tree obtained from 139 ultrametric trees based on an uncorrelated exponential relaxed clock.

<p>Blue branches indicate posterior probability values of a node below 0.5. Horizontal blue bars indicate the 95% HPD of divergence times, and the scale axis shows divergence times as millions of years ago (Mya).</p

Using phylogenetic information and the comparative method to evaluate hypotheses in macroecology

It is widely recognized that macroecological patterns are not independent of the evolution of the lineages involved in generating these patterns. While many researchers have begun to evaluate the effect of ancestor-descendant relationships on observed patterns using the phylogenetic comparative method, most macroecological studies only utilize the cross-sectional comparative method to 'remove the phylogenetic history', without considering the option of evaluating its effect without removing it. Currently, most researchers use this method without explicitly evaluating three fundamental evolutionary assumptions of the comparative method: (i) that the phylogeny is constructed without error (which implies evaluating phylogenetic uncertainty); (ii) that more closely related species tend to show more similar characters than expected by chance (which implies evaluating the phylogenetic signal) and; (iii) that the model of the characters' evolution effectively recapitulates their history (whi