Ecological processes may diffuse through evolutionary time: an example from Equidae

Body mass evolution and diversification within horses (family Equidae). Lauren Shoemaker, Aaron Clauset. 2013. Article first published online: 5 DEC 2013. Ecology Letters. DOI: 10.1111/ele.12221

One of the things that community phylogenetic approaches have tended to overlook is that how we interpret phylogenetic relationships depends on a model of evolution. For example, the assumption that closely related species also are similar in their traits is implicitly relying on a particular model of trait evolution. One downside to this approach is that different models of evolution may provide different conclusions about macroecological patterns and processes (competition, environmental filtering, facilitation). 

For example, a new paper in Ecology Letters provides an example of how patterns of trait divergence and adaptive radiation can evolve as a result of diffusion evolution, rather than from a single strong ecological pressure. The paper by Shoemaker and Clauset focuses on the Equidae (horse) family, which underwent an adaptive radiation 56 million years ago, resulting in massive increases in diversity and in trait variation, particularly in body size, habitat type and range size, diet, life span and reproductive traits. Several explanations have been proposed for this radiation and in particular the great increase in body size variation (species are estimated to have ranged between 10-1200 kg). A diversity-focused model explains body size divergence as the result of macroecological competition for niches. A limited number of niches at a given size are assumed to be available, and these niches vary in quality or attractiveness. Increasingly extreme body sizes (and presumably less desirable niches) evolve as niches are filled at more desirable sizes. The result is a correlation between diversity and body size variation, much like the one seen in Equidae. The alternative model considered suggests that trait space is filled via diffusion or a random walk, with the only assumption being that there are some physiological constraints – here a hard limit on minimum size, and an assumption of increasing extinction risk as maximum size increases.

From Shoemaker + Clauset, 2013.

Using mathematical models of Equidae body size evoluation, the authors’ results were very clear (figure below): “Using family Equidae as a model system, we found that macroevolutionary ‘diffusion’, in which selective effects on species body size vary independently of the occupation status of nearby niches, explains substantially more of the observed changes in the Equidae body mass distribution over 56 Myr (Fig. 5) than does a diversity-driven mechanism...”. The results are interesting because they are a reminder that the relationship between macroecological patterns (for example, of traits like body size) may be related to evolutionary history in a much more nuanced way than ecophylogenetic studies sometimes assume. Rather, Shoemaker and Clauset suggest that the better performance of the diffusion model--rather than indicating that competition is *not* important--may be effective at capturing many independent ecological interactions and selective effects all driving body size evolution. A macroevolutionary model of competitive effects on trait divergence is may simply be unrealistic, since competition and ecological interactions may be more localized and less generalized in their effects across the entire Equidae family.

From Shoemaker and Clauset, 2013. Left - competition model, Right - diffusion model

“A large role for diffusion does not undermine the general ecological importance of competition, but rather clarifies its role in generating broad-scale patterns for horses in particular, and for evolving systems in general. Macroevolutionary diffusion is an effective large-scale description of many roughly independent ecological interactions and evolutionary constraints on species size variation. Short-term selective effects on size for a particular species can stem from any number of specific mechanisms, including but not limited to competition over ecological niches. So long as the magnitude and direction of these effects, as defined at the species-level, are roughly independent across the taxonomic group, the large-scale pattern will be well described by diffusion. Ecological competition may thus be crucial for individual species, but its effects are more diffuse at the large scale because competition is typically a local process.”

This is a reminder that many phylogenetic hypotheses (trait divergence or convergence in communities, etc) are too simplistic in their assumptions that broad macroecological processes dominating, and instead need to recognize that ecological processes are often numerous, independent, and local, making outcomes more nuanced than usually assumed.