The Tempo and Mode of Evolution: Body Sizes of Island Mammals

Abstract

The tempo and mode of body size evolution on islands are believed to be well known. It is thought that body size evolves relatively quickly on islands toward the mammalian modal value, thus generating extreme cases of size evolution and the island rule. Here, we tested both theories in a phylogenetically explicit context, by using two different species-level mammalian phylogenetic hypotheses limited to sister clades dichotomizing into an exclusively insular and an exclusively mainland daughter nodes. Taken as a whole, mammals were found to show a largely punctuational mode of size evolution. We found that, accounting for this, and regardless of the phylogeny used, size evolution on islands is no faster than on the continents. We compared different selection regimes using a set of Ornstein-Uhlenbeck models to examine the effects of insularity of the mode of evolution. The models strongly supported clade-specific selection regimes. Under this regime, however, an evolutionary model allowing insular species to evolve differently from their mainland relatives performs worse than a model that ignores insularity as a factor. Thus, insular taxa do not experience statistically different selection from their mainland relatives. K E Y W O R D S :Body size, islands, κ statistic, Ornstein-Uhlenbeck models, punctuated equilibrium, rates of phenotypic evolution, θ statistic.Islands are often perceived as "natural laboratories" of evolution (Mayr 1967). This is because of their relatively low species richness promoting ecologically "free space," and low immigration rates compared to mainland areas allowing for drastic diversifications and radiations. Moreover, islands are well defined in space making delimiting populations and communities more straightforward, and further, islands provide natural replicates of the same evolutionary experiment, but often with interesting differences (derived from differences in e.g., area, isolation, history and community compositions) that easily lend themselves to analyses. These attributes of islands spawned extensive study of the morphological and ecological trait shifts in insular species as compared to their mainland relatives. The best-known trait shift on islands is probably body size change. Size is strongly correlated with a host of physiological and ecological characteristics (such as life-history traits, metabolic rate, home range, range size, dominance in intraspecific and interspecific encounters), and many other attributes (Peters 1983;Calder 1984;Brown et al. 2000) which make it a trait of utmost importance to both ecologists and evolutionary biologists.Insular mammals often evolve different body sizes than their mainland relatives. The relative tempo and mode of evolution of size on islands, relative to size evolution on the mainland, are believed to be well known: it is often thought that size evolves more quickly on islands than on the mainland (at least over relatively short periods, Millien 2006) and that it evolves toward the mammalian modal value (i.e., the Island

Unravelling the determinants of insular body size shifts

“…In contrast to previous studies [5] however, we find a weaker phylogenetic signal for ASR (magnitude of size shifts; electronic supplementary material), suggesting that the strength of size shifts is less clade-specific than is the directionality. Different clades may be evolutionarily predisposed to dwarfism or gigantism, but the magnitude of size changes depends more on adaptation to the local environment [21].…”

Evidence of the island rule and microevolution in white-footed mice (Peromyscus leucopus) in an urban harbor archipelago

“…While other island species have been observed undergoing microevolutionary changes, most notable Darwin's finches on the Galapagos Islands (Grant and Grant 2002), the case of the white-footed mouse is somewhat unique in that individuals rarely live for a year in the wild so microevolutionary changes may be more readily observed in a short timeframe. Generally, evolutionary changes on islands are thought to occur rapidly initially and then stabilize over time (Raia and Meiri 2011).…”

Phenotypic Convergence in Genetically Distinct Lineages of a Rhinolophus Species Complex (Mammalia, Chiroptera)

“…These traits can however also be highly convergent even in populations occupying different habitats. Phenotypic convergence by organisms in different habitats has often been attributed to adaptation to similar ecological niches [4,5]. Such convergence has often been used as evidence for adaptation but recently the causal relationship between convergence and adaptation has been questioned [5] with the realization that other factors such as random genetic drift can also lead to convergence [6].…”