Evidence for determinism in species diversification and contingency in phenotypic evolution during adaptive radiation

Abstract

Adaptive radiation (AR) theory predicts that groups sharing the same source of ecological opportunity (EO) will experience deterministic species diversification and morphological evolution. Thus, deterministic ecological and morphological evolution should be correlated with deterministic patterns in the tempo and mode of speciation for groups in similar habitats and time periods. We test this hypothesis using well-sampled phylogenies of four squamate groups that colonized the New World (NW) in the Late Oligocene. We use both standard and coalescent models to assess species diversification, as well as likelihood models to examine morphological evolution. All squamate groups show similar early pulses of speciation, as well as diversity-dependent ecological limits on clade size at a continental scale. In contrast, processes of morphological evolution are not easily predictable and do not show similar pulses of early and rapid change. Patterns of morphological and species diversification thus appear uncoupled across these groups. This indicates that the processes that drive diversification and disparification are not mechanistically linked, even among similar groups of taxa experiencing the same sources of EO. It also suggests that processes of phenotypic diversification cannot be predicted solely from the existence of an AR or knowledge of the process of diversification.

Figure 1.

Frequency of occurrence for each of (a) the coalescent and (b) standard models with highest AIC weights (see electronic supplementary material, table S1) in the posterior probability distribution composed of 5000 trees for the four New World squamate groups. Models not present indicate they were chosen with a frequency approximately = 0.

Figure 2.

Lineage through time plots (black) for each of the four squamate groups compared with simulated curves (in grey), which use ML estimates of speciation rate (λ), extinction rate (μ) and carrying capacity (K) from the time calibrated trees. These simulated curves account for diversity dependence and extinction for each group and are contrasted against lineage through time plots from the real trees (see text and Etienne et al. [42]).

Figure 3.

Frequency of occurrence for each of the morphological models with highest (a) and second highest (b) AIC weights in the posterior probability (Pp) distribution for the four New World squamate groups examined here. Differences between the highest and second highest AIC weights are displayed. Models not present indicate they were chosen with frequency = 0.

Figure 4.

Relative disparity through time estimates represented as Δ-MDI scores (see text) for each of the four NW squamate groups.