Convergence across a continent: adaptive diversification in a recent radiation of Australian lizards

Abstract

Recent radiations are important to evolutionary biologists, because they provide an opportunity to study the mechanisms that link micro-and macroevolution. The role of ecological speciation during adaptive radiation has been intensively studied, but radiations can arise from a diversity of evolutionary processes; in particular, on large continental landmasses where allopatric speciation might frequently precede ecological differentiation. It is therefore important to establish a phylogenetic and ecological framework for recent continental-scale radiations that are species-rich and ecologically diverse. Here, we use a genomic (approx. 1 200 loci, exon capture) approach to fit branch lengths on a summary-coalescent species tree and generate a time-calibrated phylogeny for a recent and ecologically diverse radiation of Australian scincid lizards; the genus Cryptoblepharus. We then combine the phylogeny with a comprehensive phenotypic dataset for over 800 individuals across the 26 species, and use comparative methods to test whether habitat specialization can explain current patterns of phenotypic variation in ecologically relevant traits. We find significant differences in morphology between species that occur in distinct environments and convergence in ecomorphology with repeated habitat shifts across the continent. These results suggest that isolated analogous habitats have provided parallel ecological opportunity and have repeatedly promoted adaptive diversification. By contrast, speciation processes within the same habitat have resulted in distinct lineages with relatively limited morphological variation. Overall, our study illustrates how alternative diversification processes might have jointly stimulated species proliferation across the continent and generated a remarkably diverse group of Australian lizards.

Diversification across biomes in a continental lizard radiation

“…This contrasts with many of the best examples of adaptive radiation, especially in lizards, which show great phenotypic diversity and clear links between phenotype and ecology (Losos ; Blom et al. ). It also contrasts with observations that at shallower phylogenetic scales, Gehyra show considerable plasticity in morphology and ecology (Doughty et al.…”

“…The two Gehyra ecomorphs do not differ in leg length, unlike Anolis and Australian Cryptoblepharus skinks, which have more strikingly divergent ecomorphs (Losos ; Blom et al. ).…”

“…; Blom et al. ). Hence, an alternative to ecological release is that diversity of radiations is shaped primarily by topographic complexity, through promoting allopatric or parapatric speciation, opportunity for niche partitioning, and persistence in refugia through oscillating climates (Moritz et al.…”

Cryptic lineage diversity, body size divergence, and sympatry in a species complex of Australian lizards ( Gehyra )

“…Such a mix of cryptic and divergent evolution within a clade is not unusual (Rundell and Price ; Blom et al. ). Body size shifts in lizards are often associated with habitat shifts or ecological release, especially in island systems (e.g., Harmon and Gibson ; Mahler et al.…”

“…In contrast to adaptive radiations (especially from islands and lakes) that featured prominently in research (Schluter ; Gavrilets and Losos ), continental radiations can have a mix of primarily allopatric, and often cryptic, divergence together with ecologically driven speciation (Blom et al. ). A substantial remaining challenge is to understand how this mix of processes, together with possible character displacement, shapes alpha and beta diversity of assemblages.…”