Australian Cryptoblepharus: Convergence across a Continent

When it comes to adaptive radiations, the diversification of Anolis is one of the most striking examples. While Anolis is therefore a well-known rock stars in the league of adaptive radiations, in this blog post I would like to introduce you to one of the mere mortal examples; skinks from the genus Cryptoblepharus.

Cryptoblepharus skinks (“Cryptos”) are small diurnal lizards that have rapidly diversified and are known for their widespread distribution with species present in the Malagasy region, on the Australian continent and on many island archipelagoes in the Indo- and wider Pacific. Furthermore, species that occur on similar substrates are notoriously difficult to identify based on morphological characteristics and a more accurate estimate of species diversity has only recently been accomplished using a widespread genetic screen with allozyme markers (Horner & Adams, 2007).

Distribution of Australian Cryptoblepharus and the three habitat specialists. (a) Topographic map of Australia with the mean point of each species’
distribution plotted and coloured according to habitat type (for complete distribution maps, see Horner & Adams (2007)). In situ photographs of (b) arboreal, (c) littoral, and
(d) rock specialists (green, blue, and red dots on the topographic map, respectively).

Whereas species within the same habitat are highly cryptic, species that occur on different substrates (‘rock’, ‘trees’ or ‘beaches’) are relatively easy to distinguish. Rock Cryptos for example, traverse the red sandstone escarpments that are iconic for the Australian outback (think ‘Uluru like’ in terms of color and rock type) and they look very different from Cryptos that occur in a more mesic or coastal habitat. In a recent paper (Blom et al., 2016) we focused on the Australian radiation and explored whether habitat specialization explains current patterns of phenotypic variation in ecologically relevant traits. Using a comparative approach, we quantified the presence of distinct adaptive peaks, the frequency of shifts between such peaks and ultimately discuss the role of ecology in promoting continental radiation.

The evolutionary history of rapid radiations is notoriously difficult to resolve due to processes such as incomplete lineage sorting of individual genes. We therefore used coalescent based methods and approximately 1200 genetic markers (exon-capture) to first infer the phylogenetic history of Australian Cryptoblepharus. The 27 Australian Crypto species rapidly diversified in two distinct clades, each with a crown age of ~5 million years. Species that occur on rocks significantly differ from tree species in terms of head height and fore- and hindlimb length. The reduction in head height enables rock species to hide in shallow crevices and elongated limbs aid rapid movement across flat vertical surfaces. Tree and beach species are similar in most traits except for the length of their hindlimbs; beach species tend to climb less and benefit from longer hindlimbs to thrust forward in a horizontal direction. Correlations such as these have been reported before in other groups of lizards, but the frequency of habitat shifts and subsequent convergence is striking.

pPCA and three-dimensional phylomorphospace plot. (a) Morphological variation among species along pPC1 and pPC2, where each dot represents a
species and is coloured by that species’ habitat type. PC1 separates the rock specialists from the other habitat specialists. (b) Size-corrected residual
scores from phylogenetic regression are plotted for each of the traits that were identified as different between habitats. Colours correspond to the habitat
type for each respective species and the phylogeny is mapped onto morphospace. Regardless of phylogenetic association, species are more closely clustered
in morphospace by habitat.

Even though the radiation is relatively recent, distinct phenotypes have evolved multiple times with at least four adaptive peak shifts (arboreal to rock substrate) and two additional peak shifts that are not completely convergent across all traits. This pattern of adaptive phenotypic divergence between habitats and morphological convergence within habitat type, strongly suggests that ecology has played an important role in promoting continental diversification. However, given that many cryptic sister lineages have emerged within similar habitat types, the Crypto radiation is not solely driven by ecologically mediated divergence (‘ecological speciation’) but rather highlights the importance of macrohabitat during radiations at a continental scale. Strong selection for convergent phenotypes in isolated populations might speed up the speciation process while ecological character displacement between diverged populations might result in distinct ecomorphs. Further research is required to test such hypotheses.

While the genus Cryptoblepharus might not harbor the remarkable phenotypic diversity of Anolis or African cichlids, the radiation of Australian Cryptoblepharus can provide important insights on the spatial and ecological context of macroevolutionary change on a continental scale.

Blom MPK., Horner P. & Moritz C. 2016. Convergence across a continent: Adaptive diversification in a recent radiation of Australian lizards. Proc. R. Soc. B. 283: 20160181

Horner P. & Adams M. 2007. A molecular systematic assessment of species boundaries in Australian Cryptoblepharus (Reptilia: Squamata: Scincidae): A case study for the combined use of allozymes and morphology to explore cryptic biodiversity. Beagle Suppl. 3: 1 – 19