Evolution 2018: the Origin of Species, Anolis Style

Two of the authors (Colin Donihue and Raphaël Scherrer) with their poster at Evolution 2018

How do new species form? At ESEB 2018, Colin Donihue uses Anolis lizards to answer this fundamental question in evolutionary biology.

Anoles are known for their adaptive radiation in the Carribean and the corresponding diversification into distinct “ecomorph” categories. Each ecomorph is associated with distinct morphologies and behaviors that allow it to live easily in a different habitat. This pattern is repeated across the Greater Antillean islands, but what we see is the end result of an adaptive radiation – each ecomorph corresponds to a separate species.

Donihue and his co-authors embarked on an ambitious project to capture the beginning of an adaptive radiation. To do so, they turned to the ubiquitous brown anole, Anolis sagrei. As Anolis sagrei is found across the Bahamas in a variety of different habitats, you might expect to see them adapting to those different habitats through changes in morphology; in other words, looking at the early adaptation of Anolis sagrei populations in different habitats is a natural experiment reflecting the early stages of ecomorph development. And since Anolis sagrei is on islands across the Bahamas, there isn’t just one experiment, but several replicated ones. Donihue et al. could therefore also question the role of contingency vs deterministic evolution though their study.

The authors captured 20 individuals from coastal scrub, mangrove, and primary coppice forest habitat across 11 islands in the Bahamas, and measured a suite of morphological traits for all individuals; these traits include the “usual culprits” of ecomorph differentiation, such as forelimb length, hindlimb length, and lamella count. This effort resulted in an enormous data set that the authors could use to test whether brown anoles had adapted to the different habitats across all the islands.

So are the Bahamian brown anoles adapting along early ecomorph lines? Well…sort of. On any given island, lizards living in different habitats have different morphological characteristics. But, looking across islands, Donihue et al. observe different patterns of morphological specialization on each island. This suggests that contingency, in this case represented by the island of origin, is playing a large role in how the lizards adapt to the three different habitats.

In an interesting twist to the project, Donihue et al. used supervised machine learning to test whether lizards could be assigned to the correct habitat categories based on morphology. They found that this algorithm could assign lizards to their habitat correctly based on the input of their morphological measurements across islands. This result implies that determinism is playing a role in the specialization of these brown anoles, but may only be detectable when looking at a lizard’s holistic phenotype rather than any individual trait measurement. Looking forward to seeing the paper on these results!