Topic: Anolis lizard ecomorphs
“A classic example of convergent evolution is the set of Anolis lizard ecomorphs of the Greater Antilles.” – Langerhans, Knouft & Losos (2006) Evolution, vol. 6, p.362
The anolid lizards (Anolis) are a group of small, insectivorous, tree-dwelling reptiles comprising >400 species, almost half of which live in the Caribbean. Anolis species inhabiting the Greater Antilles islands of Cuba, Jamaica, Hispaniola (Dominican Republic and Haiti) and Puerto Rico have received considerable attention as a “classic example of convergent evolution”. Following independent colonisations c. 25-30 million years ago, anolids on these four separate islands diversified, but in each case into remarkably similar forms, each adapted to life in a particular ecological habitat, or niche. The reason for this occurrence is that common selective pressures independently shaped common assemblages of ‘ecomorphs’ (habitat-specialised species) that converge in morphology and behaviour to an astonishing degree.
Importantly, Anolis ecomorphs have diversified independent of the degree of relatedness between species; indeed, when comparing anolids of equivalent ecomorphs on different islands, species only distantly related may resemble one another in intricate detail, whereas very closely related species from different ecomorph classes are morphologically very different. In his famous work on anolids, Jonathan Losos defined these ecomorphs according to the predominant ‘microhabitat’ (e.g. grasses, open ground, different parts of trees) of the respective anolids, labelling them as either ‘crown-giant’, ‘grass-bush’, ‘trunk’, ‘trunk-crown’, ‘trunk-ground’, or ‘twig’ forms. Unique suites of physical, or structural, environmental pressures are proposed to drive adaptation within each microhabitat and thus the emergence of convergent species assemblages. For example, ‘trunk-ground’ species such as Anolis cooki (Puerto Rico) and A. sagrei (Cuba, Jamaica) have the longest hindlimbs of all ecomorphs, adapted to sprinting and jumping to evade predators and forage effectively in the relatively open environment in which they live. ‘Twig’ anoles (e.g. A. occultus of Puerto Rico and A. insolitus of Hispaniola) possess the slightest bodies of all anoles, with short limbs, short tails, and small toe pad areas, well adapted to cling on to, and move among delicate, thin tree branches. Similar to twig anoles, ‘grass-bush’ anoles have slender bodies, but exemplary species such as A. krugi (Puerto Rico) and A. olssoni (Hispaniola) also have the longest tails, tarsi and meta-tarsi (hindlimb ‘foot’ bones) of all ecomorphs, ideally suited for stability on narrow, vertical surfaces.
Confirming the consistent patterns of convergence within the natural Anolis ecomorphs, elegant experiments using the brown anole (Anolis sagrei) were carried out, in which populations were placed in each of the ecomorph habitats, and several of their characteristics, or traits, were measured and analysed for change over time. Sure enough, the selection pressures in each habitat resulted in visible change among A. sagrei populations that mirrored the specialisations that had evolved in the indigenous Anolis present there already.
Although there are six Anolis ecomorphs, up to 14 ecologically similar species may exist on each island, leading us to question whether common ecological selective pressures in similar habitats are the only driving force behind morphological variation in anolids. Recent research supports the notion that yes, habitat-specific selection is genuinely the principal influence behind the complex patterns of Anolis convergence and divergence, but that there are other factors at play as well. Subtle island-specific variations within particular habitat types may affect independent lizard traits significantly, as in some cases can the degree of relatedness to other species. For example, close shared ancestry between Cuban ‘twig’ anoles and ‘trunk-crown’ anoles has conferred the Cuban ‘trunk-crown’ species (A. allisoni, A. porcatus) with wider jaws and shorter hindlimbs than ‘trunk-crown’ anoles on any other island. Each separate trait (e.g. body size, body shape, head shape, limb length, density of toe lamellae for grasping, diet, territoriality, sexual dimorphism) responds to ecological pressures independently of other characteristics. In this context, it has been found that multiple environmental factors (some of them unique to a given island) affect each trait, and the pattern of convergence of such traits may be variable between the species of a given ecomorphic type. Certain ‘crown-giant’ species (e.g. A. cuvieri), for example, use their unusually crested tails in social displays. Interestingly, ‘trunk-ground’ ecomorphs show the most extreme convergence, wherein all measurable characteristics of every ‘trunk-ground’ ecomorph varies in the same way, indicative of independent and yet identical (or virtually identical) selection pressures operating within the ‘trunk-ground’ habitat on each island.
We may not yet know the exact elements of each micro-habitat that have shaped the types of anolid that live there, but it is clear that the adaptive radiation of these lizards is a stunning example of convergence, within which multiple specific traits have independently responded to common selection regimes by adaptive change that consistently points in the same direction.
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Map of Life - "Anolis lizard ecomorphs"
September 21, 2020