Anolis sagrei. Photo by Melissa Losos.

Anoles are renowned for their adaptation to different habitats. One particularly well-documented and ubiquitous axis of adaptation involves the length of the hindlimbs. Both among and within species, lizards that use broader surfaces have longer legs. The adaptive explanation for this correlation appears to revolve around a locomotion trade-off: on broad surfaces, longer limbs provide greater sprinting ability, whereas on narrow surfaces, shorter legs provide enhanced nimbleness. Anoles, and particularly A. sagrei, are also known for their ability to adapt rapidly to novel conditions (but see caveat below)—experimental populations introduced to different environments differentiate in hindlimb length in ten years.  For these reasons, anoles may be a particularly good organism to examine the extent to which human-caused habitat alterations lead to evolutionary change or, looked at another way, whether a species can adapt to changing conditions in a human-altered world.

In this vein, Erin Marnocha and colleagues studied populations of A. sagrei on four islands in the Bahamas. On each island, she compared two populations, one in natural, forested habitat, the other in disturbed habitats around houses. These habitats differ both because disturbed areas have fewer trees, but also because disturbed areas have more broad surfaces, such as big trees, walls, and fenceposts, as compared to natural forest, which has lots of narrow diameter vegetation. The prediction is straightforward: A. sagrei in disturbed areas should have relatively longer legs. And that is exactly what they found.

These results may indicate that anole populations adapt very rapidly to changing environments caused by human habitat degradation. However, another possibility is that these differences result from phenotypic plasticity: lizards that grow up using broader surfaces develop longer limbs. Exactly this has been shown in lab studies, so the possibility of non-genetic causes of these differences must be further investigated, as planned by the investigators (this holds true, even though hindlimb length is a genetically highly heritable trait, the reason being that heritability estimates only hold when parent and offspring are raised in the same environment). Regardless, this study convincingly demonstrates that morphological differences occur between closely-situated populations occupying different habitats, and suggests the possibility that these differences may result from rapid evolutionary adaptation in a changing environment.

Jonathan Losos
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