Is anole evolution predictable?

By: Royce Conlin, Mark Khalil, Claire Manuszak, and Meredith Moore (Stonehill College, BIO323 Evolution, Spring 2018)

In this article, scientists examined various species of anoles. Anoles belong to what many know as the “New World” lizards, closely related to iguanas, and can interestingly change the color of their skin related to their environment.  Anoles generally thrive in warm, moist environments; in cooler weather, anoles tend to hide out and take shelter in a confined area. Most anoles live in the southeastern U.S. and the Caribbean. Many scientists have taken a strong interest in anoles due to their fascinating evolutionary histories.

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The author of this study, Robert Thorpe, investigates the question to what extent is evolution predictable or random. In order to do this, Thorpe compared two different species of anoles on the Lesser Antilles, which is a group of mountainous islands in the Caribbean Sea, and Bonaire, which lies off the northern coast of Venezuela in the southern Caribbean and has a drier climate. On these islands, researchers observed the physical coloration of anoles. The Lesser Antillean anole lives on an island that has two distinct geographic and climate differences due to a mountain range: the montane rainforest and the dry, coastal habitat. Contrastingly, the Bonaire anole lives in a much drier climate with little geographic variation.

Thorpe investigates whether the Lesser Antillean anole can be used to predict what the Bonaire anole will look like if they occupy similar habitats by looking at their quantitative traits. Quantitative traits are traits that are measurable physical characteristics. In this study, he uses color traits, such as colorlessness, blueness, and greenness, and the size of the chevron, which is a v-shaped mark on the anole.

In order to carry out this study, Thorpe first made a phylogenetic tree of the different species of anoles that inhabit Bonaire by using the mitochondrial gene, cytochrome B. Basically, what that means is that they used a gene that would tell them the most recent differences in their DNA and from that information built a tree that organized the different locations of the anole species. The researchers found three distinct groups, one in the south, one in the north, and one centrally located. They then took one species from each location to test whether or not their coloration and chevron could be predicted from the Lesser Antillean anoles. It’s important to note that the Bonaire anoles were found to have very similar quantitative traits among the three different locations.

In order to see how well Lesser Antillean anoles could predict the quantitative traits, a regression of the quantitative traits against climate was made. This would compare the Lesser Antillean anoles from the wet mountain habitat and the drier coastal habitat, and it led to a significant correlation between climate and coloration. Because Bonaire had climate similar to the drier habitats on the Lesser Antilles, it was predicted that the Bonaire anoles would have more similar quantitative traits to the Lesser Antillean anoles that live in the drier climate and that the color traits of Bonaire anoles would match the regression-predicted patterns of achroma, dorsal greenness and blueness, and chevron intensity in drier environments.

Thorpe concluded that Bonaire anoles’ quantitative traits can rapidly evolve due to climate and other geographic conditions. Since there was no geographic variation in Bonaire, all the quantitative traits were similar. The Lesser Antillean anoles, however, exhibited correlated differences in physical traits due to their habitat. And as predicted, the Bonaire anoles showed similar quantitative traits to the Lesser Antillean anoles from drier habitats. Thorpe was able to conclude that evolution can be predictable in anoles within a certain range given the climate and geography. This prediction isn’t perfect because the Bonaire anoles didn’t fall exactly on the regression line and there is room for variation, but in the end there selection prefers one extreme, due to climate. Thorpe also looked at scalation but didn’t find enough information to make any definitive conclusions. In this case, the evolution of the anoles seems to be predictable rather than random. However, the extent that microevolution can predict macroevolution cannot be answered due to this study.

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Citation: Thorpe, R.S. 2017. Predictability in evolution: Adaptation of the Bonaire anole (Anolis bonairensis) to an extreme environment. PloS ONE 12(5): e0176434.

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