Evolution in Action

Darwin was right about an amazing number of topics—not just evolution by natural selection, but also how coral reefs form and the role earthworms play in aerating the soil—so it doesn’t pay to bet against him. But one thing he got wrong was the pace of evolution, which he thought must be glacially slow. We now know that populations can evolve very rapidly to adapt to changing conditions, so fast that it’s possible to observe evolution as it occurs. In turn, this realization means that we can apply the gold standard of modern science, the controlled experiment, to study how evolution occurs. Our work on Bahamian anoles has been at the forefront in demonstrating that it is possible to conduct evolution experiments in nature; moreover, these studies have shown that when conditions change, such as the appearance of a larger predatory lizard or having to live on narrow bushes instead of big trees, natural selection pressures can be strong and lizard populations can adapt very quickly.

Adaptive Radiation

Adaptive radiation—the phenomenon that occurs when an ancestral species produces numerous descendant species adapted to use a wide variety of different ecological niches—is now a central pillar of evolutionary study. The concept had fallen into disrepute 30 to 40 years ago because it was thought to be ill-defined and untestable. My work played a major role in reviving the idea by developing an approach to identifying adaptive radiations which requires the synthesis of ecological, phylogenetic, behavioral, and biomechanical research. In addition, our work and that of colleagues has made Caribbean anoles are a model case study of adaptive radiation for a simple reason: These lizards represent four distinct and parallel adaptive radiations. Our studies have demonstrated that each of the major islands in the Caribbean (Cuba, Hispaniola, Jamaica and Puerto Rico) has been its own evolutionary theater, with one or a few ancestral species diversifying in situ to produce an ecologically and anatomically diverse ensemble of descendant species adapted to live in different parts of the environment. 

Convergent Evolution

Remarkably, anole diversification in the Caribbean has occurred very much in parallel, producing highly similar sets of species on each of the four largest islands. For example, each island has a species that predominantly uses narrow surfaces; these “twig anoles” have similar slender bodies, short tails, and long heads, relying on camouflage to avoid detection as they slowly move through the environment. This “convergent evolution” is not restricted to twig anoles, however: convergent evolution has also produced multiple species living in grass, on tree trunks, in the canopy and elsewhere. The Greater Antillean anoles have become a textbook case of convergent evolution and the best known example of convergence of entire communities of coexisting species (referred to as “replicated adaptive radiation”). The widespread occurrence of convergent evolution throughout the animal and plant kingdom is highlighted in my book, Improbable Destinies.

Athletic Performance and Evolutionary Adaptation

The repeated, independent evolution of similar traits by species living in similar environments suggests that the trait makes the species well adapted to that environment. Our work established a means to test this hypothesis: Lizard Olympics! By measuring the capabilities of species at ecologically relevant tasks such as running on narrow surfaces, jumping, or clinging to a smooth surface, we have demonstrated that anatomical differences often do provide a performance advantage to species in the environment in which they occur (twig anoles are more adept at moving on narrow surfaces than longer-legged species; the large toepads of canopy species do provide greater clinging ability on smooth surfaces like leaves).

Lizard Biology

One of the joys of my career has been spending time in nature watching what lizards do. There’s no better way to understand how a species is adapted to where it lives than to watch it go about its daily life. I’ll never forget watching a false chameleon (a species of Anolis lizard convergent in many ways with chameleons) for five hours as it slowly moved from the tree canopy to the ground to then back up, along the way crunching some sort of insect pupa and running as quickly as it short legs could carry it past another large lizard. Though most of my observations are much shorter, they have provided rich insight into the lives of these lizards, what matters to them and how they cope. 

One of my two favorite studies of all time was conducted in the cloud forest of Ecuador in the town of Mindo. There, the remarkable horned anole—notable for the long projection on the snout of males—had recently been rediscovered after not having been reported for 40 years. The only thing known about its biology is where they slept (researchers had found them by scouring trees with powerful lights at night to find them). We learned much about how this species behaves, why they were so hard to find (excellent camouflage and extremely slow, swaying movements that don’t attract attention) and what they eat (read more about this fun project here). Our Ecuadorian colleagues then took some back to the lab and discovered how the males use their horns, as displays both to other males and to females. 

Fossil Lizards

Another favorite study was not on behavior, but on fossils. For the most part, small tropical lizards do tend to turn into fossils because when they die, their bodies decompose in the moist, humid conditions too rapidly to become covered with soil and turn into rocks. However, an exception is another type of fossil—ancient remains preserved in amber (which is the hardened residue of tree sap). You may be familiar with such fossils from Jurassic Park. Insects are more common, but sometimes small lizards get preserved in amber. At the time of our research, 39 amber anole fossils were known, either in natural history museums or owned by private collectors. We were able to get access to 38 species. By CT-scanning them, we were able to study their skeletons in exquisite detail (check out the video of some of these remarkable specimens!), providing a snapshot of the community of Anolis living in the Dominican Republic 20 million years ago. Remarkably, the species appeared to be highly similar to those alive today.

Urban Evolution

We now know that species can evolve rapidly when environmental conditions change. And where are conditions more different than in cities? Though most species have disappeared in the face of urbanization, some species are persisting—even thriving—in our cities. What a perfect, albeit inadvertent, test of the idea that species adapt rapidly! In recent years, researchers in my lab have examined how urban Anolis lizards adapt to their surroundings. Perhaps not surprisingly, they are adapting, and rapidly. Moreover, comparisons across cities in Puerto Rico have shown that this adaptation is occurring convergently—different populations of the same species are adapting to urban conditions (e.g., different surfaces, different temperatures) in very similar ways.

Another ongoing project is closer to home, examining whether and how a ubiquitous urban denizen, the squirrel, is adapting to city living and whether differences in the urban landscape across a socioeconomic gradient are reflected in the behavior, diet and evolution of these rodents.

Cats

Stay tuned!