Move over, moths! There’s a new kid in town!
During the Industrial Revolution, naturalists in England noticed that the incidence of normal, light-colored peppered moths (Biston betulari) had become scarce in the vicinity of various urban centers. Instead, they were finding a melanistic (dark colored) variety. At the same, time, pollution had caused the local trees to get darker. In 1896, J.W. Tutt proposed that this change was an example of natural selection. The light moths lost their camouflage effect when they sat on the new dark trees, so they got eaten by the local birds. Hence, the moths with the gene for melanism fared better and became prevalent.
The issue was hotly debated and thoroughly investigated through the first half of the twentieth century, and has since become one of the best known and best supported examples of natural selection. You probably remember it from your high school biology class.
Well, in Puerto Rico, a band of scientists (Winchell 2016) have found another example of “urban” natural selection, this time in a lizard called the crested anole (Anolis cristatellus).
Anoles are a small lizards found throughout the Caribean area. There are many species in the Anolis genus. They are known for their colorful dewlaps (throat-flaps), but their most important quality for our purposes is their ability to climb.
Forest-dwelling anoles have longish legs and claws for gripping tree trunks, but it seems that these adaptations don’t quite cut it in the city. Like the raccoons of North America, the crested anole flourishes in urban environments. But has it remained unchanged?
The urban environment is different from the forest in many ways, but in this study the authors looked at the different kinds of perches available to the lizards. There are fewer trees in the urban jungle, and many broad, flat smooth surfaces. Buildings are made of stone or glass, and even small houses are painted with smooth surfaces that make it hard for claws to grip.
The authors looked at the morphology of the anoles in three different cities on the island of Puerto Rico (Mayagüez, Ponce, and San Juan), and in forested areas just outside each city. They found significant differences in the length of limb bones – even accounting for differences in body size- as well as in the number of toe lamellae.
The lamellae are small scales on the toes that help an anole stick to smooth surfaces. The urban anoles had more and smaller lamellae, which should make it easier for them to climb up windows.
But this study goes further than just finding morphological differences. They also took pains to ensure that the differences were heritable, by actually interbreeding captive urban and forest specimens. Plus, they took samples of tissue from the wild-caught animals and performed genetic tests. The tests confirmed that the differences in morphology were not a product of some ancient split between forest dwellers and the animals that now live in the city.
What is exciting here is that the study lays the groundwork for further tests of evolutionary effects. For example, the authors hope to perform analyses of gene flow between the urban and forest populations, and across the island. This will allow us to answer questions about the strength of selection (can the urban critters retain their differences even if they mate with forest individuals?) and about one of my favorite topics: speciation.
Speciation is the process in which new species arise by the splitting or other evolution of old species. The classic view of speciation is that a population of organisms somehow gets separated or isolated from the rest of the species, and either loses the ability to interbreed with the parent species or becomes quite different from it, or both. In fact, there are many hypothetical versions of this scenario. In some, the isolated population experiences different selective pressure and so undergoes adaptation. But this leads to the question of how they retain their reproductive isolation. In other models, reproductive isolation occurs first, and visible differences are slow to accumulate. Still other models invoke a concept called genetic drift, in which changes in gene frequency are the product of random fluctations.
With a good foundation laid, and resources close to hand, this new discovery will serve as a laboratory to study the process of speciation in detail. I look forward to watching their developments!
Winchell et. al 2016. Evolution, 11 May. DOI: 10.1111/evo.12925
Photo: http://www.flickr.com/photos/schizoform/3893782985/ CC BY 2.0