2.5: Evolution and Natural Selection Observable Today

Although this chapter primarily focuses on the past, it is important to remember that natural selection and evolution are still ongoing processes. Climate change, deforestation, urbanization, and other human impacts on the planet are influencing evolution among many contemporary species of plants and animals. One such example occurs among crested anoles (Anolis cristatellus), small lizards of the Caribbean jungle that are increasingly making their home in cities (Figure 2.15).

Figure 2.15: Puerto Rican Crested Anole photographed in Picard, Dominica. Credit: Anolis cristatellus in Picard, Dominica-2012 02 15 0339 by Postdif is under a CC BY-SA 3.0 Unported License.

As urban sprawl continues across the planet, shrinking the availability of wilderness habitat, many wild species have come to make their homes in cities. “Urbanization has dramatically transformed landscapes around the world—changing how animals interact with nature, creating “heat islands” with higher temperatures, and hurting local biodiversity. Yet many organisms survive and even thrive in these urban environments, taking advantage of new habitats created by humans (National Science Foundation 2023). A recent example of lizards in Puerto Rico demonstrates evolution happening quickly in both behavior and genes that has come about as a result of the pressures of urban life (Winchell et al. 2023). Crested anoles, who once lived only in forests, now scurry around towns and cities throughout the Caribbean. As a result of having to sprint across large open spaces, like hot streets and parking lots, they have developed longer limbs. City-living lizards also now sport longer toe pads with special scales that allow them to cling to smooth surfaces, like windows and walls (as well as the plastic patio furniture pictured in Figure 2.15), rather than to the rough surfaces of bark and rock that their forest-living relatives climb. These adaptations enhance their ability to escape predators and survive in cities.

Researchers were curious to see if these changes were the result of genetic changes in urban populations, so they captured 96 male lizards in three Puerto Rican regions and compared their genomes to each other and to forest specimens in each location. They found that members of the three city-living populations were genetically distinct from each other, as well as from forest populations in their respective regions. In total, 33 genes in the urban lizards’ genomes were different from their forest-living counterparts and were linked to urbanization. These changes are estimated to have occurred just within the last 30 to 80 generations, suggesting that selective pressures related to survival in urban environments is strong. As study coauthor Kristen Winchell put it, “We are watching evolution as it is unfolding” (National Public Radio 2023). (If you are interested in hearing more about the study, see “How Lizards Adapt to Urban Living,” an episode of Science Sessions, a free podcast from the Proceedings of the National Academy of Sciences (PNAS 2023) featuring Dr. Winchell and her work.)