In a global change scenario, the persistence of numerous animal populations is challenged by the consequences of human activities. Urbanization, for instance, represents a dramatic habitat transformation that has led to a general pattern of reduced biological diversity in these areas. Paradoxically, some species are doing very well in these new environments. This leads to the question of whether and how these populations are adapting to these new environmental conditions.
Although the number of studies providing evidence for phenotypic differences between urban and natural areas is growing fast, few studies have investigated whether and how animals might be evolutionarily adapting to the intensively modified urban habitats. Kristin Winchell and collaborators address this question in their recent publication in Evolution “Phenotypic shifts in urban areas in the tropical lizard Anolis cristatellus.” The authors studied the habitat use and morphology of forest vs. urban populations of Crested Anoles, Anolis cristatellus, from three municipalities in Puerto Rico. In short, this article provides evidence suggesting that urban anoles are under differential selective pressures as compared with those from forested habitats, and that these differences may have a genetic basis.
As the authors detail in their paper, anoles are a great system to study the morphological consequences of urbanization. This is because much information is available on the relationship between their habitat use, morphology and performance (reviewed in Losos 2009). In urban habitats, natural substrates have largely been replaced by artificial structures such as metal poles and walls. Consequently, it can be predicted that their performance on these surfaces is not optimal, as their morphology may not be suited to use these substrates. Indeed, it has been shown that lizards tend to perch on narrower, less smooth surfaces in natural habitats –a topic that has been dealt with in previous AA posts.
In this paper, the authors use field observations to show that lizards in urban areas use artificial substrates a large proportion of the time and that these urban substrates are broader and smoother than those in natural areas. Then, by X-raying lizards from the different habitats, the authors show that urban lizards have longer limbs (relative to their body size) and higher number of subdigital lamellae -which improve traction for perched lizards- than individuals from forested areas (Fig. 4). This is indeed consistent with ecomorphological predictions that anoles with longer limbs perform better on wider perches. Increased lamellae number should provide lizards with a better grip on smoother surfaces.
Finally, the authors conducted a common-garden rearing experiment in which they reared individuals from one of the three pairs of populations studied. The aim of this experiment was to rule out the possibility that morphological differences are merely the consequence of phenotypic plasticity. When measured at approximately one year of age, the first generation offspring of urban lizards showed longer forelimbs and more lamellae as compared to offspring of forest-dwelling lizards (Fig. 5). This result suggests that anoles in urban areas are under significantly different natural selection pressures and may be evolutionarily adapting to their human-modified environment.
Winchell, Kristin M., Reynolds, R. G., Prado-Irwin, Sofia R., Puente-Rolón, Alberto R., and Revel, Liam J. (2016). Phenotypic shifts in urban areas in the tropical lizard Anolis cristatellus.
Kolbe, J.J., Battles, A.C. & Avilés-Rodríguez. K. (2015) City slickers: poor performance does not deter Anolis lizards from using artificial substrates in human-modified habitats. Functional Ecology.
Losos, J.B. (2009) Lizards in an Evolutionary Tree: Ecology and Adaptive Radiation of Anoles. University of California Press, Berkeley, CA, USA.