The Quick and the Dead: Correlational Selection on Morphology, Performance, and Habitat Use in Island Lizards

Abstract

Natural selection is an important driver of microevolution. Yet, despite significant theoretical debate, we still have a poor understanding of how selection operates on interacting traits (i.e., morphology, performance, habitat use). Locomotor performance is often assumed to impact survival because of its key role in foraging, predator escape, and social interactions, and shows strong links with morphology and habitat use within and among species. In particular, decades of study suggest, but have not yet demonstrated, that natural selection on locomotor performance has shaped the diversification of Anolis lizards in the Greater Antilles.Here, we estimate natural selection on sprinting speed and endurance in small replicate island populations of Anolis sagrei. Consistent with past correlational studies, long-limbed lizards ran faster on broad surfaces but also had increased sprint sensitivity on narrow surfaces. Moreover, performance differences were adaptive in the wild. Selection favored long-limbed lizards that were fast on broad surfaces, and preferred broad substrates in nature, and also short-limbed lizards that were less sprint sensitive on narrow surfaces, and preferred narrow perches in nature. This finding is unique in showing that selection does not act on performance alone, but rather on unique combinations of performance, morphology, and habitat use. Our results support the long-standing hypothesis that correlated selection on locomotor performance, morphology, and habitat use drives the evolution of ecomorphological correlations within Caribbean Anolis lizards, potentially providing a microevolutionary mechanism for their remarkable adaptive radiation.

Natural selection on thermal performance in a novel thermal environment

“…Moreover, we detected weak positive directional selection on P max at both sites, suggesting that faster lizards were more likely to survive. This pattern is consistent with previous studies (22,23) and supports the common assertion that sprint speed is an ecologically relevant measure of performance in lizards.…”

“…Second, lizards with higher thermal optima, and therefore aboveaverage sprint capacities at the warmer transplant site, may have been more successful in outcompeting rivals for access to resources (through either interference or exploitative competition) (23). Third, individuals with higher thermal optima may have been better at evading predators (22,23). It is also possible that the thermal sensitivity of sprint speed is correlated with other measures of performance (e.g., digestive efficiency, immune function) that influenced fitness but that we did not measure directly.…”

“…Quadratic gradients were calculated by doubling quadratic regression coefficients (and their associated SEs) from a separate model that included both linear and quadratic (but not crossproduct) terms. a 3-cm diameter wooden dowel (demarcated every 5 cm), inclined at an angle of 20°to discourage hopping (22). We filmed trials with high-speed (60 frames per second) digital video.…”

“…Only those individuals that were reliably measured at four of five temperatures (including the lowest and highest temperatures) were included in selection analyses, resulting in final sample sizes of 85 and 80 males, respectively. Sprint speed is an ecologically relevant trait that is known to correlate with survival (22,23) and may also provide a representative index of whole-organism physiological performance (34). Near the end of the breeding season (late August), we returned to both sites and attempted to recapture all surviving individuals to estimate viability selection.…”

Sex-specific variation in morphology and performance in the Asian house gecko

“…Because performance is not solely determined by any one trait but rather the complex interactions among many, natural selection may cause modifications of other morphological and/or physiological traits to counteract the negative effects of sexually selected traits (Lande, 1980;Arnold, 1983;Calsbeek and Irschick, 2007;. The idea that compensatory mechanisms decrease the cost of sexually selected traits is not novel and support for such compensatory traits derives from several studies examining the relationship between male ornaments, locomotor performance and morphological variation in both birds and insects (Moller, 1996;Husak and Swallow, 2011;McCullough et al, 2012;Painting and Holwell, 2013).…”