Functional morphology, phenotypic plasticity, and geographic variation in two terrestrial ectotherms
The concept of geographic variation in phenotypes has a long history in evolutionary biology. Differences in phenotypes can stem from both local adaptation, in which differentiation occurs at the genetic level, and phenotypic plasticity, in which differentiation is a direct effect of an animal's environment. Although not mutually exclusive, each of these pathways contributes to organismal fitness. I examined the effects of environmental factors, including development, temperature, latitude, altitude, and habitat structure on the growth, body morphology and performance of two terrestrial ectotherms. Eleutherodactylus coqui is a direct-developing frog whose life history lends itself to studies of development, and Sceloporus occidentalis is a wide-ranging temperate lizard suited for studies of geographic variation.
In the laboratory, I manipulated the development of these ectotherms either by introducing a predation threat or by varying incubation temperature. I then examined the effects of these factors on the animals' phenotype at hatching and, in the case of S. occidentalis, for a number of weeks thereafter. This design allowed me to test the persistence of phenotypic plasticity. Finally, I measured the correlations among morphology, performance and habitat use in four wild populations of lizards.
I found that hatching early in response to predation can have dramatic effects on the morphology and performance of E. coqui froglets: animals that hatch early to escape predation suffer a decrease in jumping ability, resulting in a trade-off between life history stages. In S. occidentalis, incubation temperature affects morphology and performance over time, but some morphological characteristics that contribute to performance capacities, such as hindlimb length, vary instead across latitudinal and altitudinal gradients. Finally, despite morphological and performance differences among populations of wild-caught lizards, S. occidentalis represents a generalized species that can utilize many different habitat types without large corresponding changes in body form.
Here I argue that developmental responses to the environment can be adaptive, but also that traits that have direct effects on fitness tend to be more canalized in these animals. Further studies on geographic variation should include both of these factors when determining the evolutionary history and possible evolutionary trajectory of ectotherms.
0472: Organismal biology