Geographic variation in life history tactics, adaptive growth rates, and habitat-specific adaptations in phylogenetically similar species: The Eastern Fence Lizard, Sceloporus undulatus undulatus, and the Florida Scrub Lizard, Sceloporus woodi

To understand the evolutionary and ecological significance of geographic variation in life history traits, we must understand whether the patterns are induced through plastic or adaptive responses. The Eastern Fence Lizard, Sceloporus undulatus, exhibits countergradient variation (larger body sizes, et cetera, in northern, cooler environments; presumed adaptive) in life history traits across its large geographic range. However, cogradient variation (the expected result from a plastic response, although not necessarily inconsistent with adaptation) has been suggested as a null hypothesis, especially on fine geographic scales because of relatively small environmental changes. Here we focus on life history variation on a fine geographic scale to test whether cogradient variation is exhibited even though countergradient variation is exhibited at larger scales, and if so, what mechanisms are involved in the switch. We examined north and south populations (∼2° latitude between) of the S. undulatus, and the Florida Scrub Lizard, S. woodi, by measuring adult body sizes, reproduction, and hatchling body sizes over a two year period and conducting reciprocal transplants of juvenile lizards each year. Our results indicate cogradient variation (larger body size in the southern population experiencing a warmer environment) in life history traits of S. undulatus and countergradient variation, a lack of variation in adult body size, in S. woodi along the Florida peninsula. Thus, S. undulatus exhibits cogradient variation at fine geographic scales and countergradient variation at larger scales. Reciprocal transplants revealed that the larger adult body sizes in the southern population of S. undulatus could be explained by longer growth periods allowed by greater intrinsic survival. In S. woodi, the larger than expected adult body sizes in the north could be explained by faster intrinsic and extrinsic juvenile growth rates in the northern population. Because S. undulatus and S. woodi remain distinct species associated with distinct, though adjacent, habitats, we also looked for habitat-specific adaptations. The second reciprocal transplant (between species and habitats) revealed habitat-specific adaptations in juvenile growth rates, but not juvenile survival. Each native species grew faster and had a higher average probability of reaching size at maturity in their native environment than did the foreign species.