Ongoing natural selection on morphology of two rapidly evolving lizard species in White Sands

Background/Question/Methods

Studying colonization of new environments can tell us about how selection and adaptation shape biodiversity. White Sands, New Mexico, is an ideal system to study evolution after colonization. White Sands formed in the last 6,000 years and supports an ecosystem biologically distinctive from the surrounding desert. Since its formation, White Sands has been colonized independently by three lizard species that have evolved blanched coloration, broader heads and longer legs in parallel. Rapid convergent evolution of the three species indicates presence of strong divergent selection; however, natural selection has never been measured in this system. In our ongoing project, we measure the direction and magnitude of ongoing selection on ecologically important traits in White Sands lizards. We document differential survival of Holbrookia maculata, one of the three White Sands species. Recently, we have also incorporated another species, Sceloporus undulatus. We hypothesize that selection, in terms of differential survival of White Sands Holbrookia maculata and Sceloporus undulatus, acts on a suite of phenotypic traits including dorsal color and body shape. We predict: a) individuals with better crypsis (substrate matching) will be selected for and increase in frequency over seasons, and b) individuals with longer legs and broader heads will be selected for and increase in frequency over field seasons. To evaluate these predictions, we use mark-recapture to measure selection by incorporating survival as a proxy for fitness.

Results/Conclusions

Survival data and morphology measurements combined with regression analysis allows us to evaluate selection’s direction and magnitude via correlation. Our initial are promising, and suggest strong directional selection for color over just four months. Preliminary data on H. maculata show significant trends towards recapturing individuals with the highest condition (highest weight for their length), shortest legs, and brightest dorsal color.