An experimental test of the role of predators in the maintenance of a genetically based polymorphism


Correspondence: Ryan Calsbeek, Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA.

Tel: 603 646 9917; fax: 603 646 1345;



Polymorphisms provide one of the most useful tools for understanding the maintenance of genetic and phenotypic variation in nature. We have previously described a genetically based polymorphism in dorsal patterning that is expressed by female brown anole lizards, Anolis sagrei, which occur in Bar, Diamond and intermediate Diamond–Bar morphs. Previous studies of island populations in The Bahamas support a role for selection in maintaining the polymorphism, but the agents responsible for this selection remain unclear. We tested two main hypotheses regarding the importance of predation as a selective agent that maintains the polymorphism within populations. First, we tested whether correlational selection favours different combinations of morph, locomotor performance and escape behaviour by measuring morph-specific natural selection on sprint speed, running endurance and the propensity of females to either ‘freeze’ or ‘run’ in response to attempted capture. Morphs did not differ in any of these traits, nor did correlational selection consistently favour any particular combinations of morph and antipredator behaviour. Second, we experimentally excluded bird and snake predators from two entire island populations, allowed these predators access to two additional islands and then measured subsequent differences in natural selection on morphs in each population. Predators reduced the survival of Bar and Diamond females, but not of genetically intermediate Diamond–Bar females. These results provide limited evidence that predation may play a role in maintaining this polymorphism, although the functional traits that could account for differential susceptibility to predation remain unclear.