Determining the causes and consequences of speciation in western North American scincid lizards of the Plestiodon skiltonianus species complex (Squamata: Scincidae)

Date of Completion

January 2005


Biology, Ecology|Biology, Zoology




This dissertation investigates a candidate example of parallel speciation in North American scincid lizards of the Plestiodon skiltonianus species group. Parallel speciation occurs when morphologically similar but phylogenetically distant populations are more reproductively compatible than are morphologically differentiated but closely related individuals. The skiltonianus group consists of two ecotypes that differ primarily in adult body size. A molecular phylogeny based on mtDNA showed that the larger ecotype, P. gilberti, evolved multiple times independently from within the smaller ecotype, P. skiltonianus. Although clades with opposite ecomorphology appear to be reproductively isolated, circumstantial evidence indicates that gilberti clades in secondary geographic contact may not be. ^ The first two chapters provide explicit tests of the parallel speciation hypothesis. Chapter 1 describes mate compatibility experiments using wild-caught skinks that were designed to separate the effects of genetic differentiation, geographic separation and body size differentiation on mate compatibility. The difference in body size has a significant effect on the probability of copulation, whereas the other variables do not. I argue that the body size effect results from a mechanical constraint on copulation likely to be general for lizards. ^ Chapter 2 describes a complementary molecular study using AFLP markers that was designed to test whether there is ongoing nuclear gene flow between gilberti clades, which come into geographic contact in the southern Sierra Nevada. Bayesian analyses show that the degree of population differentiation increases with increasing distance from the mtDNA contact zone; this provides evidence for gene flow between independently evolved clades of gilberti. To my knowledge, the skiltonianus group provides the first example of parallel ecological speciation in which the lineages that evolved in parallel are merging in nature. ^ Chapter 3 examines patterns of morphological variation. Morphological variation is largely partitioned by sex and ecomorph. Subtle differences were also detected between the clades of gilberti, but these clades overlap substantially in morphology, emphasizing their morphological parallelism. ^ Chapter 4 investigated broad-scale patterns of morphological evolution in North American Plestiodon. Comparative phylogenetic analyses were used test alternative hypotheses of character evolution, with the goal of providing insight into the developmental mechanisms that shaped the current morphological diversity. ^