Ecological speciation of King rails (Rallus elegans) and Clapper rails (Rallus longirostris)

Abstract

Ecological speciation is a process involving adaptation to different environments leading to reproductive isolation between populations. Hybridization between Clapper and King rails (Rallus longirostris and R. elegans) is frequent but restricted to narrow bands of brackish marsh along the Gulf of Mexico and Atlantic coastlines of North America. In the second chapter I used mitochondrial and nuclear sequences to infer phylogenetic relationships in this species complex. The complex falls out into three distinct groups: South America; western North America; and eastern North America and the Caribbean. My results indicate that R. elegans as currently recognized is paraphyletic, with birds of highland Mexico sister to R. longirostris of California whereas R. elegans of eastern North America and Cuba forms a clade with eastern North American and Caribbean R. longirostris. My results support splitting the complex into four distinct species. In the third chapter I used morphological data, ecological data, and DNA sequences generated via next-generation methods to investigate the hybrid zone situated along a salinity gradient. Maximum-likelihood clines fitted to the five loci, body weight, salt gland weight, and water salinity revealed that the hybrid zone is narrow (mean genetic cline width = ~8.3 km), and that all clines are coincident and concordant with the water salinity cline. In the fourth chapter, I measured morphological characters on males collected across the hybrid zone to determine differences on opposite ends of the zone and to examine if these differences covaried and were correlated with genotypes from four nuclear markers; I also estimated dispersal distances using these characters. For four hybrid sites I found six of twelve correlations were significant, and ten of twelve estimates of covariance were greater than the expected covariance, suggesting selection against recombinants for the quantitative characters salt gland weight, bill length, and overall size. The estimates of dispersal distances for genetic and morphological characters were more than an order of magnitude lower than the only published estimate from a banding study. These results suggest that while rails are capable of dispersing across the zone, the hybrid zone constrains dispersal within it.