Evolutionary Genetics of Flycatchers. I. Sibling Species in the Genera Empidonax and Contopus

Abstract

Protein variation was analyzed at 38 presumptive genetic loci in 214 specimens of 12 species of Empidonax and 3 species of the related genus Contopus. These tyrannid flycatchers are noted for their great morphologic uniformity, species-specific songs, and distinctive nesting ecology. Intraspecific genetic variability was substantial and comparable to that shown in other recent avian studies. The average Nei's genetic distance among species of Empidonax, exclusive of euleri, was 0.070, a value somewhat higher than the average of 0.044 shown by other birds. Because euleri differs from all other species of Empidonax at large genetic distances, 0.348 to 0.465, we conclude that it should be removed from the genus; euleri is probably best placed in Cne- motriccus. Eight of the 12 species of Empidonax possessed unique alleles, ranging from 1 autapomorphy in atriceps to 10 in euleri. Within Contopus, only sordidulus had unique alleles and these were uncommon. A locus by locus comparison of alleles revealed many subsets of species which share particular alleles, but no clear groupings of taxa are evident. Distance Wagner trees and Fitch-Margoliash trees, as well as UPGMA and WPGMA pheno- grams, were constructed from the matrix of Rogers' genetic distances, showing that: (1) euleri is very distinct from all other species of Empidonax and Contopus; (2) the three species of Contopus are closely related; and (3) E. traillii-E. alnorum and C. sordidulus-C. virens are species pairs. Two of the three branching diagrams also suggest the close affinity of E. difficilis and E. flavescens, and of E. hammondii and E. atriceps. The latter relationship was unexpected because, on morpho- logic and vocal evidence, E. hammondii had been considered to be a close relative of E. minimus. The latter species and E. virescens seem to be rather strongly differentiated within the genus. Except for the component allospecies of superspecies (E. traillii-E. alnorum, E. difficilis-E. fla- vescens, and C. sordidulus-C. virens), which are relatively recently derived, the protein evidence suggests that the radiation of species of Empidonax occurred over a rather narrow span of time, perhaps in the late Pliocene or early Pleistocene. The phylogenetic picture is not one of a prolonged evolution of species via a dichotomous sequence of cladogenetic events. Therefore, the great similarity of sibling species in Empidonax and in Contopus is not evidence for their recent origin. Rather, it is a consequence of a very conservative evolution of plumage and morphology that has not kept pace with evolutionary rates for allozymes, vocalizations, and features of nesting biology. (Flycatchers; Empidonax; Contopus; sibling species; allozymes; char- acter discordance; speciation; phylogenetic inference; genetic distance.)