Evolution of Brown Towhees: Allozymes, Morphometrics and Species Limits

Abstract

Variation in the Brown Towhee (Pipilo fuscus) complex was studied by using morphometric and electrophoretic methods. Study taxa, distributed primarily in the aridlands of southwestern North America, included Abert’s Towhee (P. aberti), White-throated Towhee (P. albicollis), and two major components of the Brown Towhee (eastern fuscus group and western crissalis group); a sample of Brown Towhees from Baja California (crissalis group) was also included. Goals were to assess phenotypic (29 skeletal characters) andgenetic (allozymic) patterns of differentiation, test current species limits, estimate phylogenetic relationships, and examine concordance in morphometric and genetic variation. Evolution in skeletal size and proportions has occurred primarily in the skull; taxa are not each distinct in principal component space. Phenograms based on taxonomic distances and correlation coefficients failed to reflect currently recognized species limits. A survey of 39 genetic loci revealed typical levels of genetic variation within and among taxa. However, the genetic distance between P. albicollis and P. fuscus from Arizona was nearly zero. The fuscus and crissalis groups were genetically distinct. Branching diagrams (phenograms, distance Wagner trees, maximum likelihood trees) summarizing genetic distances derived from a matrix of allelic frequencies suggest he following ordering of taxa: Abert’s Towhee, Brown Towhee (‘&cus) plus White-throated Towhee, and Brown Towhees from California and Baja California (crissalis). Based on the genetic data, the Brown Towhee as currently recognized is paraphyletic. The two forms of the Brown Towhee should be considered distinct species (Pipilo fuscus and Pipilo crissalis).