Phylogeny and Rates of Molecular Evolution in the Aphelocoma Jays (Corvidae)

-I examined hypotheses of Aphelocoma jay phylogeny derived from allozyme data. Results from various algorithms differ in details, but the overall patterns are consistent: Scrub Jays (A. coerulescens) and Unicolored Jays (A. unicolor) were derived independently from different populations of Gray-breasted Jays (A. ultramarina). Within Scrub Jays, the californica subspecies group was derived from the populations of interior North America (woodhouseii group). One Unicolored Jay population and two Scrub Jay populations, all strongly differentiated, are placed consistently at the base of the phylogeny, but phenotypic, biogeographic, and theoretical evidence suggests that these populations represent rapidly evolving populations derived from within populations of their respective species. Because analyses of rates of molecular evolution demonstrate significant rate heterogeneity, I suggest that the application of a molecular clock to date-splitting events in the Aphelocoma jays is not a valid approach. Received 18 February 1991, accepted 3 July 1991. THE THREE species of Aphelocoma jays range throughout western and southern North America and northern Central America (Fig. 1; Pitelka 1951). Scrub Jays (A. coerulescens) range from Oregon and Wyoming south to the Isthmus of Tehuantepec, with disjunct populations on Santa Cruz Island off the coast of southern California and in peninsular Florida. The 15 subspecies form five groups, each characterized by unique combinations of plumage, morphological, and behavioral characters (Fig. 1; Pitelka 1951, Peterson 1991a): (1) woodhouseii group (Wyoming and southeastern Oregon south through Great Basin and along both sides of Rocky Mountains, and then along interior slopes of Sierra Madre Oriental and Sierra Madre Occidental of northern Mexico to southern Chihuahuan Desert and vicinity of Mexico City); (2) californica group (western Oregon, California, and Baja California); (3) sumichrasti group (southern Mexico); (4) coerulescens group (peninsular Florida); and (5) insularis group (Santa Cruz Island). The Gray-breasted Jay (A. ultramarina) ranges throughout the mountains of northern and central Mexico and the southwestern United States. The seven subspecies fall into three groups characterized by unique combinations of morphological and behavioral characters (Fig. 1; Pitelka 1951): (1) potosina group (Sierra Madre Oriental); (2) wollweberi group (Si' Present address: Department of Zoology, Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, Illinois 60605, USA. erra Madre Occidental); and (3) ultramarina group (Transvolcanic Belt). Finally, the Unicolored Jay (A. unicolor) consists of five allopatric populations, each a separate subspecies, in southern Mexico and northern Central America (Pitelka 1951). The Aphelocoma jays have been the subject of numerous comparative studies. Evaluations of social systems in the genus have led to advances in understanding ecological factors important in the evolution of sociality (Woolfenden and Fitzpatrick 1984, Fitzpatrick and Woolfenden 1986, Brown 1987, Peterson and Burt, in press). Differential habitat use in Scrub Jays (Peterson and Vargas 1992) is correlated with geographic variation in beak shape, suggesting that beak shapes have responded to natural selection (Peterson, in prep.). Integration of phylogenetic information into such investigations will allow an important new dimension of understanding (Brooks and McLennan 1990). Hence, I have attempted to estimate the phylogeny of the differentiated forms in the genus. Rates of molecular evolution. -Since the publication of the influential paper of Zuckerkandl and Pauling (1962), the idea of a "molecular clock" has been controversial in molecular biology and systematics. The clock concept is based on the assumption of a uniform rate of molecular evolution in a group. If the uniform rate assumption were correct, the accumulation of genetic differentiation between sister taxa would be time-invariant, and divergence times could be estimated from genetic distances. The clock concept is an important feature of