MARILYN NIELSON,1 KIRK LOHMAN,1,2 AND JACK SULLIVAN3
1Department of Fish and Wildlife Resources, University of Idaho, Moscow, Idaho 83844-1136 2E-mail: [email protected]
3Department of Biological Sciences, Box 443051, University of Idaho, Moscow, Idaho 83844-3051
Abstract. Tailed frogs are distributed in high-gradient streams within the disjunct mesic forests of the Pacific North-west and represent the basal lineage of the anurans. We sequenced 1530 nucleotides of the mitochondrial cytochrome b and NADH dehydrogenase subunit two genes from 23 populations and used parsimony, maximum-likelihood, and nested-clade analyses to estimate relationships among populations and infer evolutionary processes. We found two divergent haplotype clades corresponding with inland Rocky Mountain populations and coastal populations and sep-arated by up to 0.133 substitutions per site. Within the coastal assemblage, haplotypes formed clades by mountain range with 0.010–0.024 substitutions per site divergence among populations. Inland haplotypes exhibited minimal genetic structure, with the exception of 0.021 substitutions per site distance between populations from the East Fork of the South Fork of the Salmon River and all other inland haplotypes. The magnitude of divergence between inland and coastal populations, as well as the paleobotanical record, suggest isolation of these lineages occurred during the late Miocene to early Pliocene, probably in response to the rise of the Cascade Mountains. Genetic structure within coastal and inland populations is consistent with isolation in refugia during the late Pliocene and early Pleistocene. Closely related inland haplotypes reflect range expansion following glaciation. The depth of divergence between inland and coastal populations supports the persistence of mesic forests within the inland Pacific Northwest throughout the Pleistocene and is congruent with patterns found in several other mesic forest species. Based on mitochondrial divergence and previous allozyme and morphological data, we recommend recognition of inland populations as a distinct species, Ascaphus montanus.
Regional phylogeography examines the effects of geo-graphic history on genetic variation and thus is essential to understanding evolutionary processes (reviewed in Avise 1994). Comparing the phylogenies of codistributed taxa pro-vides insight into how factors such as population structure and life history influence the genetic signature left by geo-graphic events. Beyond descriptive biogeography, under-standing the factors that affect the evolution of taxa in a region allows for the development of testable predictions about patterns of genetic diversity. In turn, regional phylo-geography provides insight into the response of biotic com-munities to historic events (e.g., Sullivan et al. 2000).
The Pacific Northwest region of North America (40–528 N, 113–1268 W) provides abundant opportunities to examine phylogenetic patterns in the context of historic geology and climate. The region has experienced dramatic geological events including the uplift of several mountain ranges, mul-tiple glaciations, and scouring postglacial floods (Alt and Hyndman 1995). Furthermore, biotic communities in this re-gion range from temperate rain forest to xeric sagebrush steppe (Daubenmire 1975). In this paper, we begin to address regional phylogeography in the Pacific Northwest by ex-amining patterns of molecular variation in the tailed frog,
This species represents an excellent starting point for the examination of regional phylogeography in the Pacific North-west for several reasons. First, Ascaphus inhabits forested headwater streams in the coastal ranges and Cascade Moun-tains from British Columbia south to northern California. Disjunct populations occur in the northern Rocky, Blue, Wal-lowa, and Seven Devils Mountains (Metter and Pauken 1969; Nussbaum et al. 1983; Green and Campbell 1984; Fig. 1).
Although local distribution depends on a number of variables, Ascaphus populations are sensitive to the increased siltation and water temperatures that may accompany timber harvest; thus, they are found most often in old growth reaches (Bury 1983; Corn and Bury 1989; Welsh 1990; Walls et al. 1992). This has generated concern over the loss and fragmentation of old growth habitat in the Pacific Northwest and the effect this may have on populations of tailed frogs (Bury 1983; Corn and Bury 1989; Welsh 1990; Walls et al. 1992; Blau-stein et al. 1994).
Second, several ecological characteristics of Ascaphus may influence genetic variation. Savage (1960, 1973) suggested that the progenitors of tailed frogs inhabited the North Amer-ican continent by the Jurassic, thus genetic structure within this species may reflect ancient events. In addition, strong geographic structure might be expected because these frogs are restricted to stream habitats and have limited dispersal abilities (Daugherty 1979). Conversely, a long generation time (6–8 years) and low metabolic rate may result in a slow evolutionary rate and therefore reduced genetic divergence (Kocher et al. 1989; Martin and Palumbi 1993; Rand 1994).
Third, the tailed frog is among 156 species, including bryo-phytes, fungi, vascular plants, worms, insects, and verte-brates, endemic to the Pacific Northwest that share this dis-junct distribution in association with mesic coniferous forests (compiled from McCune 1984; Johnson 1987; Lorrain 1988). Numerous authors (e.g., FAUNMAP Working Group 1996; Avise et al. 1998; Bernatchez and Wilson 1998) have focused on Pleistocene glaciations and their effect on the fauna of North America, however, earlier events undoubtedly influ-enced intraspecific patterns (Riddle 1996a,b; Zink 1996; Waits et al. 1998). Savage (1960, 1973) included tailed frogs as members of the ‘‘old northern element’’ and postulated that this element evolved in close association with the mesic flora of the Miocene, responding in concert as the range of this flora contracted in response to geologic events. Others have agreed that elements of the mesic forest community may have responded similarly to changes in climate (Johnson 1987; Lorraine 1988).