Sources of genetic structure in the woody perennial Betula occidentalis

We designed a transect-based survey of allozyme variation in water birch, B etula occidentalis, to investigate causes of genetic structure common to ma ny temperate woody perennials: large-scale Holocene range shifts, landscape barriers to gene flow, and introgressive hybridization. We sampled subpopu lations at low, medium, and high elevations at approximately every degree o f latitude along the eastern Rocky Mountain front. Genetic diversity was re latively even across the latitudinal range of the species (H-e = 0.19), but this was a result of two opposing forces. Introgressive hybridization with the more northerly distributed hexaploid, Betula papyrifera, has apparentl y enriched variation in that direction. But native variation decreased in t he same direction because of a loss of rare alleles along this known postgl acial migration route. Two latitudinal frequency clines were attributed to an inter- and an intraspecific source, but clinal variation was not a featu re at other loci. High levels of gene flow were detected at scales of up to 660 km in a hierarchical analysis of gene flow. This is a pattern expected of highly outcrossing species with wind-dispersed pollen and seed. In ripa rian-distributed species such as water birch, wind dispersal is probably mu ch more common within mountain valleys than between mountain valleys. Inste ad, pairwise estimates of historical gene flow,(M) over cap followed the op posite pattern. Elevational barriers within valleys seem to be stronger tha n topographical barriers between valleys. We tested for isolation by distan ce using subsets of the data that either removed or maximized the effect of elevation. When population pairs were restricted to the same elevation, th e isolation by distance model explained 58% more variation than that of the entire data set, but when pairs were selected from maximally different ele vations, the slope of the regression was not significantly different from z ero. Thus, systematic sampling across a predictable barrier to gene flow in troduced noise into our data set. Such noise may be present in many other d ata sets, in which researchers have haphazardly sampled across ecological b arriers to gene flow.