Founder effects and the genetic structure of Coulter pine

Mean expected heterozygosity at 33 isozyme loci decreased with latitude from 0.193 near the southern extreme of Coulter pine's range to 0.107 at its northern extreme. This decrease was paralleled by a loss of alleles north of the Peninsular Ranges of southern California. Fifteen alleles dropped out along the roughly lineal range, at points coincident with large gaps in the species' distribution. The pattern may reflect a cascading series of founder events as Coulter pine invaded the Trans. verse Ranges and the South Coast Ranges from Pleistocene refugia. Alleles were not replaced following colonization, probably because migration, Nm, among populations is only 0.74-1.27, depending on estimator, the lowest values reported in any pines. Wright's (F(ST)) indicated that 16.5% of the total genic diversity is among populations. The fixation index, (F(IS)) of 0.072 indicated only a moderate excess of homozygotes. However, the northernmost outlier had significant excess homozygosity (F = 0.253). Hybridization may also play a role in the genetic structure of Coulter pine: 16 alleles were novel, or private, occurring only where Coulter pine was sympatric with Jeffrey pine, particularly at San Benito Mountain. Some of these novel alleles could be the result of introgression from Jeffrey pine, or possibly represent hybrizymes, products of intragenic recombination between genomes.