Genetic variation in a widespread, epiphytic orchid: Where is the evolutionary potential?

Breeding system, population dynamics, and the underlying genetic structure of orchid populations should reflect their evolutionary potential. We exami ned the genetic structure of populations representing a widespread, obligat ely outbreeding epiphyte of the Greater Antilles, Tolumnia variegata. We sa mpled 14 populations from Puerto Rico, Dominican Republic, and Cuba for iso zyme variation using horizontal starch-gel electrophoresis, We resolved and scored variation for twelve loci from six enzymes, and found that all loci were polymorphic at the species level and allele frequencies were signific antly different among all populations. Variation at the population level wa s also high, as the average percent polymorphic loci was 71%, the expected mean heterozygosity (H-e) was 0.21, and the average number of effective all eles per locus was 1.43. Nearly all genetic variation in T. variegata occur red within populations (H-I = 0.22; D-ST = 0.03; G(ST) = 0.11). Average gen e flow estimates among populations were moderate (Nm(S) = 1.34 and Nm(W) = 2.07). Within islands, populations were very similar to one another and the re was a significant negative correlation between geographic distance and e ither genetic identity or Nm(W) among populations. Among islands, gene flow was more restricted but there was no obvious relationship between genetic distance and population differentiation. Patterns of genetic and morphologi cal variation matched. Some minor differentiation has occurred among island s, but gene flow estimates reinforce the notion that all populations sample d represent a single species. Evolutionary potential is high because of sub stantial levels of genetic and morphological variation, but evolutionary di versification among these populations would likely be a slow process.