Breeding system, population dynamics, and the underlying genetic structure of orchid pop- ulations should reflect their evolutionary potential. We examined the genetic structure of populations repre- senting a widespread, obligately outbreeding epiphyte of the Greater Antilles, Tolumnia variegata. We sampled 14 populations from Puerto Rico, Dominican Republic, and Cuba for isozyme 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 significantly different among all populations. Variation at the population level was also high, as the average percent polymorphic loci was 71%, the expected mean heterozygosity (He) was 0.21, and the average number of effective alleles per locus was 1.43. Nearly all genetic variation in T variegata occurred within populations (HT = 0.22; DST = 0.03; GST = 0.11). Average gene flow estimates among populations were moderate (Nms = 1.34 and Nmw = 2.07). Within islands, populations were very similar to one another and there was a significant negative correlation between geographic distance and either genetic identity or Nmw 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 morphological variation matched. Some minor differentiation has oc- curred among islands, but gene flow estimates reinforce the notion that all populations sampled represent a single species. Evolutionary potential is high because of substantial levels of genetic and morphological variation, but evolutionary diversification among these populations would likely be a slow process.
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Ackerman et al. (1999) studied this question.