GENETIC VARIATION, DIFFERENTIATION, AND EVOLUTION IN A SPECIES COMPLEX OF TROPICAL SHRUBS BASED ON ISOZYMIC DATA.

An isozyme investigation of the Lisianthius skinneri (Gentianaceae) species complex in central Panama assayed levels of genetic variation within and among isolated populations and was used to reconstruct phylogenetic relationships within the complex. The widespread and low elevation L. skinneri and one derived cloud forest endemic species, L. habuensis, are depauperate in genetic variation. Three other endemic cloud forest species exhibiting larger population sizes and apparently more outcrossed breeding systems have higher levels of heterozygosity but retain low levels of allelic diversity. More than 90% of the genetic variation in the species complex is confined to among-population differentiation rather than witnin-population variation. Isozyme-based relationships within the species complex using both genetic divergence values (Fitch and Margoliash algorithm) and shared allelic states (Nelson and Van Horn algorithm) are identical. This network is not entirely congruent with a previous DNA-based network. Geographical isolation, small population size, low allelic diversity, and high levels of among-population differentiation suggest that repeated instances of founder events and genetic drift have been important in the evolution of this tropical shrub complex.