Plant Species Disjunctions

Abstract

The use of molecular data in the study of plant species disjunctions is reviewed and evaluated. The major reason for employing molecular information is to estimate genetic divergence between morphologically similar disjunct species. Flavonoid chemistry offers few advantages over morphology because it is difficult, if not impossible, to infer genetic divergence from the arrays of flavonoid compounds sequestered by two species. Also, flavonoids can, like morphological characters, undergo stasis. Rather direct evidence for this comes from the fact that extant and fossil species may have identical or nearly identical flavonoids. Enzyme electrophoresis is useful for estimating divergence between disjunct species at gene loci encoding soluble enzymes. Disjunct species pairs in several genera are highly divergent at isozyme loci despite their morphological similarity. Restriction site analysis of chloroplast DNA (cpDNA) has proven useful for measuring divergence between disjunct species. The conservative rate of nucleotide substitutions in cpDNA allows one to estimate (albeit with several assumptions) sequence divergence between the DNAs. Whether isozyme and cpDNA data can be used to estimate divergence times with reasonable confidence remains an open question. In two studies employing both methods, similar divergence times were calculated with each. As two species become more divergent at isozyme loci, the variance in estimates of divergence times becomes larger, and the calculated times become less certain. Despite limitations, enzyme electrophoresis and cpDNA restriction site data are valuable for estimating genetic divergence between disjunct species. Future studies of plant species disjunctions will likely include nucleic acid sequence data. The molecular information should always be part of a broader study of species disjunctions, including detailed investigations of morphological features, chromosome numbers, ecology, and the geological histories of the species.