Systematics Section / ASPT
Baldwin, Bruce G. .
A phylogenetic perspective on cryptic biological species, chromosome evolution, and putative peripatric speciation in the California tarweed genus Holocarpha (Compositae--Madiinae).
Phylogenetic data from external and internal transcribed sequences of nuclear ribosomal DNA allowed resolution of multiple evolutionary lineages within chromosomally heterogeneous "morphological species" of Holocarpha, a genus of obligately outcrossing, annual tarweeds from low-elevation grasslands in California. Phylogenetic structure and minimal noise (homoplasy) within rDNA clades of H. heermannii and H. virgata may reflect minor potential for gene flow among populations of each taxon; Clausen, Keck, and Hiesey and R. Palmer found that H. heermannii and H. virgata have each undergone such extensive chromosomal repatterning that members of different populations are generally intersterile and represent distinct, cryptic biological species. Rates of chromosomal and morphological change appear to have varied greatly during the evolutionary history of Holocarpha. For example, based on the rDNA trees, H. heermannii, with chromosomally differentiated, intersterile populations, is evidently sister to H. obconica, which has been shown to exhibit high interfertility among populations throughout its range -- the two sister taxa are widely sympatric and morphologically and ecologically similar. In an allopatric, second pair of taxa, rDNA trees indicate that the interfertile populations of one (H. macradenia) diverged more recently than the intersterile populations of the other (H. virgata). Clausen, Keck, and Hiesey suggested that H. macradenia and H. virgata were products of geographic (vicariant) speciation; the rDNA trees uphold R. Palmer's alternative hypothesis that the rare, narrowly distributed H. macradenia descended from the widespread (paraphyletic) H. virgata and appears to represent another tarweed example of rapid morphological and ecological divergence of a peripheral isolate (i.e., peripatric speciation). Moderate interfertility between H. macradenia and some members of H. virgata should allow exploration of Carlquist's suggestion that morphological differences between the two taxa may be the result of heterochronic change in the evolutionary history of H. macradenia.
1 - University of California, Berkeley, Integrative Biology, University and Jepson Herbaria, 1001 Valley Life Sciences Building, Berkeley, California, 94720-2465, USA
nuclear ribosomal DNA
Presentation Type: Paper
Location: Cottonwood A (Snowbird Center)
Date: Wednesday, August 4th, 2004
Time: 3:15 PM