CONTRASTING PATTERNS AND PROCESSES OF EVOLUTIONARY CHANGE IN THE TARWEED–SILVERSWORD LINEAGE: REVISITING CLAUSEN, KECK, AND HIESEY'S FINDINGS1

Abstract Jens C. Clausen, David D. Keck, and William M. Hiesey's biosystematic research on continental tarweeds (Madiinae; Compositae) provided diverse examples of evolutionary change for Clausen's synthesis, Stages in the Evolution of Plant Species. Subsequent anatomical work by Sherwin Carlquist demonstrated that the tarweed lineage also includes a spectacular example of adaptive radiation, the Hawaiian silversword alliance. Molecular phylogenetic data and evidence from genetic and hybridization studies have allowed additional perspectives on Clausen et al.'s and Carlquist's hypotheses of tarweed–silversword evolution. In Californian Layia, Clausen et al.'s evidence for gradual allopatric diversification for the n = 7 taxa accords with patterns of molecular divergence and decay of interfertility across lineages inferred from a rate-constant rDNA tree. In contrast, recent evidence on patterns and timing of diversification in an n = 8 Layia clade indicates multiple examples of accelerated phenotypic evolution, unresolved by Clausen et al., that evidently reflect rapid "budding off" of morphologically distinct lineages in ecologically novel settings. In rDNA trees of Californian Holocarpha, lineages representing different cryptic biological species, documented by Clausen, appear to predate the origin of a morphologically and ecologically distinctive taxon (H. macradenia (DC.) Greene) that retains interfertility with relatives of ancestral phenotype; at fine-scale levels of divergence, a disconnect is evident between evolution of intrinsic, post-mating reproductive barriers and phenotypic evolution in Holocarpha. Clausen's evidence for strong intersterility barriers between the mostly annual, continental species of the “Madia” lineage contrasts with Gerald D. Carr and Donald W. Kyhos's subsequent finding of partial to full interfertility between the phenotypically disparate, insular species of the Hawaiian silversword alliance, a monophyletic group that descended from continental ancestors in the “Madia” lineage. Molecular phylogenetic data indicating major ecological changes associated with diversification, a brief timeframe for diversification, and a shift to woodiness in the ancestry of the silversword alliance uphold Carlquist's hypothesis of adaptive radiation of the group and help explain the lack of substantial, internal barriers to gene flow across lineages therein. Results of recent investigations have shown that highly dynamic evolutionary change in Madiinae, both in phenotypic characters and in modes and patterns of diversification, extends to even finer-scale evolutionary levels than indicated by Clausen et al.'s elegant studies. In general, current evidence on diversification in Madiinae appears to be consistent with Clausen et al.'s views concerning the importance of ecological factors in incipient evolutionary divergence. Phylogeny of Madiinae is no longer the intractable problem perceived by Clausen; relatively little is known about the biological basis for the extreme evolutionary propensities of tarweeds.