ADAPTIVE RADIATION AND GENETIC DIFFERENTIATION IN HAWAIIAN BIDENS

Abstract

The genus Bidens (Asteraceae) has undergone extensive adaptive radiation on the Hawaiian Islands. The 19 species and eight subspecies endemic to Hawaii exhibit much more morphological and ecological differentiation than the continental members of the genus. However, the Hawaiian taxa have the same chromosome number and retain the capacity to interbreed in all possible combinations. Twenty-two populations of 15 Hawaiian taxa and four populations of American taxa were compared at 21 loci controlling eight enzyme systems. Populations of Hawaiian taxa are highly polymorphic. However, little genetic differentiation has occurred among taxa in spite of the high levels of genetic variability. Genetic identities calculated for pairs of populations show that populations of the same taxon are genetically more similar than are populations belonging to different taxa, but all values are high. The level of genetic differentiation that has occurred among the species of Hawaiian Bidens is comparable to the level of genetic differences found among populations within single continental plant species. Moreover, there is no correlation between the isozyme data and morphological data. No groups of taxa are evident in the genetic data, although morphological groups exist. Genetic differentiation at isozyme loci has not occurred at the same rate as the acquisition of presumably adaptive morphological and ecological characters in Hawaiian Bidens. Adaptive radiation may be limited to a few genes controlling morphological and ecological characters.

Genetic evidence for recent and incipient speciation in the evolution of Hawaiian Metrosideros (Myrtaceae)

“…Lowrey & Crawford (1985) found very little isozymic divergence among seven species of Tetramolopium, with a mean genetic identity of 0.95. Similar results were obtained for six species from two of the three recognized subgenera of the genus Dendroseris which is endemic to the Juan Fernandez Islands, Chile (Crawford et a!., 1987a (Helenurm & Ganders, 1985). It has been suggested that congeneric species of insular plants may differ by more conspicuous morphological features than their counterparts on continents in spite of very little genetic divergence (Crawford et a!., 1 987b).…”

“…Narrow genetic divergence at isozyme loci among different insular taxa was also reported for Terramolopium (Lowrey, 1981), Bidens (Ganders & Nagata, 1984;Helenurm & Ganders, 1985) and the silversword alliance of species belonging to the genera Argyroxiphium, Dubautia, and Wilkesia (Carr, 1985a,b;Witter & Carr, 1988) from the Hawaiian islands. Lowrey & Crawford (1985) found very little isozymic divergence among seven species of Tetramolopium, with a mean genetic identity of 0.95.…”

New World tetraploid cottons contain Old World cytoplasm

“…If this view is correct, as the cpDNA data suggest, then evolutionary diversification and spread of the tetraploid taxa after polyploidization has been relatively rapid. This interpretation would not be unprecedented, as others have noted an "uncoupling" of molecular and morphological evolution with respect to both cpDNA (28,39) and isozyme (45)(46)(47) divergence.…”

DNA diversity in Hawaiian endemic plant Schiedea globosa

“…This is probably because it is generally assumed that intraspecific diversity in the populations of island endemic plants is very low (Barrett and Husband, 1990), which seems quite logical, given that the lack of resolution in DNA-based phylogenies is a common problem in phylogenetic studies of island plant radiations. Hence, the intraspecific genetic diversity data are mostly based on allozyme diversity (Helenurm and Ganders, 1985;Weller et al, 1996;Marr and Bohm, 1999), which may be quite misleading due to possible non-neutrality of charge changing amino-acid replacements creating allozyme diversity (Hudson et al, 1994;Veuille and King, 1995;Eanes et al, 1996;Filatov and Charlesworth, 1999). Here, we reported one of the first DNA sequence-based estimates of genetic diversity in Hawaiian plants.…”