A revolution is brewing in contemporary systematics, especially at or near the level of species differences (1). Data on genetic variability within and between closely related populations can be used to establish highly sensitive indices of genetic similarity (2). A review of the biochemical variation within and between the members of the Drosophila willistoni group of species has recently appeared. A direct relationship between indices of genetic similarity and certain systematic designations was proposed (3). If identity is taken as 1.0, local geographic populations of a species were shown to have mean genetic similarities of 0.97, subspecies and semispecies about 0.8, sibling species 0.52, and nonsibling species 0.35. The present paper reports genetic similarities within and between a pair of partially sympatric, very closely related species endemic to the Island of Hawaii. We introduce the use of indices based on chromosomal variability and compare them with indices based on allozymes. Biochemical similarity both within and between the species is high, although they are strongly differentiated morphologically and chromosomally. These species evidently are very new in the historical biological sense. We propose that their biochemical similarities are directly correlated with this circumstance.
Genetic studies of two closely related endemic Hawaiian species show that in one area of sympatry about 2 percent of the naturally occurring individuals are hybrids. More than 20 times this many would be expected if the population consisted of a single panmictic unit. Despite hybridization, natural selection appears to maintain the essential integrity of each separate gene pool.
The species are endemic to the newest island in the archipelago and are broadly sympatric. They are easily distinguished morphologically in both sexes. Using standard electrophoretic procedures, we have examined 25 loci encoding for structural proteins from 539 silvestris and 325 heteroneura collected at three widely-separated localities where the two species are sympatric. Pairwise comparisons within and between the species show the following coefficients of genetic identity (Nei's I): within silvestris, 0.961 ± 0.01; within heteroneura, 0.949 ± 0.02; between silvestris and heteroneura, 0.939 ± 0.01. Neither the differences within nor between the species are significant. There are no fixed allelic differences either within or between the species. At the three areas of sympatry, the species show gene frequency differences (P < 0.05) at 9, 11 and 13 loci respectively. This is not much different from the variation within either one of the species across the three localities. The two species have similar heterozygosity (H) levels (silvestris, 0.083; heteroneura, 0.089) and percent of polymorphic loci (both 0.37). It is suggested that despite their morphological divergence, these species are much more newly formed than classical sibling species. Significant allozymic differences may not have had time to accumulate.
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