Chromosomal Tracers of Founder Events (1970)

Recently published chromosomal data on certain of the large Drosophila species of Hawaii are briefly presented and discussed. In particular, chromosomal similarities between certain species from the islands of Maui and Hawaii are emphasized. As Hawaii is geologically more recent than Maui and is separated from it by an abysmal channel, it appears that Hawaii must have been populated from Maui, rather than vice versa. The chromosomal data confirm this view; it is possible to write the inversion formula for the polytene chromosomes of five interisland species founders going from Maui to Hawaii. Despite the recency of these evolutionary events, each founder case has resulted in from one to three new species on Hawaii. No instances of subspecies colonizers have been recognized between these islands. Little adaptive shift appears to occur, as indicated by comparisons of Maui and Hawaii species. It is suggested that the founder effect and its accompanying random genetic drift are particularly conducive to species formation. Adaptive evolution, in these founder cases, may be expected to proceed principally after the new species have been formed. WHEN AN ANIMAL or plant species first reaches an isolated island it frequently gives evidence of being descended from a very small number of founder individuals. In extreme cases, only a single founding propagule may be involved. When a propagule carries along with it a competent genetic system, it is clear that many species may be formed in the populations descended from such a founder. The present paper examines some of the recently published data on the chromosomal characteristics of selected species of Drosophila endemic to Hawaii (Carson and Stalker 1968, 1969; Carson et al. 1970). Particular attention is focused on the comparison of the polytene sequences of certain species from the Island of Maui and the geologically more recent nearby Island of Hawaii. These comparisons shed some light on the genetic nature of founder individuals. The data will be discussed with particular regard to the problems of how adaptations arise in insular forms derived from what was apparently a single founder. Since 1963, a cooperative research effort has been devoted to the evolutionary biology of the Drosophilidae of Hawaii (for review and references, see Carson et al. 1970). Among the more than 450 described species of the family are about 100 very large and biologically distinct species. Because most of them have a series of dark maculations on the wings, they are loosely referred to as the picturewinged flies. They belong to the genus Drosophila (subgenus Drosophila) and are generally characterized by large body sizes, very large, favorable salivary gland chromosomes, and unusual sexual dimorphism and courtship patterns. Unlike Drosophila species from the rest of the world, the males display a territorial or lek behavior. Most are confined to wet forests above 200 meters elevation. They breed primarily on the decaying small branches of various genera of Hawaiian woody plants, including Clermontia, Cheirodendron, Pisonia, and Freycinetia. Giant chromosome sequences of 69 species of this group of flies have been mapped. In each case, the banding sequence of each of the five major polytene chromosomes has been determined. The sequences found in Drosophila grimshawi Oldenberg have been used as a Standard and each species can be described in terms of the degree of difference from this Standard which it displays. The differences are almost entirely due to paracentric inversions. The species usually differ from each other and from D. grimshawi by one or more fixed inversions. Some related species, although morphologically distinct, are nevertheless homosequential, that is, the gene order is identical in all chromosomes. This condition is fairly frequent in Hawaiian Drosophila but is rare in mainland forms, where even sibling species are usually different in basic gene order (see Carson et al. 1967). All 69 species have a metaphase chromosome group of 2n 12 (Carson et al. 1970). BIOTROPICA 2(1): 3-6. 1970 3 1 This work has been supported by the Evolution and Genetics of Hawaiian Drosophilidae Project, Grant No. GM10640 to the University of Hawaii from the National Institutes of Health, and by GB3147 to Washington University (St. Louis) and by GB7 11 to the University of Texas from the National Science Foundation. 2 Paper presented as a contribution to the Symposium on Adaptive Aspects of Insular Evolution sponsored by The Association for Tropical Biology and held at the Mayagiiez Campus of the University of Puerto Rico, June 15-19, 1969. This content downloaded from 207.46.13.84 on Tue, 05 Jul 2016 04:48:06 UTC All use subject to http://about.jstor.org/terms Not only are all of the species which have been characterized chromosomally endemic to the six main islands of the Hawaiian archipelago, but single-island endemism is very high. As has been fully documented (e.g., Stearns 1966), Maui, Molokai, and Lanai were joined during the Pleistocene and have only recently been separated by the existing shallow channels. These three islands (the Maui complex) may be considered as a single biological island for the purposes of the present discussion. If this is done, only two of the 69 species for which full data are available are found as such on more than one island. These are: D. crucigera Grimshaw (Kauai and Oahu) and D. grimshawi (from all but Hawaii). The situation with regard to the islands of Maui and Hawaii is especially significant for the present discussion. The two islands are separated by a channel (Alenuihaha Channel) which is 48 kilometers (30 miles) wide and more than 2000 meters in depth. Of the five volcanoes on the Island of Hawaii, McDougall (1964) has shown by K-Ar dating that the oldest rocks (in the Kohala Range) are no greater than 800,000 years old. Mauna Loa, Mauna Kea, Hualalei, and Kilauea show rocks which are still younger. On Maui, comparable studies indicate ages ranging from 1.0 million to 1.3 million years. Accordingly, it appears that Hawaii is the younger of the two islands. Of the 69 species of picture-winged flies studied, 17 are endemic to Hawaii; this island, as far as is known, does not share any of these species with adjacent Maui. The question arises: what is the immediate origin of these Hawaii species? The picture-winged species can be arranged according to their inversion similarities. Thus, those that have inversions in common may be grouped together; in fact, all species may be united in a single phylogenetic diagram based on inversion-sharing (see Carson et al. 1970, for a summation and updating of the data given in Carson and Stalker 1968,1969). Without outside information, an inversion phylogeny cannot predict a starting point in evolutionary time. Thus, one may arbitrarily designate any one species as ancestral and proceed to derive all other species in the diagram. With regard to the Hawaiian situation, however, several useful kinds of outside information are available. In the first place, direction can be given to the phylogeny because of the findings of Stalker (1968). He discovered that the polytene sequence of chromosome 5 of the Kauai species D. primaeva Hardy & Kaneshiro is closer to that of certain mainland forms than is that of D. grimsAawi and 66 others of the picture-winged species. According to this information, then, all 17 of the picture-winged species of the Island of Hawaii are terminal in the phylogeny, that is, the D. grimshawi Standard, which especially flowered on Maui, stands between them and D. primaeva of Kauai. Secondly, we may cite the fact, established earlier, that although both islands are probably referable to the Pleistocene, the Island of Hawaii is definitely younger than Maui. This fact may thus also be used independently to specify the direction of the evolutionary movement of the fauna. The chromosomal formula for D. grimshawi may be written: X 2 3 4 5. This species, as well as a number of other species closely related to it, seems to be centered on the Maui complex, from which 30 species of picture-winged flies have been analyzed. The Standard occupies a central position in the phylogeny (see Carson et al. 1970). Table 1 gives the chromosomal formula of five Hawaii species (in groups of 2 and 3, respectively) and compares each of the groups with certain Maui species. Thus, D. silvarentis Hardy & Kaneshiro and D. hawaiiensis Grimshaw are homosequential; they differ from Standard by four particular inversions. A Maui species, D. recticilia Hardy & Kaneshiro, is quite different from them morphologically but is nevertheless homosequential with them, having the same four fixed inversions. The inference may be TABLE 1. Comparison of chromosome sequences of certain Drosophila species from the islands of Maui and Hawaii (after Carson et al. 1970). Island Species Chromosomal formula Maui D. recticilia Xa2 2b 3g 4u 5 Hawaii D. silvarentis Xa2 2b 3g 4u 5 D. hawaiiensis Xa2 2b 3g 4u 5 Maui D. orphnopeza X 2 3/3o 4 5 D. balioptera Xg 2 3 4 5 Hawaii D. engyochracea Xg 2 3 41 5 D. ciliaticrus Xg 2 3o 4 5 D. murphyi Xg 2 3/3o 4 5