To: <[EMAIL PROTECTED]>, <[EMAIL PROTECTED]> From: <[EMAIL PROTECTED]> Mailing-List: list [EMAIL PROTECTED]; contact [EMAIL PROTECTED] Delivered-To: mailing list [EMAIL PROTECTED] Date: Sat, 25 Nov 2000 21:19:06 +0100 (CET) Reply-To: [EMAIL PROTECTED] Subject: [>Htech] BIO: The Rapid Origin of Reproductive Isolation http://www.sciencemag.org/cgi/content/full/290/5491/462 ECOLOGY: The Rapid Origin of Reproductive Isolation Nick Barton* The part that natural selection plays in the origin of species has long been debated. It is easy to see that if two populations are kept separate--by mountains or ocean, for example--they will eventually become so different that they can no longer interbreed successfully. Their differences may have evolved by natural selection, but their reproductive isolation is merely a side effect of changes that emerged for other reasons. This view seems unsatisfactory to those who emphasize the positive aspect of selection in evolution. Both Alfred Russell Wallace (1) and Theodosius Dobzhansky (2) argued that natural selection would reinforce reproductive barriers between diverging populations. There has been little evidence, however, that selection has in fact contributed directly to the formation of new species (speciation) in this way. Reports by Higgie et al. (3) and Hendry et al. (4), on pages 519 and 516 of this issue, provide examples from fruit fly and sockeye salmon populations showing that selection can produce the kind of isolation that separates species in the wild (3), and moreover, that it can do so within a very short time (a dozen or so generations) (3, 4). The best evidence that selection has reinforced mating barriers as an adaptation to reduce interbreeding has been indirect: Where two species encounter each other in nature, their preference for their own kind is typically stronger than for species whose ranges do not overlap (5). In their report, Higgie et al. (3) give the first direct evidence that such a pattern can be generated by selection, and that it can be generated very quickly. They worked with Drosophila serrata and Drosophila birchii, fruit flies that are almost indistinguishable in morphology and produce viable and fertile hybrid offspring in the laboratory. These sister species are found together in northeastern Australia, yet they rarely interbreed. Where their ranges do overlap, the two species differ in the mix of hydrocarbons on their cuticle (see the figure, below). The strong correlation between mate choice and hydrocarbon profiles in hybrid offspring, and in flies perfumed with hydrocarbons from the other species, shows that mate choice is largely due to the scent of these chemicals (6). Most important, in southeastern Australia, beyond the range of D. birchii where only D. serrata is found, the hydrocarbons of D. serrata change abruptly, and there is a corresponding weakening of its mating preference (3). In flagrante delicto. Gas chromatographic profile of hydrocarbons in the cuticle of the fruit fly Drosophila serrata. Individual hydrocarbons that are important for mate recognition are labeled 1 to 10. (Inset) Photograph of a male and female fruit fly (D. serrata) mating. CREDIT: HYDROCARBON PROFILE COURTESY OF M. HIGGIE; PHOTOGRAPH COURTESY OF A. O'TOOLE/UNIVERSITY OF QUEENSLAND Higgie et al. (3) set up experimental populations containing D. birchii together with D. serrata from either the north or the south of its range. After nine generations, Higgie et al. compared the cuticular hydrocarbons of D. serrata with those of control populations in which only one species was present. Little change was seen in D. serrata taken from the north, within the range of D. birchii; in contrast, D. serrata taken from further south, where D. birchii is absent in nature, tended to evolve hydrocarbons more similar to those of the northern D. serrata. (Females from three replicate populations evolved in this direction, as did males from two replicates. However, males from the remaining replicate evolved in the opposite direction.) The investigators did not test the consequences for mate preferences, but the strong correlation between hydrocarbons and mate choice in previous experiments suggests that selection has acted so as to reduce cross-mating between the species. The interpretation is simple: D. serrata in the north had long been exposed to the presence of its sister species, and so did not evolve in response to the presence of D. birchii in the laboratory. In contrast, D. serrata from the south evolved in the laboratory in the same way as northern populations presumably had in the past. Selection for a shift in mate choice is strong: When D. birchii is present, the proportion of D. serrata males from the south that successfully inseminate females of their own species is reduced by nearly 50%, whereas there is no significant interference with insemination by males from the north. Thus, the speed of the response to selection is not surprising. This work is important mainly because it opens up the possibility of a detailed ecological and genetic analysis of how mating systems evolve in nature. Species can evolve differences in mate choice by direct selection on that choice (as in the Higgie experiment) or as a side effect of other changes. As with so much else in the study of speciation, these possibilities are difficult to distinguish because we usually see only an accumulation of changes, most of which may have evolved long after speciation itself. In their study, Hendry et al. (4) describe a remarkable example of reproductive isolation that has evolved in 13 generations or less. Like other freshwater fish, salmon often evolve distinct "ecotypes" that are adapted to spawn in different habitats. Such ecotypes have repeatedly evolved in postglacial lakes within the last ~10,000 years. Hendry et al. describe an example from Lake Washington in the northwestern United States, where, in 1937, a large population of sockeye salmon was established in a river feeding into the lake. Salmon were found spawning at a lake beach in 1957, and they now have a morphology distinct from the fish that breed in the river: River females are larger, allowing them to dig deeper nests, and river males have shallower bodies, allowing them to swim more efficiently. Hendry et al. show that nearly 40% of fish breeding at the beach were born in the river, and yet they differ genetically, at six microsatellite loci, from the native beach fish. These differences are small (of the same order as those typically found between neighboring populations within a species) but could hardly be maintained if the beach and river fish interbreed at random, and their offspring are fully viable and fertile. Thus, within half a century, both adaptive differences in morphology and some degree of reproductive isolation have evolved. It is well established that natural populations can respond rapidly to selection (7). It is also well known that in the laboratory, selection for reproductive isolation can produce a rapid response, provided that it is not opposed by genetic exchange between the diverging populations (8). However, attempts to create species in the laboratory, by selection on a single interbreeding population, have usually failed (8). Neither of the examples presented by the Higgie and Hendry groups provide evidence that reproductive isolation evolved by the splitting of a single population; indeed, such evidence is almost impossible to obtain. But, because reproductive isolation can evolve so quickly, to levels that allow further divergence even after contact, this difficulty is perhaps not important. Populations must often become temporarily isolated for a few tens of generations, and this may suffice to allow divergence within an essentially continuous geographic range. The two reports provide strong evidence for the rapid evolution of reproductive isolation. This raises the question: Why do we not see more species? It may well be that new species (that is, reproductively isolated populations) do form often, but that only rarely do they evolve sufficiently to be recognized as separate species by biologists or such that they find a distinct ecological niche. For ecologists, the question is then whether the number of established species that we see is determined by a balance between the rate of speciation and the rate of extinction (9), or instead is set by the range of distinct niches that are available. The realization that evolution occurs on time scales accessible to experiment and observation may help bring together evolutionary and ecological approaches to address such questions. References 1.A. R. Wallace, Darwinism (Macmillan, London, 1889). 2.T. Dobzhansky, Am. Nat. 74, 312 (1940). 3.M. Higgie, S. F. Chenoweth, M. W. Blows, Science 290, 519 (2000). 4.A. P. Hendry, J. K. Wenburg, P. Bentzen, E. C. Volk, T. P. Quinn, Science 290, 516 (2000). 5.J. A. Coyne, H. A. Orr, Evolution 51, 295 (1997); M. A. Noor, Am. Nat. 149, 1156 (1997). 6.M. W. Blows, R. A. Allan, Am. Nat. 152, 826 (1998). 7.A. P. Hendry, M. T. Kinnison, Evolution 53, 1637 (1999) [abstract]. 8.W. R. Rice, E. E. Hostert, Evolution 47, 1637 (1993). 9.S. Hubbell, The Unified Neutral Theory of Biodiversity and Biogeography (Princeton Univ. Press, Princeton, NJ, in press). -----BEGIN TRANSHUMANTECH SIGNATURE----- Post message: [EMAIL PROTECTED] List owner: [EMAIL PROTECTED] List home: http://www.egroups.com/community/transhumantech/ Alt archive: http://excelsior.planetx.com/transhumantech/ Old archive: http://excelsior.planetx.com/transhumantech/threads.html -----END TRANSHUMANTECH SIGNATURE----- == You are subscribed to the Europa Icepick mailing list: [EMAIL PROTECTED] Project information and list (un)subscribe info: http://klx.com/europa/
