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Familiar fruit flies emerge in new guise.

Familiar fruit flies emerge in new guise

Scientists studying the genetics of a pesky little fruit fly appear to have settled a century-old dispute about how new species can arise.

Ever since Darwin, conventional wisdom has held that geographic isolation is a key ingredient to speciation: Only when two populations of a given species are separated from each other -- by large expanses of water or by mountain ranges, for example -- will interbreeding be sufficiently interrupted to allow the gradual developement of separate and distinct traits. Over time, these differences may become significant enough to make the two populations incapable of producing fertile offspring when they interbreed. As such, they are deemed separate species.

Since the 1860's, however, a determined handful of scientists have argued for the feasibility of "sympatric speciation" -- speciation in the absence of geogrpahic isolation. Rhagoletis pomonella -- the No. 1 insect pest in U.S. apple orchards -- provided scientists the perfect opportunity to test that hypothesis. Once found only in hawthorn trees, many of the fruit-burrowing flies have taken a liking to apple trees since apple seeds were introduced into eastern North America. In recent decades, subpopulations of Rhagoletis have developed a clear preference for one kind of tree or the other.

To see whether tree preferences might be leading to truly differenct species, scientist analyzed the enzymes of flies found on the two types of trees in sites containing both. Although no physical barriers separated the two populations, the researchers found significant and consistent biochemical differences between the two fly populations over several years, indicating that members of the two populations were mating somewhat separately. Laboratory studies show the two races can still interbreed when forced to share a room. But the stable biochemical differences between the two groups in nature strongly suggest they are becoming distinct species.

The scientists conclude that such factors as host-plant preferences and mating-time asynchrony [fruit fly reproductive cycles are coordinated with fruit ripening times, which occur several weeks apart in apples and hawthorns] are sufficient "gene flow barriers" to allow the development of distinct races -- and perhaps species -- at least in insects.

The research, reported in the Nov. 3 NATURE, was led by Jeffrey L. Feder at Michigan State University in East Lansing; Bruce A. McPheron at the University of Illinois at Urbana-Champaign; and D. Courtney Smith at the Univeristy of Utah in Salt Lake City.
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Author:Weiss, Rick
Publication:Science News
Date:Nov 5, 1988
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