After just nine generations, certain male fruit flies alter their biochemical calling cards to correctly target females within their own species. Similarly, river fish placed in a Washington State lake differentiated according to specific habitats within a mere 13 generations, researchers report 20 October in the international journal, Science.
These findings-described in two separate studies by Australian and U.S. research teams-show that natural selection can help new species emerge much faster than scientists ever imagined.
Instead of geographic features like mountains and rivers, physiological reasons provide the barriers that prevent certain populations of flies and fish from reproducing with other members of their species. The rapid evolution of this situation, called "reproductive isolation," among fish and fruit flies sets the stage for realistic, experimental tests to answer long-standing questions about the role of natural selection in species evolution.
Until now, studies have provided no firm evidence for natural selection's direct role in the evolution of mate recognition systems, the specific mechanisms that males and females of a species use to identify each other. But, research on flies led by Megan Higgie of University of Queensland, Australia, counters this and shows that after nine generations, Drosophila serrata males had increased their mate selectivity by altering their pheromones to improve their chances of actually inseminating a female of their own species. These perfume-like pheromones resembled those of males that already co-existed with a sister species in the wild.
The sole dependence of these flies on pheromones for mate recognition makes them an ideal model organism to study how the frequency of certain characters evolve when faced with a natural selection pressure.
In a second Science study, sockeye salmon adapted to river and lake-beach habitats and evolved partial reproductive isolation over 13 generations. Thus, the fish may provide some insight as to how quickly reproductive isolation may evolve in the wild, according to Andrew P. Hendry of University of Massachusetts and colleagues.
Since their introduction to Lake Washington in 1937, the salmon differentiated genetically and morphologically according to the nature of their breeding environments, Hendry reports. Fish associated with the river or lake breeding areas bred successfully when in their respective habitats. Although fish swam between the sites, the rate of gene flow did not correspond to immigration. Morphologically, fish that breed in the river consist of larger females that dig deeper nests, and river males have shallower bodies that allow them to cut through stronger currents than in the lake beach.
Recently introduced populations of organisms, in different environments, offer an opportunity to study the processes of rapid speciation in nature, because much of the observable reproductive isolation can arise soon after a population's initial divergence, or deviation from the ancestral form, notes Hendry.
Considering the short evolution time for reproductive isolation, many questions still remain, such as, why don't more species exist? The studies of Higgie and Hendry provide a basis for conducting more detailed ecological and genetic experimentation that examines how speciation may occur naturally across taxonomic groups.
With Higgie, collaborators on the Science study were Steve Chenoweth of Griffith University and Mark W. Blows of University of Queensland, both in Australia. Research was supported by a grant from the Australian Research Council.
With Hendry, collaborators on the Science study were John K. Wenburg and Paul Bentzen and Thomas P. Quinn of University of Washington and Eric C. Volk of Washington Department of Fish and Wildlife. Research was supported by a Natural Sciences and Engineering Research Council of Canada postgraduate scholarship and the Keeler Endowment.
To receive a copy of these Science papers, call 202-326-6440, or send e-mail to scipak@aaas.org.
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