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does evolution occur through very small genetic changes involving a very large number of genes, or does it occur through changes associated with a smaller number of genes? This question has nagged researchers for decades now. They have been studying the possible mechanisms underlying evolution but the challenge has been to find means of applying what is tested and proved in the laboratories to the biosystem of nature. Now, researchers at the Stanford University, usa, have come up with a solution. David Kingsley, professor of developmental biology at the university, and his colleagues have developed a gene-linkage map of the chromosomes of a small spiny fish called the three-spine stickleback. According to them, similar maps may be the tools that evolutionary biologists have been wanting for years (www.newswise.com, December 21, 2001).

The researchers have developed a map of the inheritance patterns showing the links between genes from different generations. For such a mapping, what was important was to find two populations, that unlike laboratory bred mice and rats, would have traits that had evolved naturally and yet could still be crossbred. It was also important to find two species, Kingsley said, that could produce viable offsprings in the laboratory even if they would not naturally mate in the wild. The sticklebacks met all these criteria.

During their experiments, the researchers first designed a marking system that would allow them to follow the inheritance patterns of the genes. Using the markers, the team crossbred two populations

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