( By tracing the 30-million year history ...)UCSD biologists find new evidence for one-way evolution

Their study, published this week in an early online edition of the journal Proceedings of the National Academy of Sciences, offers new support for the idea that the loss of complex traits, like eyes, wings or in this case a reproductive mechanism, is often irreversible. In other words, once lost, the traits never revert to their original state.

"This is the strongest evidence yet to support irreversibility," said Joshua Kohn, an associate professor of biology at UCSD who headed the study. "If we had not used the genetic data coding for this reproductive mechanism and only inferred the pattern of evolution based on the traits of living species, we would have come to the opposite conclusion and with high statistical support -- that the trait evolved more than once."

The scientists examined existing variation in the gene used by many members of the Solanaceae family, which include tomatoes and tobacco, to recognize and reject their own pollen, thereby avoiding self-fertilization and the harmful effects of inbreeding. This ability is sometimes lost, as is the case for garden tomatoes, which can set seed by self-fertilization. Apparently, once lost, the ability to reject pollen in order to prevent self-fertilization is never regained.

Irreversible loss of complex traits, which result from the combined interaction of several genes, is an old and at times controversial scientific question. While the late evolutionary biologist Stephen Jay Gould popularized the hypothesis of irreversibility, known as Dollo's Law, studies that use current methods to reconstruct the evolution of complex traits often fail to support it. This is because it is often difficult to reconstruct characteristic
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Source:University of California - San Diego