A fundamental tenet of Darwin's theory of evolution was that new species arise from older ones. Biologists typically define species on the basis of the ability of individuals to mate and produce fertile offspring; if two organisms can accomplish this feat, they belong to the same species, and if they cannot, they belong to separate species. Theoretically, a species can split into two when genetic barriers arise that keep two geographically mixed groups (populations) of the species from mating with each other. But what sorts of genetic barriers might arise in a living population that could lead to this kind of reproductive barrier?
Scientists Jun-Yi Leu and Andrew Murray at Harvard University have recapitulated what may be early stages of speciation by evolving populations of brewer's yeast in the laboratory. The researchers took a single strain of yeast and genetically engineered it to create an evolving population, which was subject to special selective forces, and a reference population, which was not.
At the beginning of the experiment, a yeast cell in the evolving population was equally likely to mate with a cell in either the reference population or the evolving population. To change the cells' preferred partners, the researchers mixed the two populations, allowed them to mate, and then used engineered genes to create a selective force such that offspring from a mating of two cells from the evolving population were selectively allowed to survive while offspring from a cross between the evolving population and the reference population, or a cross between two yeasts from the reference population, died. By repeating this selection process for 36 generations, the researchers produced evolved populations that were five times more likely to mate to other evolved cells than they were to the reference population.
Analysis of the evolved populations suggested that this change was the result of several different mutations, and that one important consequence of these mutations was to change how quickly cells mated. Subtle, genetically based differences in the timing or mechanism of mating could well represent a barrier to mating between otherwise closely related populations, thereby contributing to genetic isolation that leads to speciation. This work provides an intriguing insight into one of the fundamental features of evolution and paves the way for a detailed understanding of the genetic changes that turn one species into two.
The researchers include Jun-Yi Leu and Andrew W. Murray of Harvard University in Cambridge, MA. This work was supported by grants from the National Institutes of Health (A.W.M.) and a fellowship from the Damon Runyon Cancer Research Foundation (J.Y.L.).
Leu et al.: "Experimental Evolution of Mating Discrimination in Budding Yeast." Publishing in Current Biology 16, 280-286, February 7, 2006, DOI 10.1016/j.cub.2005.12.028 www.current-biology.com