Mammals' genomes getting smaller.

• Position: Animal Diversity - Brief article

Evidence buried in the chromosumes of animals and plants suggests strongly that only one group--mammals--had its genomes shrink after the dinosaurs' extinction. What's more, that trend continues today, maintain scientists from Indiana University, Bloomington. This finding might seem counterintuitive, given that mammals have expanded in diversity and number, as well as in dominance in a wide variety of ecological roles throughout the last 65,000,000 years. Yet, it is precisely their success in numbers that could have led to this contraction of their genomes.

"Larger population sizes make natural selection more efficient," explains evolutionary biologist Michael Lynch. "If we are correct, we have shown how to bring ancient genomic information together with the paleontological record to learn more about the past." Moreover, Lynch indicates that human genomes still are undergoing a contraction--though noticeable changes in our chromosomes should not appear for a few million years yet.

Lynch's group examined the genomes of seven mammals, eight nonmammalian animals, and three...