Mammals and many species of birds and fish are among evolution’s “winners,” while crocodiles, alligators and a reptile cousin of snakes known as the tuatara are among the losers, according to new research by UCLA scientists and colleagues.
“Our results indicate that mammals are special,” said Michael Alfaro, a UCLA assistant professor of ecology and evolutionary biology and lead author of the research.
The study, published July 24 in the early online edition of Proceedings of the National Academy of Sciences, also shows that new species emerge nearly as often as they die off.
Alfaro and his colleagues analyzed DNA sequences and fossils from 47 major vertebrate groups and used a computational approach to calculate whether the “species richness” of each group was exceptionally high or low. The research allows scientists to calculate for the first time which animal lineages have exceptional rates of success.
Among the evolutionary winners are most modern birds, including the songbirds, parrots, doves, eagles, hummingbirds and pigeons; a group that includes most mammals; and a group of fish that includes most of the fish that live on coral reefs, said Alfaro, an evolutionary biologist.
A group with the scientific name Boreoeutheria, which consists of many mammals, has diversified about seven times faster than scientists would have expected, beginning about 110 million years ago, Alfaro and his colleagues calculated. The group includes primates and carnivores, as well as bats and rodents. Pouched mammals, such as kangaroos, are not as richly varied as other mammals, Alfaro said.
Modern birds have diversified about nine times faster than expected, starting about 103 million years ago, and the group of fish that live on coral reefs has diversified about eight times faster than expected, he said.
Who are the evolutionary losers?
Crocodiles and alligators are nearly 250 million years old yet have diversified into only 23 species, Alfaro said. They are diversifying a staggering 1,000 times slower than would have been expected. “Their species richness is so low, given how old they are,” he said.