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 its losers, according to new research published July 24 in the online early edition of Proceedings of the National Academy of Sciences (PNAS).
"Our results indicate that mammals are special," said Michael Alfaro, UCLA assistant professor of ecology and evolutionary biology, and lead author of the research.
The study 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 of these 47 groups is exceptionally high or low. The research allows scientists to calculate for the first time which animal lineages have exceptional rates of success.
Evolutionary winners include most modern birds, including the songbirds, parrots, doves, eagles, hummingbirds and pigeons; a group that includes most mammals; and a group of fish including most of the fish that lives on coral reefs, said Alfaro, an evolutionary biologist.
A group of many mammals, with the scientific name of Boreoeutheria, 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, he said.
Modern birds have diversified about nine times faster than expected, starting about 103 million years ago, Alfaro said.
The group of fish that lives 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, Alfaro said, yet have diversified into only 23 species. They are diversifying a staggering 1,000 times slower than Alfaro would have expected.
"Their species richness is so low, given how old they are," he said.
The tuatara, which lives in New Zealand and resembles lizards (though, actually distant cousins), has only two species.
"In the same period of time that produced more than 8,000 species of snakes and lizards, there were only two species of tuatara," Alfaro said.
Why are there not thousands of species of tuataras?
"That is one of the big mysteries about biodiversity," Alfaro said. "Why these evolutionary losers are still around is a very hard thing to explain. They have been drawing inside straights for hundreds of millions of years. It's a real mystery to biologists how there can be any tuataras given their low rate of speciation. They must have something working for them that has allowed them to persist. In species richness, these are losers, but in another sense, this highlights how unique they are. There are incredibly disparate patterns of species richness."
Tuataras were a bit more diverse in their heyday; there may have been a few dozen species of them, most of which have gone extinct, Alfaro said.
In contrast, there are more than 9,000 species of birds, more than 5,400 species of mammals, approximately 5,500 species of frogs, some 3,000 species of snakes, and 5,200 species of lizards Alfaro said.
The number of frog species, although it sounds high, is about what Alfaro would expect, given how old they are — approximately 250 million years old.
"Our analysis suggests we should not be surprised to see a group with that many species in that amount of time," Alfaro said.
There are almost 60,000 species of jawed vertebrates. Alfaro and his colleagues report evidence for exceptional diversification rates in nine taxonomic groups of jawed vertebrates. Interestingly, their findings do not coincide with traditional scientific explanations for why there are so many mammals, birds and fish.
"The timing of the rate increases does not correspond to the appearance of key characteristics that have been invoked to explain the evolutionary success of these groups, such as hair on mammals or mammals’ well-coordinated chewing ability or feathers on birds," Alfaro said. "Our results suggest that something more recent is the cause of the biodiversity. It may be that something more subtle explains the evolutionary success of mammals, birds and fish. We need to look for new explanations."