The loser, the winner in the race evolved

Mammals and many birds and fish are the winners in the evolutionary race, while African crocodiles, American crocodiles, and a relative of the tuatara snake are the losers. According to a new study conducted by UCLA scientists and colleagues.

The study, published on July 24, in the online version of the Proceedings of the National Academy of Sciences, shows that new species appear at the same speed as they lost.

Alfaro and his colleagues analyzed DNA sequences from fossils from 47 basic vertebrate groups, using a computerized method to calculate whether the density of species in each group is dense. high or low. This study allows scientists for the first time to calculate which animal family has a great success rate.

According to Alfaro - an evolutionary biologist - among those who were earlier in the evolutionary race were mostly modern birds (including songbirds, parrots, doves, eagles, hummingbirds) , a group consisting of most mammals and a group of fish species (including mostly coral reef fish).

A group of animals whose scientific name is Boreoeutheria includes many mammals that are 7 times faster at diversifying than scientists think, the branch division process starts from about 110 million years ago, according to Alfaro's calculations with his colleagues. The group also includes primates and predators, including bats and rodents. Pocket animals, such as kangaru, are not as diverse as mammals.

Modern birds have a faster rate than experts' calculations nine times, starting about 103 million years ago . The group of coral reef fishes also have variable rates up to 8 times faster.

So who are the losers in the evolutionary process?

African crocodiles and American alligators with a lifespan of nearly 250 million only have a level of diversity that stops at 23 species, according to Alfaro. Their speed is surprisingly slow - 1000 times - compared to scientists' predictions. Alfaro said: 'Their species diversity is very slow even though they have been around for a long time.'


Tuatara living in New Zealand has a similar shape to the lizard - although it is only a distant relative - only two species. According to Alfaro, 'in the same spawning period of more than 8000 snakes and lizards, only 2 species of tuatara appear' . The question is: why not thousands of species of tuatara appear?

Crocodile. (Photo: Michael Alfaro / UCLA)

Alfaro said: 'It is one of the great mysteries of biodiversity. Why these losers still exist, this is a difficult thing to explain. Biologists cannot explain why the tuatara species still exists even though they have a very slow rate of species division. There must be something that allows them to exist. In terms of species diversity, they are losers. But on the other hand, this emphasizes their uniqueness. It is true that there exists a different species diversity model. '

Tuatara has somewhat changed compared to its heyday: there have been several dozen tuatara species in existence, but most are extinct.

In contrast, there are over 9,000 species of birds, over 5,400 species of mammals, approximately 5,500 species of amphibians, about 3,000 species of snakes and 5,200 species of lizards.

The number of amphibians, although it sounds like a lot, but according to Alfaro's calculations, has shown us the time when they appeared on the earth - nearly 250 million years ago. 'Our analysis suggests that we should not be surprised to see a group of animals that have so many species in such a time.'

Approximately 60,000 vertebrate species have jaws. Alfaro and his colleagues have produced evidence for this remarkable species diversity rate through nine vertebrate animal groups and jaws. Interestingly, their findings do not overlap with the scientific explanations that explain why there are many mammals, birds and fish.

Alfaro said: 'The time of increasing growth rate is not consistent with the appearance of important characteristics. It is these characteristics that have been suggested to explain the evolutionary success of the above mentioned animal groups, such as the hair on the mammalian body, or the ability of mammals to chew, or feathers on birds' bodies.

'Our research results show that there is a more new cause explaining that biodiversity. It may be an undetectable cause to explain the evolutionary success of mammals, fish and birds. We need to seek new explanations'.

The PNAS co-author is Luke Harmon (professor of biological sciences at the University of Idaho), Francesco Santini (postdoctoral scholar at UCLA works for Alfaro's lab), Chad Brock (graduate student studying at the University of Washington), Hugo Alamillo (bioengineering student at Washington University), Alex Dornburg (working for Alfaro Laboratories, currently a graduate student at Yale University), Daniel Rabosky (graduate student Department of Biology at Cornell University), and Giorgio Carnevale (postdoctoral scholar at the University of Pisa, Italy).

The research is funded by the National Science Foundation.

Alfaro's lab also studies why some animal groups have large diversity in shape and others do not even though those groups have many species. He and his colleagues used DNA sequences to analyze evolutionary relationships, fossil analysis and perform many other complex statistical analyzes.

Alfaro said: 'We are interested in understanding the causes of biodiversity. We are working to find an explanation for the incredible diversity of coral reef fish and other vertebrates. Our analysis emphasizes the current great extinction rate, compared to the rate that history has experienced. '