There are more than 400,000 species of beetles and only two species of the tuatara, a reptile cousin of snakes and lizards that lives in New Zealand. Crocodiles and alligators, while nearly 250 million years old, have diversified into only 23 species. Why evolution has produced "winners" including mammals and many species of birds and fish and "losers" is a major question in evolutionary biology.
Scientists have often posited that because some animal and plant lineages are much older than others, they have had more time to produce new species (the dearth of crocodiles notwithstanding). This idea that time is an important predictor of species number underlies many theoretical models used by biologists. However, it fails to explain species numbers across all multi-cellular life on the planet, a team of life scientists reports Aug. 28 in the online journal PLoS Biology, a publication of the Public Library of Science.
"We found no evidence of that," said Michael Alfaro, a UCLA associate professor of ecology and evolutionary biology and senior author of the new study. "When we look across the tree of life, the age of the group tells us almost nothing about how many species we would expect to find. In most groups, it tells us nothing."
Another idea, that some groups are innately better or worse at producing species, similarly fails to explain differences in species number among all of the major living lineages of plants and animals, the life scientists found.
"We know that some groups, like flowering plants or cichlid fishes, have been exceptionally good at producing species during certain periods of their evolutionary history," Alfaro said. "However, when we look at the ages of all of the major groups of plants and animals, these differences in speciation rate are not sufficient to explain the differences in species number that exist in extant groups."
Alfaro and his colleagues studied 1,397 major group
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University of California - Los Angeles