How did marine animals become so diverse?
Upending an evolutionary theory proposed in the 1950s, scientists have found that the groups most resistant to extinction also contain the greatest ecological diversity – their members perform a larger number of different functions in ecosystems.
Why do some groups of animals contain many more species than others? Like the tale of the tortoise and the hare, it turns out the groups more resistant to extinction have succeeded over the long run, rather than the groups most prone to forming new species, according to research.
Upending an evolutionary theory proposed in the 1950s, scientists have found that the groups most resistant to extinction also contain the greatest ecological diversity – their members perform a larger number of different functions in ecosystems. The research, published Feb. 28 in Science, was led by a team at the University of Hawai‘i at Hilo and co-authored by paleontologist Jonathan Payne, a professor of geological sciences at Stanford’s School of Earth, Energy & Environmental Sciences (Stanford Earth).
“The big advantage of examining the fossil record in addition to the modern world is the chance to directly study extinction and its effects,” Payne said. “We have many biological and ecological intuitions that are based on what we see in the world around us today, and some of those intuitions are correct and have applied over all of evolutionary time, but many others turn out to be based on the specific circumstances of the modern world – they are not universal.”
The researchers examined nearly 20,000 fossil marine animal genera – groups of species – across the past 500 million years, in addition to about 30,000 genera of living marine animals. The analyses also show the most ecologically diverse animal groups – those that possess the most functional abilities – contain a majority of the genera in the modern oceans, according to lead author Matthew Knope, an assistant professor of biology at UH Hilo.
This close link between species richness and functional differentiation was not always the case in marine ecosystems, according to Payne. For much of the first 300 million years of animal evolution in the oceans, the most diverse groups were those with the highest rates of species formation or speciation, and they did not necessarily have a higher number of different ecosystem functions than less diverse groups.
“Most paleontologists would already know that extinction and speciation rates tend to be correlated – the groups that have a high rate in one tend to have a high rate in the other, and the differences are subtle,” Payne said. “But with lots and lots of time, subtle differences will still have large effects. If you have a little bit more speciation than extinction, given 500 million years, you can end up with a lot of species.”
It was only during and after the major mass extinction events that the current association developed, due to the greater extinction resistance of the more functionally diverse groups – a fact that could only be determined through direct examination of the fossil record, Payne said.
“Being a member of an ecologically flexible group makes you resistant to extinction, in particular during mass extinctions that primarily impacted ecologically homogenous groups,” said Knope, who began research for the study as a postdoctoral researcher with Payne at Stanford.
Understanding patterns of biodiversity and evolution has always been a major pursuit in the field of biology. While some deductions can be made by simply by examining species alive today, adding extinct animals to the equation is notoriously difficult, according to Rosemary Gillespie, a professor of evolutionary biology at the University of California, Berkeley, who was not involved in the study.
“This study represents some of the most detailed and careful analyses of the fossil record to date, showing very clearly the importance of the ‘slow and steady’ development of lineages through time has been a key factor in dictating which lineages have achieved the highest diversity,” Gillespie said.
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