Mass Extinctions Change the Rules of Evolution

A reinterpretation of the fossil record suggests a new answer to one of evolution’s existential questions: whether global mass extinctions are just short-term diversions in life’s preordained course, or send life careening down wholly new paths.

Some scientists have suggested the former. Rates of species diversification — the speed at which groups adapt and fill open ecological niches — seemed to predict what’s flourished in the aftermath of past planetary cataclysms. But according to the calculations of Macquarie University paleobiologist John Alroy, that’s just not the case.

“Mass extinction fundamentally changes the dynamics. It changes the composition of the biosphere forever. You can’t simply predict the winners and losers from what groups have done before,” he said.

Alroy was once a student of paleontologist Jack Sepkoski, who in the 1980s formalized the notion that Earth has experienced five mass extinctions in the 550 million years since life became durable enough to leave a fossil record. Graphs of taxonomic abundance depict lines rising steadily as life diversifies, plunging precipitously during each extinction, and rising again as life proliferates anew.

As the fossil record is patchy and long-term evolutionary principles still debated, paleobiologists have historically disagreed about what these extinctions mean. Some held that, in the absence of extinctions, species would diversify endlessly. The Tree of Life could sprout new branches forever. Others argued that each taxonomic group had limits; once it reached a certain size, each branch would stop growing.

Sepkoski’s calculations put him on the limits side of this argument. He also proposed that, by looking at the rate at which each group produced new species, one could predict the winners and losers of each mass extinction’s aftermath. Groups that diversified rapidly would flourish. Their destiny was already established.

“It’s a clockmaker vision of evolution. Each group has fixed dynamics, and if there’s an extinction, it just messes it up a bit,” said Alroy. “That’s what I’m challenging in this paper. There are limits, and that’s why we don’t have a trillion species. But those limits can change.”