Study links mass extinctions to a race life often loses

Environmental shifts can become deadly when they move faster than organisms can evolve, and a new model suggests that same mismatch helps explain mass extinctions across Earth’s history. Researchers at MIT and the University of Leicester say the approach also offers a way to gauge modern extinction risk as ocean carbon dioxide rises.

The work, by MIT geophysicist Daniel Rothman and University of Leicester applied mathematician Sergei Petrovskii, was published June 24 in Physical Review Letters, according to MIT. The researchers tested whether the speed of global environmental change could be compared with the speed at which animal groups adapt, then used that comparison to estimate how severe extinction events became.

A model for adaptation under stress

According to MIT, Rothman and Petrovskii built a theoretical model for the distribution of adaptation rates among animal groups. In the study, adaptation means changes within a species over times far longer than a generation that help the species persist as conditions shift.

The model rests on a standard idea in evolutionary theory, MIT reported: adaptation requires several conditions, including variation within a population, inheritance of useful traits and greater reproductive success for organisms better suited to new conditions. If any required condition fails, the population may disappear.

Rothman and Petrovskii translated that chain of requirements into a mathematical curve. MIT said the resulting bell-shaped pattern describes a world in which many animal groups adapt at middle-range speeds, while fewer groups adapt at very slow or very fast rates.

Testing the idea against the fossil record

To compare the model with Earth’s past, the researchers examined paleontological and geochemical records from 27 episodes in the past 450 million years when the carbon cycle changed substantially, according to MIT. Scientists commonly use carbon-cycle disruption as an indicator of broad environmental change.

The team then compared those rates of environmental change with extinction levels drawn from earlier work by paleobiologist John Alroy, MIT said. The model matched the severity of most major extinction events, estimating the share of animal life that could not adapt quickly enough and died out.

MIT said the findings support a “rate-mismatch” hypothesis at a global scale: extinction risk rises when environmental change exceeds the adaptive capacity of a significant fraction of life. That idea had been proposed in the mid-20th century by American geologist Norman Newell for extinctions, after earlier debates that traced back to Georges Cuvier’s 18th-century argument that catastrophes could wipe out species.

Rothman and Petrovskii also found that the spread of adaptation rates among animal groups appears broadly similar to the spread of rates at which the environment can change, according to MIT. Rothman said the pattern suggests life’s capacity to adapt may correspond in some way to the range of stresses it has faced.

Modern risk

The researchers applied the model to past events rather than making a direct forecast, MIT said. One case discussed in the study is the end-Permian extinction, when rapid ocean acidification likely exceeded organisms’ ability to evolve defenses, contributing to the loss of more than 80% of marine species worldwide.

Rothman said today’s rise in ocean carbon dioxide, when rescaled for comparison, is near rates of carbon-cycle change just below those linked with major extinction events in the past, according to MIT. He said that pattern suggests current environmental change may be nearing speeds at which adaptation becomes harder for life.

The paper is titled “Relating Rates of Global Change, Evolutionary Adaptation, and Extinction.” Its DOI is 10.1103/62jn-xgqy.

This story draws on original reporting from Phys.org.