Understanding the interplay between different drivers of evolution depends on the stage a group is at in its evolutionary trajectory, suggests a report in Nature this week. The modelling study provides an explanation for the apparent contradiction between the seemingly inevitable decline of ageing taxa and randomness of extinction implied by one of the most influential ideas in evolutionary biology since Darwin: the Red Queen’s hypothesis.
The Red Queen’s hypothesis, which describes the ongoing evolutionary arms race between species, was proposed by Leigh Van Valen as an explanation of the law of constant extinction, which he formulated after having found that a taxon’s probability of extinction bore no relationship to the length of time that it had existed. This finding is at odds with much of the fossil record, which reveals ‘hat-like’ distributions of taxa abundance, diversity or geographic range through time - taxa are often scarce at the beginning, peak in the middle and then are scarce at the end.
Indre Zliobaite and colleagues show that the paradox can be resolved if the peak of a taxon’s expansion is considered, rather than its final extinction. They suggest that the factors limiting a species at its peak are more likely to do with competition, whereas those limiting its initial diversification and eventual extinction are more likely to be related to stochastic abiotic effects. This means that when seeking to discover when or if a species is likely to become extinct, instead of considering its final decline and extirpation, one should instead look at when it is just past its peak.