Why Are There So Many More Species Of Insects? Because Insects Have Been Here Longer

In a study appearing in the April issue of the American Naturalist, McPeek and Brown show that many insect groups like beetles and butterflies have fantastic numbers of species because these groups are so old. In contrast, less diverse groups, like mammals and birds, are evolutionarily younger.

This is a surprisingly simple answer to a fundamental biological puzzle. They accumulated data from molecular phylogenies (which date the evolutionary relationships among species using genetic information) and from the fossil record to ask whether groups with more species today had accumulated species at faster rates.

Animals as diverse as mollusks, insects, spiders, fish, amphibians, reptiles, birds, and mammals appear to have accumulated new species at surprisingly similar rates over evolutionary time. Groups with more species were simply those that had survived longer. Their analyses thus identify time as a primary determinant of species diversity patterns across animals.

Given the unprecedented extinction rates that the Earth's biota are currently experiencing, these findings are also quite sobering. We are rapidly losing what it has taken nature hundreds of millions of years to construct, and only time can repair it.

Mark A. McPeek and Jonathan M. Brown, "Clade Age and Not Diversification Rate Explains Species Richness among Animal Taxa" The American Naturalist, volume 169 (2007), pages E97--E106 DOI: 10.1086/512135