Predators stifle rapid evolution of prey

Evolution, normally viewed as a slow, steady process, can occur in rapid fits and starts with one species splitting into several lineages in a relatively short period of time. Now scientists have identified two factors that influence these bursts of new species.

Called adaptive radiation, the relatively swift emergence of new species is known to occur in isolated ecosystems, such as remote islands, or following mass extinctions. But details about what drives this process have remained murky.

Two new studies, detailed in the March 22 issue of the journal Nature, suggest predator-prey relationships, as well as the timing and relative order of a species' arrival into a new environment, can greatly affect how rapidly this branching process occurs.

The studies involved microbes, in part because they can be easily manipulated in the lab and, with such short life cycles, they evolve quickly over time.

Justin Meyer and Rees Kassen of the University of Ottawa in Canada looked at the effects of a predator, a single-celled microbe called Tetrahymena thermophila, on the diversification of the soil bacterium Pseudomonas fluorescens.

They found that under some circumstances, the predators helped cull the bacteria population. This prevented overcrowding, which, in turn, reduced bacterial competition for food resources. With less interspecies competition, the bacteria were less inclined to spread out and fill new niches or experiment with new foods. Thus, predators appear to stifle speciation under some circumstances.

“There’s less need for prey to evolve into different types because the environment’s not saturated,” Meyer explained.

The findings suggest predation plays a prominent, yet often overlooked, role in the evolutionary histories of many species. It also helps explain why organisms that find their way to remote islands often undergo a speciation explosion and branch rapidly into different lineages. A classic example is Darwin’s finches in the Galapagos. Scientists think these 13 finches evolved from a common ancestor that found its way to the islands about 3 million years ago.

On an island with plentiful resources, few competitors and no predators, the ancestral finch was in evolutionary paradise and diversified to fill available niches. The ancestral finch ate seeds and spent most of its time on the ground; today, the Galapagos finches eat insects, grubs, seeds, fruit and even blood. There’s even a finch that uses small twigs as tools.

The birds are named after Charles Darwin, who at age 26 visited the islands and collected the birds. The finches were one of the  inspirations for Darwin’s theory of evolution by natural selection.