Australian frogs create new species in record time

10/03/2007 00:00:00 Choosy female frogs in a small rainforest in Australia have evolved a new species in just 8,000 years, according to scientists from the University of Queensland, the University of California, Berkeley, and the Queensland Parks and Wildlife Service. This is as fast as any known evolutionary step yet discovered.

The as yet un-named new species arose after 2 isolated populations of the green-eyed tree frog made contact less than 8,000 years ago and discovered that their hybrid offspring were less viable. To avoid breeding with the wrong frogs and ensure healthy offspring, one group of females chose mates from their own lineage. Over a few thousand years, this behaviour created a reproductively isolated population - basically a new species - that is unable to mate with either of the original frog populations.

This example suggests that rapid speciation can be driven by re-contact between isolated populations. Standard evolution can produce small variations over millions of years, whereas re-contact can amplify the difference over several thousands of years to generate a distinct species. The Green-eyed tree frog, Litoria genimaculata, is found in the Wet Tropics area of northeast Queensland, a wild tropical region of northern Australia. The frog, which is green with reddish-brown splotches, is common around streams and grows to about 2 1/2 inches in length.

Because of geographic isolation began 1-2 million years ago when the retreat of rainforest to higher elevations led to 2 separate frog lineages developing in different parts of the species' coastal habitat - only to be reconnected under 8,000 years ago as the climate got wetter and warmer and the rainforest expanded.

The northern and southern male frog mating calls, which attract females, became different from each other. Despite this difference, noted in the call's duration, note rate and frequency, the two lineages could still breed with one another.

The southern females, however, were choosier about their mates than the northern females. And in one small contact area that became isolated from the southern range, the southern females were very picky, so they virtually never mated with northern males.
Biologists found the reason for this pickiness: Although northern and southern populations could breed successfully, they diverged sufficiently during their million-year separation that offspring of southern females and northern males did not develop beyond a tadpole. And crosses involving northern females and southern males did successfully produced frogs, but the offspring developed more slowly than is normal.

Field studies confirmed the laboratory results. Researchers could found no hybrid frogs in the contact areas that were the offspring of southern mothers.

Because southern females have the most to lose from such cross-breeding, there may have been selection pressure to create a mating strategy to minimize low success mating with northern males. It appears that this occurred in the contact zone where a group of the southern lineage became isolated from the rest of its lineage and developed a preference for particular male calls. The male frog call in this population diverged significantly from both the northern and southern lineage calls.

This so-called reinforcement has been controversial since Charles Darwin, with some biologists claiming that it requires too many steps for evolution to get it right.

Interestingly, evolutionary theory would predict that the southern and northern frog populations would drift apart into two distinct species. In the case of the green-eyed tree frog a subpopulation of the southern species unexpectedly drifted away not only from the northern species, but also from the southern.

Geographic isolation in this tiny parcel of rainforest in Australia has split many species, and that reinforcement at zones of re-contact may be generating other new species.