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Frog research sheds light on evolution

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Natural selection against hybridization has driven the evolution of a population of a rainforest frog into a new species, according to research at the Australian National University.

Biologist Conrad Hoskin has been studying populations of the green-eyed tree frog from the north coast of Queensland in a bid to answer big questions about the forces behind evolution.

Genetics studies suggest a single species, Litoria genimaculata, once ranged in rainforest from Cooktown to Cairns, but it was split into two populations when its habitat was fragmented. During this period of isolation, the fragmented populations of the mottled brown and green frog diverged into two distinct lineages - a northern one and a southern one.

The populations were reunited about 7,000 years ago when warmer, more humid conditions promoted the expansion of the rainforest.  In the area where they reunited (a ‘contact' or ‘hybrid' zone), a population of the southern lineage evolved into a new species, Litoria myola, that now lives alongside its Litoria genimaculata relatives.

Hoskin and colleagues wanted to gauge the extent to which natural selection against hybridization had forced the evolution of this new species. Hybridization can produce unviable offspring, but the extent to which it drives evolution is hotly contested and difficult to gauge. The green eyed tree frog populations abutting each other in contact zones near Cairns have the potential to provide answers. "Contact zones have been described as natural laboratories for studying species formation," says Hoskin.

With all populations looking much the same, avoiding hybridization through mate choice relies mostly on female preference for a single trait - mating call. "The mating call is largely genetically determined, and it is a way frogs recognise their own species," Hoskin says. "The calls differ between the lineages and if females choose the correct call they avoid hybridization."

His team used genetic analyses, experimental crosses, field observations, analyses of morphology and call characteristics, and studies of female response to mating calls in the research, and reported its results in the British journal Nature.

At night, the male frogs, which measure about four centimetres long, sing their staccato love song of a repeated tone from the lower branches of trees along streams in a bid to lure the six-centimetre-long females in from the surrounding rainforest.

Hoskin's team found that the females with southern ancestry were fussier than those with northern roots, preferring the up-tempo call of their own kind over the more laid-back call of the northerners.
"The southerners are very particular about call choice," says Hoskin. Of all the southerners, Litoria myola is the most discerning, he adds. "Litoria myola females have a strong preference for the distinct calls of their own males."

Clues to the reason the southerners have a distinct call and are more choosy come from the results of hybridization experiments. Tadpoles from crosses involving northern females and southern males survived whereas tadpoles produced in the union of southern females and northern males died. This suggests that the impetus to avoid hybridizing is stronger for southern than northern females.

Hoskin, an Australian Research Council Postdoctoral Fellow in the School of Botany and Zoology, is continuing the research. Attracted to the ANU by its strength in evolutionary biology, he expects to complete the project in 2010.

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