Friday, 15 September 2017

Frog Shocker!

Atlantic Coast leopard frog, Rana kauffeldi

Source: Wikipedia
New study contradicts assumption that true frogs diversified as they expanded their range around globe | The University of Kansas

One of the nice things about science is that, properly done, it can throw up some surprises, even overthrowing some preconceptions and axioms.

Science demands we constantly question and reassess our assumptions and preconceptions. It is not for those who prefer simple certainties over difficult truths.

For example, it is generally assumed that when a species colonises new geographic regions with new ecological niches, it will tend to adapt and radiate into several new species; that range extension itself is a major driver of speciation.

But this is not always so, it seems.

Spotted Leaping Frog, Indirana diplostict

Source: Wikipedia
For example, as the true frogs diversified, quite rapidly as it turned out, what should have happened if all went according to plan was that there should have been a corresponding increase in the rate of diversification into new species. However, as Kin Onn Chan and Rafe M. Brown from the University of Kansas Department of Ecology and Evolutionary Biology showed, in a paper appearing in Royal Society Biology Letters, there was not only not the expected increase but the rate remained about the same. In one group it actually decreased.

True frogs are descended from survivors of the Eocene–Oligocene extinction event (EOEE) so were an ideal group to test the prediction that range expansion triggers increased net diversification, but first, the researchers had to identify where the true frogs arose, when they began to diversify and how long it took them to spread to their current distribution in every continent except Antarctica.

To do this they compiled the most densely sampled phylogeny of frogs to date from 402 genetic samples obtained from GenBank, an online genetic database. These represented 292 of the known 380 true frog species in the world. From this it was clear that the group originated in southeast Asia in the area of Thailand, Vietnam, Cambodia and Burma. From there they radiated over a period of some 40 million years during the late Eocene.

Having constructed a 'family tree' of species, the researchers then calibrated this using frog fossils of know ages, dated by the age of the rock deposits in which they were found. Using date from paleontological studies they were able to identify the place in the phylogeny where the fossil best fitted and so date-stamp the family tree.

Using this method, Chan and Brown believe they have shown, surprisingly, that the rich diversity seen in the Ranidae was not due to range extension per se, nor to simple niche-filling made possible by the EOEE mass-extinction, but to simple diversification over time, influenced by a host of factors. As Kin on Chan, one of the co-authors said:

Our conclusion is kind of anticlimactic, but it’s cool because it goes against expectations. We show the reason for species richness was just a really steady accumulation of species through time — there wasn’t a big event that caused this family to diversify like crazy.

Kin Onn Chan, co-author,
Biodiversity Institute and Department of Ecology and Evolutionary Biology,
University of Kansas, Lawrence, KS, USA

In other words, they found exactly the opposite of what they expected but that in itself is a result worth writing up and publishing.

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