July 28, 2014-By Lauren
Ingeno, George Washington University’s ON-line News
Service
There are more than 7,000 known species of
amphibians that can be found in nearly every type of ecosystem on six
continents. But there have been few attempts to understand exactly when and how
frogs, toads, salamanders and caecilians have moved across the planet throughout
time.
Armed with DNA sequence data, Alex Pyron, an
assistant professor of biology at the George Washington University, sought to
accurately piece together the 300-million-year storyline of their
journey.
Dr. Pyron has succeeded in constructing a
first-of-its-kind comprehensive diagram of the geographic distribution of
amphibians, showing the movement of 3,309 species between 12 global ecoregions.
The phylogeny—or diagram of evolutionary relationships—includes about half of
all extant amphibian species from every taxonomic group.
"There have been smaller-scale studies, but they
included only a few major lineages and were very broad,” Dr. Pyron said. “What
we needed was a large-scale phylogeny that included as many species as possible.
That allows us to track back through time, not only how different species are
related, but also how they moved from place to place.”
His findings, which appear in the journal Systematic
Biology, suggest that, contrary to popular belief, certain groups of
amphibians may have swam long distances from one landmass to another within the
past few million years.
Biologists have long hypothesized the distribution
of extant lineages of amphibians has been driven by two major processes:
vicariance and dispersal.
Vicariance occurs when a population is separated
following a large-scale geophysical event. After the fragmentation of
supercontinent Pangaea and the subsequent split of the Laurasian and Gondwanan
landmasses, certain groups of amphibians were able to “hitch a ride” from one
continent to another, Dr. Pyron explained. The researcher’s biogeographic
analysis supports this hypothesis, showing that continental movement can explain
the majority of patterns in the distribution of extant species of
amphibians.
Dr. Pyron also found that dispersal during the
Cenozoic Era (66 million years ago to the present), likely across land bridges
or short distances across oceans, also contributed to their
distribution.
Given their ancient origin, it is unsurprising that
the history of amphibians is a mixture of both vicariance and dispersal. But the
third and final distribution pattern that Dr. Pyron notes in his study was an
unexpected finding.
Past studies have assumed that long-distance over
water dispersal was essentially impossible for amphibians due to salt
intolerance. However, when Dr. Pyron began completing his analysis, he noticed a
number of cases of distribution that could not be explained by old
age.
For instance, one group of frogs found in Australia
and New Guinea (pelodryadine hylids) that originated around 61 to 52 million
years ago is deeply nested within a group of amphibians that exist only in South
America. By the time pelodryadines originated, all major continental landmasses
occupied their present-day positions, with South America and Australia long
separated from Antarctica.
“They’re 120 million years too late to have walked
to Australia,” Dr. Pyron said.
So how could this group of South American amphibians
be related to frogs on the other side of the world?
“You wouldn’t think that frogs would be able to swim
all the way there, but that seems like one of the more likely explanations for
how you could have such a young group nested within South America and have it
somehow get to this other continent,” Dr. Pyron said.
In his study, Dr. Pyron points two other instances
of long-distance oceanic dispersal.
“What you have is this mixture of processes. You
have vicariance, which over 300 million years has put certain groups in Africa,
some in Australia and others in South America,” Dr. Pyron said. “But even more
recently, within the last few million years, you have these chance events of
long distance dispersals across the ocean, which can influence distribution
patterns.”
Dr. Pyron’s next research question is whether there
is any specific quality, such as tolerance to salt water, which allows some
groups of amphibians to be better dispersers. He has also begun to conduct a
similar analysis with lizards and snakes to see if the same distribution
patterns hold up. And as new species are discovered, Dr. Pyron will continue to
revise his model.
These findings not only provide evidence for the
unlikely hypothesis of long-distance oceanic dispersal, but they also provide a
model for explaining the distribution of other species and learning about the
geographic diversity of different groups. For example, an endangered frog in
South America unconnected to any other major lineages would need to be a high
conservation priority.
“That’s something we can only learn from a
biogeographic analysis,” Dr. Pyron said
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