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Pseudophilautus poppiae, a microendemic shrub frog from
Southern Sri Lanka that only occurs in a few hectares of
cloud forest. Photo credit: Alex
Pyron
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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.
Citation
Pyron, RA. 2014. Biogeographic Analysis Reveals Ancient
Continental Vicariance and Recent Oceanic Dispersal in Amphibians. Systematic Biology, DOI:10.1093/sysbio/syu042