PhD student Zijian Sim just published a
paper in Molecular Ecology which outlines new understanding of the phylogeography of the iconic North American
thinhorn sheep. Congrats to Sim!!!
The University of Alberta Press Release reads:
Kristy Condon,
Communications Associate / (780) 492.5344 / kristy.condon@ualberta.ca
Edmonton, Alberta, Canada /
www.science.ualberta.ca
FOR IMMEDIATE RELEASE:
A
tale of two subspecies
DNA
analysis rewrites the story of thinhorn sheep during the last ice age
(EDMONTON) - Evolutionary biologists
studying the lineages of thinhorn sheep have found evidence suggesting that the
species diverged hundreds of thousands of years earlier than previously
thought.
During the last ice age, most of North
America was covered by inhospitable glaciers, forcing plants and animals to
seek refuge in ice-free regions known as refugia. In some cases, individuals
from the same species were separated in different refugia where they accumulated
variances between populations over time, sometimes resulting in the formation
of new species.
An iconic symbol of the mountains of
western North America, the ancestors of the two dominant varieties of thinhorn
sheep were believed to have weathered the last ice age together in the
Beringian refugium, which today spans Alaska and the northern Yukon. After the glaciers melted some 10 thousand
years ago, this theory posits, the species diverged into the white Dall’s sheep
and the dark Stone’s sheep that we know today.
When scientists looked into the DNA of both
subspecies, however, they found it told a different story.
“The separation is hard to date, because
with glaciation, fossilization is a hard thing to achieve,” explains Sim Zijian,
PhD candidate in the Department of Biological Sciences and lead author on the
study. “Traditionally when you’re trying to figure out what species came first
and how they split, you use fossils. But that’s not possible here because these
sheep live on top of mountains, where it’s rocky. Fossils just don’t form on
rock.”
Instead, scientists turned to genetics,
conducting a phylogenetic study—the study of lineages—into both subspecies of
sheep. Looking at samples of modern animals, they found that, based on how the
lineages were split, the division between the white Dall’s sheep and the dark
Stone’s sheep is deeper than would be expected if they had both survived in the
Beringian refugium. The findings support the idea that there was likely a
second smaller refugium located south of Beringia that sheltered one of the two
thinhorn groups through the glacial advance.
“We used to think they were all in one
place. Now we think they were in two places, and based on the typology of this
phylogenetic study, we can show that it is this survival in different refugia
that gave rise to the two different subspecies that we see today,” says
Zijian. “So that gives us the confidence
to say that one subspecies, in this case the Stone’s sheep—so the dark
one—probably survived in that smaller ice-free refugium.”
The findings will be reflected in an update
of the subspecies distribution map, which includes a third, somewhat more
nebulous group known as Fannin’s sheep, which can vary dramatically in colour
from mostly light to mostly dark.
“We’ve always known that the Fannin’s sheep
were there, but we didn’t really know what they were,” says Zijian. “Are they
just a type of Stone’s sheep? Are they just a type of Dall’s sheep? Why are
there so many colour variations in this one area?”
The answer, it turns out, is none of the
above. Fannin’s sheep are a hybrid of Dall’s and Stone’s sheep—a product of the
two subspecies reuniting following their separation over the last ice age.
Though this may seem like a relatively
small takeaway, Zijian emphasizes the importance of maintaining an accurate
picture of subspecies distribution. “In today’s management framework, the
ability to define groups and what these groups are truly made of is very
important,” he explains, adding that having clearly defined groups are critical
when establishing conservation frameworks.
The study, “Genome-wide set of SNPs reveals
evidence for two glacial refugia and admixture from postglacial recolonization
in an alpine ungulate,” was recently published in Molecular Ecology. Samples for the study were provided in part by
hunters and hunting outfitters. The study was supported by the Yukon Department
of Environment, the BC Ministry of Forests, Lands and Natural Resource
Operations, the Wild Sheep Foundation, the Wild Sheep Society, and the Habitat
Conservation Trust Foundation of BC.
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The
University of Alberta Faculty of Science is a research and teaching powerhouse
dedicated to shaping the future by pushing the boundaries of knowledge in the
classroom, laboratory, and field. Through exceptional teaching, learning, and
research experiences, we competitively position our students, staff, and
faculty for current and future success.
For Interviews:
Sim
Zijian
PhD Candidate, Department of Biological Sciences
Faculty of Science, University of Alberta
E-mail: sim.zijian@gmail.com
Phone: (780) 975-7208
Photos:
Caption: Three Fannin's sheep rams in their
winter range. (Photo: Sim Zijian)
Caption: Two Dall's sheep rams in their
winter range. (Photo: Sim Zijian)
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