By Erling Friis-Baastad
The fossil record suggests mountain goats immigrated to North America across the Bering land bridge from Eurasia sometime in the early Pleistocene epoch, or late Pliocene. Today, scientists like Aaron Shafer, a fourth-year PhD student at University of Alberta, are able to read fascinating chapters from the subsequent 2.5-million-year mountain goat history in North
America from the DNA held in a library of tissue samples taken from contemporary goats.
“I’m working on the evolutionary history and population structure of mountain goats,’” says Shafer, who adds that writing the history requires answers to some tantalizing geographical questions. For one: What happened to Oreamnos americanus when much of North America was covered by ice? “We had no idea what the mountain goats did. Did they go to Beringia or to the Pacific Northwest, or did they stay in refugia along the coast or remain among the ice sheets?” he says.
Then there were questions about population structures. “What that means is: Are there distinct populations or is everyone freely mating? We had no idea with mountain goats. We knew they exist in distinct bands or herds but the level of gene flow and differentiation between herds: we had no idea.”
Shafer and his colleagues obtained ear-plugs of tissue from mountain goats, provided by wildlife managers and hunters, over a wide study area. From these plugs they took samples the size of a pencil-tip and extracted the DNA. “Molecular techniques allow us to target a particular region of a genome and get a sequence from it. I have tissue samples from all across western North America: Alaska, Yukon, NWT, BC, Alberta, Idaho, Montana and Washington.”
By focusing on a particular gene scientist are able to create “evolutionary trees” and if these have a geographic structure to them, that is if they correspond to a particular population of a particular region, educated inferences can be made – for instance, that the populations became separated at some time. “The really neat thing about doing these evolutionary trees is that you can date them, calibrate your tree with fossils, and then you can date the branches,” says Shafer.
“So I can say, the north branch separated from the south branch 200,000 years ago.” Records of major glaciation in the area at that time suggest the herds were separated by ice sheets, he adds.
The two largest ice-age refugia occurred in the Pacific Northwest just below the US border and in the Beringia area of Yukon and Alaska. Smaller populations were known to have existed on Vancouver Island. Recent work by Shafer and his advisers suggests that there may have been another major refugium in Southwest Yukon and adjacent Alaska. More research is needed to confirm the size and significance this one. As well, a small population of goats currently lives in the Mackenzie Mountains of Yukon/NWT.
And then there are the challenging nunataks, mountain tops that jutted above the ice sheets during periods of maximum glaciation. They functioned much like the low-level refugia, but on a much smaller scale, says Shafer. “An alpine specialist like the mountain goat doesn’t want to cross valleys if it can avoid that. That’s where they get preyed upon.” They stick to the bare mountain tops and as they breed they create distinct genetic trees according to mountain ranges.
Nunataks are difficult to study in part because they are so exposed to the elements that the fossil record is virtually non-existent. Bones are ground down by silt-laden winds. Even pollen samples, a means of calibrating geological strata and corresponding animal populations over time, are nearly impossible to come by on the peaks. Genetic data from living goats help fill in the gaps.
Mountain goats are a game animal. That is a major impetus for the research. Until recently, management of the herds was somewhat arbitrarily based more on geography than biology. By concentrating on the genetics scientists can determine diversity. This is the crux: The more genetically diverse, the healthier these populations are.
“When you have genetic diversity, that allows evolution to occur. It’s like a genetic bank account that’s large enough to allow populations to change and cope with change to climate,” says Shafer. “Problems occur when the diversity is lost through inbreeding. The ability to change is seriously impaired.”
Meanwhile, as with all scientific research from toads to exoplanets, answered questions lead to yet newer questions. Shafer, who has been part of the mountain goat study since 2007 is looking forward to tackling two specific additional issues. One is: “What habitat variables in the wild affect gene pools?” This means “seeing if there are certain characteristics of habitat, like vegetation, affecting gene flow between populations.”
As well Shafer and his colleagues will be looking at mountain goat immune genes. “Diversity is good, again,” says Shafer, “If you have diverse immune genes then you have the repertoire to respond to a whole host of pathogens, whereas if you don’t have that diversity, then potentially, parasites can be a problem.”
So how are our mountain goats faring? Last summer Shafer attended a conference in Oregon on mountain goats and wild sheep. Overall, he says, the goats were reported to be doing well, though pneumonia outbreaks have plagued some sheep populations.
In the Yukon, the Mackenzie mountain goat herd is small and more vulnerable, but the population in the southern Yukon appears to be in good shape as far as gene diversity and health go. Shafer cautions, however, that mountain goats have not been studied or managed as extensively as wild sheep; there is much we don’t yet know, and that’s reason for caution when making pronouncements about the fate of these high-level acrobats in a time of rapid climate change.
This column is co-ordinated by the Northern Research Institute at Yukon College with major financial support from Environment Yukon and Yukon College. The articles are archived at www.taiga.net/yourYukon