Rethinking Polar Bear Evolution
A new study in the Proceedings of the National Academy of Sciences (Early Edition on-line), is shaking up science's view of polar bear evolution. Previously, it had been suggested that the white bear of the north was a relatively young species that diverged from brown bears during the last glacial period. That glaciation started after the Eemian epoch (~125,000 years ago), peaking around 25,000 years ago. New genetic analysis pushes that estimated divergence back to 4-5 million years ago, though there seems to have been a significant level of interbreeding between the two species over time. Another important finding is that the polar bear population underwent a significant contraction around 500,000 years ago. According to this new information the polar bear has been around for much longer than previously thought, implying that it has survived many interglacial warm periods. In other words, those who think the polar bear cannot survive the shrinking of Arctic ice packs are dead wrong.
The polar bear has served as a poster child for species threatened by global warming for decades. There is no denying the attraction of the big white bears, favorites in zoos and coke commercials. I myself am a great fan of polar bears, one of them is a major character in my forthcoming science fiction novel, Parker's Folly. In “Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change,” a large group of researchers led by Webb Miller, of the Center for Comparative Genomics and Bioinformatics at Pennsylvania State University, has rewritten the book on polar bear evolution. Here is the PNAS paper's abstract:
Polar bears (PBs) are superbly adapted to the extreme Arctic environment and have become emblematic of the threat to biodiversity from global climate change. Their divergence from the lower-latitude brown bear provides a textbook example of rapid evolution of distinct phenotypes. However, limited mitochondrial and nuclear DNA evidence conflicts in the timing of PB origin as well as placement of the species within versus sister to the brown bear lineage. We gathered extensive genomic sequence data from contemporary polar, brown, and American black bear samples, in addition to a 130,000- to 110,000-y old PB, to examine this problem from a genome-wide perspective. Nuclear DNA markers reflect a species tree consistent with expectation, showing polar and brown bears to be sister species. However, for the enigmatic brown bears native to Alaska’s Alexander Archipelago, we estimate that not only their mitochondrial genome, but also 5–10% of their nuclear genome, is most closely related to PBs, indicating ancient admixture between the two species. Explicit admixture analyses are consistent with ancient splits among PBs, brown bears and black bears that were later followed by occasional admixture. We also provide paleodemographic estimates that suggest bear evolution has tracked key climate events, and that PB in particular experienced a prolonged and dramatic decline in its effective population size during the last ca. 500,000 years. We demonstrate that brown bears and PBs have had sufficiently independent evolutionary histories over the last 4–5 million years to leave imprints in the PB nuclear genome that likely are associated with ecological adaptation to the Arctic environment.
The dataset used included deep sequence coverage for the entire genomes of a polar bear, three brown bears and a black bear, plus less complete coverage of 23 additional polar bears, including a 120,000-year-old individual. The researchers noted that very few vertebrate species have such comprehensive genomic resources available. Using this vast amount of data, the scientists discovered that polar bears are far more ancient than suggested by previous studies. Indeed, a recent study placed the species' age at only 600,000 years old, but that analysis looked only at small segments of DNA.
That report, “Nuclear Genomic Sequences Reveal that Polar Bears Are an Old and Distinct Bear Lineage,” which appeared in Science, obtained information from nuclear DNA indicating that polar bears actually evolved in the mid Pleistocene. This provides much more time for the polar bear ancestors to colonize and adapt to the harsh conditions of the arctic. Based on studies of mitochondrial DNA, polar bears had earlier been considered an example of surprisingly rapid adaptation of a mammal to colder climates. “In fact, the polar bear genome harbors a lot of distinct genetic information,” said Frank Hailer, the lead author, “which makes a lot of sense, given all the unique adaptations in polar bears.”
The newer PNAS report extends the polar bear's evolutionary timeline even further, from 600,000 to more than 4 million years. The significance of this finding is rather understated by the summary in the abstract. A number of the paper's authors have commented on the work in interviews for a ScienceDaily news article, “Polar Bear Evolution Tracked Climate Change.”
“We showed, based on a consideration of the entire DNA sequence, that earlier inferences were entirely misleading,” said study co-lead author Webb Miller, a Penn State professor of both Biology and Computer Science & Engineering. “Rather than polar bears splitting from brown bears a few hundred thousand years ago, we estimate that the split occurred 4-5 million years ago.” The genetic overlap among bear types identified by the new study is shown in the illustration below:
Shared alleles among bear types.
Instead of looking at short fragments of DNA the new study looked for alleles. An allele is one of two or more forms of a gene or group of genes. The terminology used here defines genomic positions where two different nucleotides could be confidently identified as SNPs. The study identified 13,038,705 SNPs when they aligned the samples from all 27 individuals. The other term, SAPs, stands for single amino acid polymorphisms and they are derived from comparing SNPs looking for differences of just a single amino acid. The analysis was helped by comparing bear DNA with dog DNA, since the DNA of dogs has been extensively studied (dogs are a more distant relative of bears). All of this DNA data was used to construct a phylogenic tree showing the relationships among different types of bears.
Phylogenetic reconstruction of relationships among bears.
The investigators found interesting contrasts between the subject bears DNA and mitochondrial DNA (mtDNA), which is inherited from only females. The new analysis uncovered more genetic similarities than previously known between polar bears and an isolated group of brown bears from southeastern Alaska, referred to as ABC brown bears. This suggests that these animals have exchanged genes since becoming separate species. The paper likens the clear discordance between mitochondrial and nuclear genomes in the ABC brown bears to that found in the evolutionary histories of archaic and anatomically modern humans.
What makes a polar bear a polar bear? Not too long ago biologists would have said it was obvious, polar bears have physical differences from brown bears that let them survive in an Arctic climate. White fur (actually transparent tubular hairs) over black skin provides camouflage and absorbs warmth, large well insulated paws help movement on snowy surfaces, and a less omnivorous diet adapted to meat and fat reflects conditions on the frozen icepack. Most of these traits are morphogenic, having to do with the physical characteristics of the creature. Modern evolutionary biologists look more to what's inside, the genetic makeup of polar bears.
“The ABC brown bears' mitochondrial sequences are much more like polar bears' than like other brown bears',” Miller said in the ScienceDaily interview. “This made us wonder what other parts of their genomes are 'polar-bear-like.' We mapped such regions, which constitute 5 to 10 percent of their total DNA, onto the genomes of two ABC brown bears. As such, brown/polar bear hybridization, which has been observed recently in Arctic Canada, has undoubtedly contributed to shaping the modern polar bear's evolutionary story.”
Distilling all of the genetic information down to a more accessible form yields a diagram showing the relationships between the three main types of northern hemisphere bears. Also shown are some of the proposed transfers of mitochondrial DNA caused by interbreeding.
The family relationship between bears.
“This means polar bears definitely persisted through warming periods during Earth's history,” added Charlotte Lindqvist, from the University of Buffalo and the paper's senior author. These intimate relations may be tied to changes in Earth's climate, with the retreat of glaciers bringing the two species into greater contact as their ranges overlapped, she explained.
It seems that polar bears are using interbreeding with their brown bear cousins as an evolutionary response during warm periods like interglacials. When the ice retreats the bears grow amorous and willing to breed with brown bears as well as their own kind. This “strategy” would allow the bears to respond quickly to a warming climate. No wonder that polar bears, or at least their ancestors, were able to ride out more than a dozen interglacial periods. The recent sightings of polar bear/brown bear hybrids in northern Canada no longer seem so surprising.
This is not to say that the polar bear and bears in general have not suffered from the ascent of man as Earth's dominant predator species. The PNAS paper notes that bear populations in general have been declining since the beginning of the Pleistocene. Whether this is due to climate change or human predation is unknown but there was a significant dip in the polar bear population beginning about 500,000 years ago that continues to this day (see the chart below).
Bear demographic history during the Pleistocene.
From the population plot it is easy to see that polar bears do better during a glacial period, holding stead or even increasing as the did since the Eemian. There also seems to be a trade-off between brown bears and polar bears since the Eemian, with the brown bears increasing as the climate warms and polar bears doing better as things get colder. This supports the interbreeding as survival strategy thesis stated above.
“Hybridization is a widespread evolutionary phenomenon that can play a role in speciation, in particular among closely related species,” the report concludes. “Today we know that evolutionary progression can continue even while species undergo "genomic invasions" from other species.” Something similar has been happening with the North American coyote, which has cross bred with gray wolves and domestic dogs to create a bigger, stronger animal. The new hybrid coyote is moving into the top predator slot left vacant since humans drove wolves and mountain lions effectively into extinction. It seems that the polar bear is not the only species to use cross breeding as a survival mechanism.
Still, green activists and climate alarmists continue to bewail the imminent demise of the polar bear, claiming that a warming world has no place for the great white bear. Balderdash. The latest scientific findings show that the polar bear has weathered worse climatic storms in the past. In fact, if asked to bet on who will survive the great global warming scam—the climate change alarmists or the polar bear—my money is on the polar bear.
Be safe, enjoy the interglacial and stay skeptical.