One big question I asked before writing my book on polar bear evolution was this: did interbreeding with grizzlies, aka brown bears, profoundly impact polar bear history, as geneticists insist? Or is something else going on?
Virtually all genetic studies done in recent years, which I review in my book, conclude that hybridization with grizzlies has happened to various degrees over the course of polar bear history (e.g. Cahill et al. 2013, 2018; Cronin et al. 1991; Edwards et al. 2011; Hailer 2015; Kumar et al. 2017; Miller et al. 2012). Two of the most recent studies claim the most complicated hybrid ancestry for polar bears yet, invoking tales of “extensive” past hybridization events between the two species (Lan et al. 2022; Wang et al. 2022).
But does their interpretation of the genetic data represent reality or does it simply fit the authors’ preferred but false narrative that climate change is to blame for recent hybridization events and therefore likely to happen more often in a warmer world? And if, as I argue in my book, grizzly hybridization isn’t needed to explain polar bear evolutionary history, what does that say about similar claims that there has been a significant amount of Neanderthal interbreeding with humans in our past? Put another way, are geneticists everywhere going overboard with claims of interspecies hanky-panky?
Questions I had to ask
And their short answers (long answers in the book):
- Is polar bear hybridization common in the wild? No. We have scientific evidence of an isolated event that spanned more than a decade, involving one female polar bear and two male grizzlies (Pongracz et al. 2017). There is no genetic evidence of any other hybridization events in modern polar bears, despite testing of individuals across the Arctic (e.g. Cronin et al. 2012). Offspring of this mating have polar bear mitochondrial DNA (mtDNA) inherited from their mother. Numerous suspected hybrids have proven to be blonde grizzlies.
- What are the barriers to hybridization in the wild? Considerable, including a mismatch in mating times but also differences in aggression between the two species. A hybrid cross can only occur between a very late breeding polar bear and a very early breeding grizzly, and it seems very few such individuals are present in any population (e.g. Smith and Aars 2015). Also, grizzlies are considerably more aggressive than polar bears, such that even very large male polar bears actively avoid contact with much smaller grizzly females outside the breeding season (Miller et al. 2015). This means that mating between a polar bear female (the less aggressive species) and a male grizzly is much more likely to occur in the wild. Polar bear males do not expect their mating partners to be as aggressive as a grizzly female is by nature, and are more likely to run than copulate.
- What do hybridization events between captive (zoo) bears tell us? Two documented hybridization events happened in zoos that involved a male polar bear X female grizzly, the opposite cross to the example documented in the wild (Davis 1950; Preuß et al. 2009). In both cases, the individual bears were housed together or lived side-by-side, giving them years to become acquainted. In one case, mating occurred only after 24 years of being housed together, suggesting it took considerable forced familiarity to break down the natural avoidance behaviour of the polar bear male to the grizzly female. Offspring of this mating have grizzly mtDNA inherited from their mother.
- Has a male polar bear ever mated with a female grizzly in the wild? Not that we know about. There has been no grizzly mtDNA found in wild polar bears (Cronin et al. 2012).
- Would rare hybridization events be easy to detect in a population genetic study of modern animals? No. Even if a hybridization event like the one documented in 2006 occurred once every hundred years or so, it is very unlikely that descendants of the individuals involved would be selected for sampling, even when hundreds of samples are used. That’s another way of saying that rare hybridization events will have no significant impact on the genetic history of a species.
- Would rare hybridization events be easy to detect from ancient samples? No. It would be even more unlikely to find mtDNA evidence of a rare hybridization event when there are only a few samples to examine. Detecting past hybridization events based on nuclear DNA analysis is a numbers game, where statistical probabilities are all you get for an answer.
- Is there another, equally plausible, explanation for finding modern polar bear DNA in some grizzly bear genetic samples? Yes. It’s called “shared common ancestry.” The simplest explanation is that both modern and ancient polar bears, and some modern grizzlies, are descendants of an ancient linage of brown bears that was once wide-spread around the world but is now extinct. That is, polar bears evolved from that ancient lineage into a new species and ABC Island brown bears that live in Southeast Alaska (Heaton et al. 1996; Talbot and Shields 1996) are an isolated relict population of that ancient lineage (Crockford 2023).
- Is ‘shared common ancestry’ a known and accepted phenomenon? Yes. Several polar bear genetic studies acknowledge it’s importance as an alternative explanation to hybridization (e.g. Cronin et al. 1991, 2014; Davison et al. 2011; Kumar et al. 2017; Kutchera et al. 2014; Kitchener et al. 2020). But this has been routinely dismissed or ignored as a viable option in favour of the hybridization scenario, especially after the recent hybridization event in Canada occurred in the early 2000s.
- Is shared common ancestry acknowledged as an alternative explanation in human/Neanderthal hybrid studies? Yes, but rarely (e.g. Erikson and Manica 2012; Meneganzin and Bernardi 2023; University of Cambridge 2012), even though shared genes from their common ancestor is the more plausible explanation. As for polar bears, the interbreeding narrative is given more weight in the human evolution field, probably because it makes for a more interesting story that’s ready-made for media promotion (e.g. Stringer and Crété 2022). What scientist isn’t thrilled to have their work splashed across the pages of newspapers world-wide, when the alternative is obscurity?
Geneticists prefer the polar bear hybridization story but that doesn’t make it the truth.
As for the human/Neanderthal example, it simply makes a more interesting story and provides a way to frame otherwise boring genetic research as something the general public will find compelling. Interspecies sex sells, whether recent or in the past!
More recently, genetic studies have been framed around a preferred narrative that falsely blames recent hybridization on human-caused climate change and concludes that even warmer Arctic temperatures, as predicted by climate models, will cause the extinction of polar bears through interbreeding with grizzlies (e.g. Lan et al. 2022; Wang et al. 2022). This scary hypothetical scenario has been repeated often by the media, the message reaches a public who would never read the jargon-heavy scientific papers (e.g. Cassella 2022; Dickie 2017; Popescu 2016; Stephens 2022; White 2022).
I contend that extinct and living ABC brown bears have a genetic similarity to modern polar bears because they are both descendants of an ancient linage of brown bears that was once wide-spread around the world, and because polar bears, having evolved only about 140kya, are an especially young species. There is no reason to invoke hybridization to explain polar bear or human evolutionary history (Crockford 2023).
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