Changing the Guard: Extinction and Migration in Ice Age America — The Extinctions

The final species to be investigated is the grey wolf (Canis lupus). Grey wolves are known from North America since the Middle Pleistocene (Irvingtonian, 600-250kya) and there is no paucity in their fossil record (32). Despite this, considerable evidence suggests that the current population may be derived from a recent arrival, perhaps at the end of the Pleistocene.

The first line of evidence that could suggest a recent colonisation are the morphological differences between Pleistocene and modern grey wolf populations. Several Late Pleistocene specimens exhibit traits associated with megafauna hunting such as enlarged carnassials and larger skulls (18). This morphotype, commonly called the ‘Beringian wolf’ has been discovered in North America on both sides of the CLIS (24). However, populations of grey wolf similar in skull morphology to modern day animals are also known from Late Pleistocene continental North America (18). The relationship of these wolves with the modern populations is yet untested. Remains of the Beringian wolf are not known to have co-existed with the smaller contemporary morphotypes (8, 18, 24).

Further elucidating the distinction between modern grey wolves and their Pleistocene counterparts is limb morphology. Simply put, modern North American wolves are significantly more cursorial than specimens known from the Pleistocene (33). This trait, however, does not appear until well into the Holocene epoch, nor is it entirely ubiquitous within modern American wolves as both the Mexican wolf (Canis lupus baileyi) and the extirpated Great Plains wolf (Canis lupus nubilus) are short-legged (33). Limited stock can then be placed in morphology to track the movement of modern wolves into America, instead phylogenetics must be relied on.

A 2010 study found that modern North American Grey wolves likely originated from three distinct waves from Eurasia. A timeframe is not provided, though it is probably best explained as various interglacials. The Mexican wolf is suggested to originate from an initial dispersal due to its basal position (35), Great Plains wolves from a second wave and Mackenzie valley wolves (Canis lupus occidentalis) from a recent colonisation – Possibly at the onset of the interglacial (35).

More recent genetic analyses of dogs and wolves suggest that all modern wolves underwent a recent genetic bottleneck during the Late Pleistocene (7, 11, 14, 20, 31). This bottleneck is variously dated at 20-45kya (11, 14, 20), if so, several colonisation events are unlikely given the narrow corridor of dispersal.

An extensive study was carried out by Loog et al in 2019 with extensive sampling of up to 50ky old aDNA. The study pinned down the location of the bottleneck to Beringia and estimated it as occurring 24kya. If true, modern North American wolves must have been unable to reach the mid-continent until after the emergence of the ice-free corridor (20, 27). An alternative scenario is presented by Koblmuller et al 2016 which dated the split between North American and Eurasian populations sometime before 30kya, suggesting a colonisation event prior to the LGM (7, 14).

The sequencing of two Pleistocene wolves from the Late Pleistocene of Western Beringia (14.1kya & 16.8kya respectively), suggested that they may cluster with modern North American wolves, rather than Eurasian (28), this genetic proximity could indicate a colonisation event close in time to the two specimens.

Resolving the interrelationships of wolves and quantifying when present populations diverged is exceedingly difficult due to interbreeding between grey wolves and other canids – particularly golden jackals (Canis aureus) in Eurasia, and coyotes (Canis latrans) in America (28).

Throwing further complication into the equation is the Mexican wolf, which does not cluster with other North American wolves in either the study by Koblmuller et al 2016 or Loog et al 2019. A more recent 2021 paper by Wilson & Rutledge found that Mexican wolves in fact cluster closely to the extinct Beringian wolves, suggesting a degree of interbreeding during the Late Pleistocene (36). Caution should certainly be taken in interpreting these results, because the bottleneck is so recent few genetic differences have accumulated and so these phylogenetic analyses are sensitive to not just interbreeding but also high rates of evolution driven by natural selection or drift (14).

Nevertheless, the Mexican wolf adds a huge question mark to the narrative of a single migration at the end of the Pleistocene, and future studies will need to clear up the confusion surrounding this sub-species before a clear picture can emerge. If the Mexican wolf constitutes a separate colonisation or a hybridisation event with incumbent populations, then it becomes fairly evident that other American wolves are descended from a single migration, either before the LGM or at the very end of the Pleistocene.