Evolutionary biogeography

The three maps above attempt to summarize much of what's relevant to understanding the current population-level genetic diversity evident among today's brown and grizzly bears--all included within the single species Ursus arctos. Most of what's presented in these maps is a result of the synthesis published by Davison and his colleagues in 2011. Parenthetically, the notion of "subspecies" has passed out of favor in application to brown bears, and been replaced instead with the concept of "clades," each of which represents enough genetic differentiation to warrant speculations about reasons for the divergence. This recent convergence by taxonomists on the notion of clades is in stark contrast to a 1918 publication of C.H. Merriam in which he described over 70 "species" of grizzly bears in North America alone. The shift has been a result of both increased reliance on information from the genome along with increased appreciation for how plastic brown bear morphology can be (morphology of the skull was the main basis for Merriam's distinctions).

 

One key to understanding the current genetic diversity of brown bears resides in understanding the distribution of this species during the last Ice Age, which was, in turn, a reflection of vegetation and climate--especially the distribution of ice sheets and exceptionally harsh polar climates. Geneticists speculate that the main explanation for current differences in genomes of bears living in different parts of Eurasia and North America has to do with where their ancestors found refuge--often in isolation--during the different Ice Ages of the Pleistocene, and how they moved and mixed during warmer intervals, especially the current warm period that led to terminal melt of ice sheets between 18,000 and 6,000 years ago.

 

With that in mind, the map in Panel A, above, shows a plausible reconstruction of ice sheets and vegetation during the height of the last glaciation, roughly 25,000-15,000 years ago. You can see that much of the main range of brown bears in Eurasia was covered by polar desert and what many have called steppe tundra--a kind of tundra with substantially more grass than is common in boggy shrub-dominated tundras of today. This abundance of grass meant that a corresponding abundance of large herbivores could live there, including horses, mammoths, rhinos, giant bison, and more. Overall, though, the main range of Eurasian brown bears (the vast majority of the brown bears alive at the time) was quite dry and bitterly cold.

 

The map in Panel B shows a speculative reconstruction of the Ice Age distributions of the various modern-day clades, each denoted by its own number (see  Evolutionary relations for more details). As you can see, Clade 1 was hunkered down in the southern part of Europe while Clade 3 was distributed throughout the steppe tundra of Asia. Clades 5 and 6, which persist as the genetically and morphologically distinct bears of the Tibetan Plateau and Gobi Desert, were thought to be more or less isolated in high-elevations of south-central Asia. Note that Clade 4 is the sole representative south of the North American ice sheet. More on where and when Clade 4 got there a little later. Also of interest, the green arrows in Panel B show how each of the clades spread and colonized during the late Pleistocene and early Holocene (our current warm period). Clade 1 moved north to occupy most of Europe, but Clade 3 was the big winner, with bears of this lineage spreading into eastern Europe and surging once again into Alaska via Beringia. Clade 3 bears also moved south in North America to mingle with Clade 4 bears moving north in what is now Alberta.

 

The final map above, in Panel C, shows (in green) how the distribution of brown bears settled out after all of the surges and retreats that occurred with warming and ice melt--by roughly 1000-2000 years ago. Of note, brown bears occurred in the Atlas Mountains of Africa as well as in Mediterranean coastal mountains of the Middle East. They also spread south into Mexico in North America and ended up retreating to an eastern boundary on this continent that aligned with the Great Plains (for more details on all of this see the section on History).

The map to the left provides a little more detail on when and how the various clades of grizzly bears represented in North America arrived. The earliest colonists were apparently of Clades 2 and 4, along with a sprig of Clade 3 called 3c. Clade 4 continued south, occupying west-central North America prior to closure of the last ice-free corridor, which some think might have happened as early as 70,000 years ago (kyBP), although with a brief opening perhaps around 55 kyBP. During this same period bears of Clade 2 (specifically, 2a) made it to the ABC (Admiralty, Baranof, Chichigof) Islands of Alaska. The early southward movement of brown bears into the region encompassing Yellowstone is evidenced by current distributions of the various clades, as well as by a single find of skeletal remains near Edmonton, Alberta, that dates to roughly 32 kyBP (corrected for bias in carbon dating).

 

Interestingly, of these early colonizing clades, 2a and 4 survived in their interior continental and island refuges whereas 2c and 3c eventually disappeared.

 

The final colonists, all across Beringia into and via Alaska, were of Clade 3. Clade 3b comprised an earlier wave of colonization that occurred perhaps during and immediately after the last glacial maximum, whereas Clade 3a represents the most recent and last wave of migrants, arriving just prior to when the land bridge of Beringia disappeared. (Remember from the Map in Panel B above, Clade 3a bears had farther to go compared to Clade 3b bears before reaching Beringia.) Modern-day grizzly bears in eastern Alaska consist of descendants of the Clade 3b colonists, whereas those in western Alaska consist of descendants of the Clade 3a new-comers.