Evolution

and the biogeography of evolution

 

Evolution is a potentially all-encompassing topic that covers not only the descent and relatedness of species and other genetic variants, but also how they came to exhibit the distinctive form and function that reflect a unique niche. Essentially all of the ecological and biological disciplines are relevant to elucidating and explaining evolutionary outcomes and dynamics, including morphology, physiology, behavior, genetics, taxonomy, and phylogeny. All of these topics are explored on this web site with the intent of clarifying what makes grizzly bears unique, when they came to be that way, and why. The approach is necessarily comparative given that a defining notion of evolution is "niche," and that the emergence and maintenance of a niche is necessarily understood in context of other niches and the taxonomic entities that occupy them.

 

Insofar as brown and grizzly bears are concerned (all of which are part of the species Ursus arctos), it's not too hard to describe features that make them and other bears truly unique. First is their large size, which by itself engenders all sorts of ecological and biological consequences--including a reduced reproductive rate. They are, as a group, the largest terrestrial carnivores around. Second is their obesity and related ability to accumulate fat stores. These stores provide bears with an unmatched ability to survive long periods of privation. This ability directly undergirds a third distinctive feature, which is a version of hibernation that allows them to maintain the health of muscles, bones, kidneys, and other organs while being essentially immobile and without sustenance in a den for (in the case of grizzly bears) 4-6 months of every year. The physiological processes associated with hibernation in bears are truly miraculous and as yet poorly understood in terms of root causal mechanisms. Hibernation is also closely intertwined with a fourth distinctive feature, which is the comparatively small poorly-developed young that females give birth to, possible only in the seclusion of a den. Finally, bears have the most flexible forelegs and paws of any large carnivore, which means that they have a well-developed ability to extract and manipulate foods with their paws. These flexuous forelimbs and paws are also powered by exceptionally powerful muscles. The muscles surmounting and surrounding the scapula of brown bears are particularly well-developed, hence their distinctive hump. As a complement, grizzlies also have claws uniquely configured to facilitate digging. The price of flexibility is the greater energetic costs of movement for bears and their comparatively lesser maximum running speed. This means that, although they can efficiently access all sorts of foods, including roots, rodents, spawning fish, insects, and berries, they are unable to pursue and capture most prey over any sort of distance. 

With this as an introduction, the scope of this section on Evolution is actually quite limited. Here I only attempt to describe patterns of relationship and descent among bears and the various subtaxa of brown bears, along with some biogeographical phenomena of the last Ice Age that help explain the emergence and varied distributions of different brown bear clades (more on clades can be found under Evolutionary relations). The many facets of the grizzly bear niche--all of relevance to an evolutionary perspective--are dealt with in other sections of this site, including Morphology, Physiology, History, and Foods