In The Origin of Species, Charles Darwin (who is, according to Steve Gould, the first great methodologist of historical science) based his one long argument for evolution on a consilience of inductions comprising: 1. artificial selection (e.g., breeding (genetic engineering) of homing pigeons, dogs, etc.), 2. natural variation (finches of the Galapogos Isles), 3. fossils, 4. biogeography, and 5. comparative anatomy. I will illustrate the fossil record by using the case of the Polar Bear.
Sometime during the mid-Pleistocene period (roughly 100,000 to 250,000 years ago), a number of brown(same as grizzly) bears (Ursos arctos) probably became isolated by glaciers. Many probably perished on the ice; however, they apparently did not all disappear. Some survived due to the fact that "organisms vary" (Gould, 1977); that is, every litter of grizzlies has a variation in coat thickness, coat color, etc., which imparted a slight evolutionary advantage to some individuals of each litter. Successive, successful individuals repeated this simple process, yielding a rapid series of evolutionary changes (driven, presumably, by the combination of small population, and extreme selection pressure) in order to survive. Note that these new variants were not necessarily "better" in any absolute sense, or on any absolute "bear" scale of perfection: they were simply more in keeping with their new environment than their immediate ancestors or their more unfortunate siblings. Today, polar bears are adapted to their harsh northern environment.
Hecht (in Chaline, 1983) describes polar bear evolution: the first "polar bear", Ursus maritimus tyrannus, was essentially a brown bear subspecies, with brown bear dimensions and brown bear teeth. Over the next 20,000 years, body size reduced and the skull elongated. As late as 10,000 years ago, polar bears still had a high frequency of brown-bear-type molars. Only recently have they developed polar-bear-type teeth.
Kurten (1976) describes bear transitions: "From the early Ursus minimus of 5 million years ago to the late Pleistocene cave bear, there is a perfectly complete evolutionary sequence without any real gaps. The transition is slow and gradual throughout, and it is quite difficult to say where one species ends and the next begins. Where should we draw the boundary between U. minimus and U. etruscus, or between U. savini and U. spelaeus? The history of the cave bear becomes a demonstration of evolution, not as a hypothesis or theory but as a simple fact of record." He adds, "In this respect the cave bear's history is far from unique."
Ursus minimus (Pliocene) First little bear, with very bearlike molars, but still had the first premolars and slender canines: shows gradual tooth changes and increase in body size as the ice age approached. Gave rise to the modern black bears (U. americanus & U. thibetanus), which haven't changed much since the Pliocene, and also smoothly evolved to the next species, U. etruscus: Ursus etruscus (late Pliocene) A larger bear, similar to our brown bear but with more primitive Dentition: molars large & square; canines were stouter. In Europe, they gradually evolved into...
Ursus savini (late Pleistocene, 1 Ma) Was very similar to the brown bear. Some individuals didn't have the first premolars at all, while others had little vestigial premolars. Had tendency toward domed forehead; slowly split into a European population and an Asian population.
U. spelaeus (late Pleistocene) The recently extinct giant cave bear, with a highly domed forehead. Clearly derived from the European population of U. savini, in a smooth transition. The species boundary is arbitrarily set at about 300,000 years ago.
U. arctos (late Pleistocene) The brown ("grizzly") bear, clearly derived from the Asian population of U. savini about 800,000 years ago; spread into Europe, to the New World.
U. maritimus (late Pleistocene) The polar bear. Very similar to a local population of brown bear, U. arctos beringianus, which lived in Kamchatka about 500,000 years ago (Kurten 1964).
Revised 2011. If you want to know about my main areas of research, go here