There's a host of interrelated reasons. I'll enumerate a few. 1) Large animals can move longer distances and overcomelarger natural barriers than small ones. A salamander might be stymied by a mile of arid foothills. A deer can easily make it over the hills to new environs. This means that a large animal can travel over a larger percentage of the species total range than a small one. Example: Eastern Leopard Frogs normally stick to the pond where they were born. Bobcats wander hundreds of miles from their origin. This means that the mates of large animals tend to be from a less-restricted area than the mates of small animals. A leopard frog is likely to mate with another frog from his own pond, tending to inbreed and exaggerate differences from other ponds. A bobcat from southern Utah may well mate with a bobcat from northern Utah, combining gene pools and tending to homogenize the species.
2) Large animals breed more slowly than small animals. Because of this, large animals tend to not change much over time. Small animals evolve more swiftly. For instance, there is a different species of mouse unique to each of the Shetland Islands. It is believed that these mice are derived from ordinary house mice left there by Vikings who landed there. In only a few centuries, the house mouse has evolved into entirely different species on each island.
3) Large animals have fewer members of their species than small animals. Hence, the bell-curve of variability has fewer critters out on the extreme ends than do smaller animals. This means large animals are less likely to speciate than those smaller.
4) Large animals are generally less specialized than small animals. A wolf can eat anything from grasshoppers to moose. A lacewing feeds almost exclusively on aphids. Because of this lack of specialization, large animals tend to not get crammed into narrow ecological niches (with a few remarkable exceptions, such as anteaters and humans). This tends to keep them more generalized in appearance, whereas you'll find one type of rodent in a grassy field, another in a forest, yet another on a hillside, etc.
>So what did the pleistocene nearctic look like?
Horses, lions, bison, rhinos, deer, wolves, bear, camel, large piglike animals, (i.e., high llamas). A lot like the present nearctic, BUT with the additional of megafauna -- mammoths, sabertooths, etc. The overlap is not complete. I have yet to see evidence that ground sloths or titanotheres exist in Genertela.
Sandy P.
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