Ecological Release: Grizzlies and North American Indians

How I wondered, did such large animals diversify so quickly? The answer lies not only in the richness of the continent but in the 'ecological release' experienced by the bears in their new homes. With few competitors and a huge variety of resources they quickly diversified and adapted to local conditions. Indians of course do not represent incipient new subspecies as grizzlies do, but for millennia before 1492 they were adapting through cultural change to local conditions even more rapidly than grizzlies were through natural selection. As a result both Indians and grizzlies have developed exceptional diversity in a very short time.

Whenever a species arrives in a new habitat, a series of evolutionary forces come into playthat have a dramatic effect upon it. The nature of a founding population has a considerable effect on the process. Such populations are never truly representative of the population from which the are drawn, and this leads to a founder effect. It may have been, for example, that there were no expert bow and arrow makers among the first people to cross into North America and this craft my have been lost to the New World through this founder effect. [Surely this is correct, but even if one subscribes to a Clovis first theory of the population of North America, the oldest evidence of the bow anywhere that I know of, from the Mesolithic Natufian complex in the Middle East, and is from at least a millenium after the the latest the Clovis people were in the Americas.]

If the new homeland that a species invades is essentially an open field, with few competitors but lots of food, then the species goes through a period of ecological release. For creatures such as birds this can result in a population that is more varied (in beak shape, leg length or size) than the parent population. This happens because in the absence of competition almost all variants can make a living [and find mates] of some sort. This phase of the evolutionary process is typically brief, occurring over a few decades or centuries. Then as the open field is filled, individuals in the variable population begin to be selected for various traits and begin to adapt to local conditions. Long beaks and long legs may be favored in one environment and short beaks and short legs, or some combination of both, in another.

Thus begins the long and final phase of the adaptive process that makes species. Known as evolution by natural selection, the process can be thought of as a great centrifuge, throwing apart the geographically separated portions of a once similar people or species, creating diversity out of uniformity. The richer and more diverse the environment is, the faster the centrifuge can be thought of as turning. The centrifugal force is felt most strongly after the closing of the frontier, and for organisms like large mammals its results via natural selection are evident only after thousands or tens of thousands of years. For humans, however, who adapt through learned cultures, the process can happen swiftly.

From The Eternal Frontier. by Tim Flannery (William Heinemann: London, 2001) pp. 237-238