|MadSci Network: Evolution|
In the fossil record there are many examples of rapid morphological changes in a series of species, followed by a long period during which little or no change takes place. Of course, relative to the geological time scale, a "rapid change" may still have taken a million or more years. It is usually assumed that rapid evolutionary events take place when the environment is changing. The environment of a species includes both the physical components that may affect the species, for example, the climate and the soil, and the biological components, for example, the predators, the parasites, the competitors and the food the species consumes. So, the rate of evolution of a population of a species can increase if there is a significant and persistent change in its environment. But can that rate increase if the environment is stable? Probably not. Once a species evolves following a period of rapid change, it may enter a period during which it exists in a more or less stable relationship with its environment. During that period, natural selection may act to preserve rather than change the species. However, such a population could still evolve due to a process known as random drift. Random drift (or genetic drift) refers to the random changes (as opposed to changes caused by natural selection) in gene frequencies between generations. Coin tossing provides a rough analogy. If you toss a coin 1000 times, you expect to get 500 heads and 500 tails. Actually that's what you would get on the average. In any series of 1000 tosses, you may actually end up with, for example 560 heads and 440 tails. If you toss a coin only 10 times, a large deviation from the expected average becomes more likely. Likewise, the gene frequencies in a small population may change simply by chance. And if two populations of a species cannot exchange genes, because they are physically separated from each other, they may evolve in different directions as a result of random drift of their respective gene frequencies even when their environments are identical and stable. But in a stable environment random drift could not suddenly increase the rate of evolution. There is an introductory discussion of the rate of evolution in chapter 17 of John Maynard Smith's classic The Theory of Evolution (first published in 1958 by Penguin Books, revised and reprinted many times since). You can find a more detailed and technical discussion in chapter 20 titled Rates of Evolution in Mark Ridley's textbook Evolution (second edition, Blackwell, 1996). Aydin Írstan
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