Re: Does evolution have a rate?

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 Örstan