Macroevolution and accumulated microevolution
People arguing with creationists sometimes claim that macroevolution (changes in the large) is just the accumulation of small changes (microevolution). The claim and the thought behind the claim is superficial; for the most part small changes do not accumulate.
The big thing that stops small changes from accumulating is natural selection. Creationists often (correctly) argue that natural selection is conservative. There are really two arguments that they use, one incorrect and one correct. The incorrect one is to appeal to examples from artificial selection; continued selective breeding for improvements in a trait quickly reaches an upper limit. The reason for this is well known; the extant variation in the population is exhausted.
The correct one (insofar as it goes) is that divergences from the population mean are selected against in natural selection. In general the species of life are fit, i.e., they are adapted to their environment, their ecological niche, and their reproductive strategy. In consequence populations exist in basins of attraction; divergences from the population norm are selected against.
At this point it is natural to point out that environments change regularly and, with it, fitness. It would seem that adaptation would therefore track the changes in environment. However matters are not that simple; one has to consider time scales. Rapid changes in the environment cannot be tracked as such. The Galapagos finches are a good example. In wet years finches with one beak shape are favored; in drought years another beak shape is favored. As the climate shifts back and forth between wet and dry the relative proportions of different beak shapes in the population shift also. The adaptive response of the species is to maintain variability with respect to that feature in the populations. The upshot is that although the gene frequencies of the relevant alleles fluctuate the fluctations are bounded over time. Change back and forth does not (necessarily) produce cumulative change.
Slow change in the environment, on the other hand, can be met with migration (or extinction.) The response to long term climate changes is typically that species change their ranges. Changing ranges is a more successful response to environmental change than staying still because useful mutations are, after all, usually not forthcoming.
Now all of this is, as I am confident that my readers are aware, a quick run through of the standard arguments made in punctuated equilibrium theory. It is, or is purported to be, a fact that paleontological species last for periods of time that are significant on a geological scale, e.g., 1-20 millions of years. In other words stability in the perceptible phenotype is the norm.
On the other hand it also observable that extant species diverge across their ranges and can diverge quite rapidly. The pace at which evolution could operate is enormously greater than the pace at which it does operate when averaged over time.
It should be clear that "changes" here refers to heritable changes in the perceptible phenotype and not to all genetic change. It is fairly clear that neutral changes cumulate; the molecular clock ticks.
To put it baldly, we do not know why species are stable for long periods of time or why they successfully diverge rapidly when they do. The truth is that we do not understand how evolution works in the large. To say that macroevolution is cumulated microevolution, even it is true genetically, is a fairly useless thing to say because it does not capture the determinants of the process.
This page was last updated June 1, 2005.