The causes of evolutionary change

Evolutionary changes in populations, of humans and all other organisms, depend on five factors.

The first and perhaps the most essential is mutation. Evolution depends on the fact that genetic material does not replicate precisely, and that errors are inevitably introduced as genes are passed from one generation to the next. In the absence of mutation, evolutionary change would slow and eventually stop.

The effects of mutations are not necessarily correlated with the sizes ofthe mutational changes themselves. Single changes in the base sequence of DNA can have no effect or profound effects on the phenotype - the allelic differences that affect skin colour, as discussed in Section 3.1, can be traced to a single alteration in a base from G to A, changing one amino acid in the protein from an alanine to a threonine. At the other end ofthe spectrum, entire doublings of chromosome number, which take place commonly in plants and less often in animals, can disturb development dramatically - human babies who have twice the normal number of chromosomes die soon after birth. But such doubling can sometimes have little effect on the organism.

A fascinating source of mutation-like changes has recently been discovered. Viruses and other pieces of DNA can transfer genes from one animal, plant, or bacterial species to another, a process known as horizontal gene transfer. Such transfers appear to have played little part in our own recent history, but they have been involved in the acquisition of important, new capabilities in the Past: the origin of our adaptive immune system is one remarkable example (Agrawal et al., 1998).

The most important mechanism that decides which of these mutational changes are preserved and which are lost is natural selection. We normally think of natural selection as taking place when the environment changes, But environmental change is not essential to evolution. Darwin realized that natural selection is taking place all the time. In each generation, even if the environment is unchanged, the fittest organisms are the most likely to survive and produce offspring. New mutations will continue to arise, a few of which will enable their carriers to take greater advantage of their environment even if it is not changing.

It is now realized that natural selection often acts to preserve genetic variation in populations. This type of selection, called balancing selection results from a balance of selective pressures acting on genetic variation. It comes in many forms (Garrigan and Hedrick, 2003). Heterozygote advantage preserves the harmful sickle cell allele in human populations because people who are heterozygous for the allele are better able to resist the effects of malaria. A more prevalent type of balancing selection is frequency-dependent selection, in which a mutant allele may be beneficial when it is rare but loses that benefit as it rises in frequency. Such selection has the capability of maintaining many alleles at a genetic locus in a population. It also has the intriguing property that as alleles move to their internal equilibrium frequencies, the cost of maintaining the polymorphism goes down. This evolutionary "freebie" means that many frequency-dependent polymorphisms can be maintained in a population simultaneously.

Three other factors play important but usually subordinate roles in evolutionary change: genetic recombination, the chance effects caused by genetic drift, and gene flow between populations.

Arguments have been made that these evolutionary processes are having little effect on our species at the present time (Jones, 1991). If so, this is simply because our species is experiencing a rare halcyon period in its history. During the evolutionary eye blink of the last 10,000 years, since the invention of agriculture and the rise of technology, our population has expanded dramatically. The result has been that large numbers of individuals who would otherwise have died have been able to survive and reproduce. I have argued elsewhere (Wills, 1998) and will explore later in this chapter the thesis that even this halcyon period may be largely an illusion. Powerful psychological pressures and new environmental factors (Spira and Multigner, 1998) are currently playing a major role in determining who among us reproduces.

It seems likely that this halcyon period (if it really qualifies as one) will soon come to an end. This book examines many possible scenarios for such resurgence in the strength of natural selection, and in this chapter I will examine how these scenarios might affect our future evolution.

Continue reading here: Environmental changes and evolutionary changes

Was this article helpful?

0 0