Drift occurs because the genetic variants, or alleles, present in a population are a random sample of the alleles that adults in the previous generation would have been predicted to pass on, where predictions are based on expected migration rates, expected mutation rates, and the direct effects of alleles on fitness.
If this sample is small, then the genetic composition of the offspring population may deviate substantially from expectation, just by chance. This deviation is called genetic drift.
Drift becomes increasingly important as population size decreases. The key feature of drift that distinguishes it from the other evolutionary forces is the unpredictable direction of evolutionary change.
Anything that generates fitness variation among individuals (that is, variation in the ability of individuals to survive and reproduce) will increase the magnitude of drift for all genes that do not themselves cause the fitness variation. Because of their indeterminate growth, plants often vary greatly in reproductive potential because of local environmental variation, and this magnifies genetic drift.
For example, the magnitude of drift inmost annual plants is more than doubled by size variation among adults. This makes sense if one considers that larger individuals contribute a larger number of offspring to the next generation, so any alleles they carry will tend to be over-represented.
Fitness variation caused by selection will also increase the magnitude of drift at any gene not directly acted upon by the selection. If an individual has high fitness because it possesses one or more favorable alleles, then all other alleles it possesses will benefit. This is called genetic hitchhiking.
This is a potent source of evolution because the direction of change at a hitchhiking gene will remain the same for multiple generations. However, it is not possible to predict in advance what that direction will be because where and when a favorable mutation will occur cannot be predicted.
The opportunities for drift to occur are greatly influenced by gene flow. Most terrestrial plants are characterized by highly localized dispersal.
Thus, even in large, continuous populations, the pool of potential mates for an individual, and the pool of seeds that compete for establishment at a site, are all drawn from a small number of nearby individuals known as the neighborhood. If the neighborhood is sufficiently small, genetic drift will have a significant impact on its genetic composition.
For these and other reasons, population size alone is not sufficient to predict the magnitude of drift. The effective size of a population, Ne, is a number that is directly related to the magnitude of drift through a simple equation. Thus,Ne incorporates all characteristics of a population that influence drift.
Loss of Variability
The long-term consequence of drift is a loss of genetic variation. As alleles increase and decrease in frequency at random, some will be lost. In the absence of mutation and migration, such losses are permanent.
|Loss of Variability|
Eventually, only one allele remains at each gene, which is said to be fixed. Thus, all else being equal, smaller populations are expected to harbor less genetic variation than larger populations.
An important way in which different plant populations are not equal is in their reproductive systems. With self-fertilization (selfing), or asexual reproduction, genetic hitchhiking becomes very important. In the extreme cases of 100 percent selfing or 100 percent asexual reproduction, hitch-hiking will determine the fates of most alleles.
Thus, as a new mutation spreads or is eliminated by selection, so too will most or all of the other alleles carried by the individual in which the mutation first arose. This is called a selective sweep, and the result is a significant reduction in genetic variation.
Which alleles will be swept to fixation or elimination cannot be predicted in advance, so the loss of variation reflects a small Ne. Consistent with this expectation, most populations of flowering plants that reproduce partly or entirely by selfing contain significantly less genetic variation than populations of related species that do not self-fertilize.
Mutations that decrease fitness greatly outnumber mutations that increase fitness. In a large population in which drift is weak, selection prevents most such mutations from becoming common. In very small populations, however, alleles that decrease fitness can drift to fixation, causing a decrease in average fitness. This is one manifestation of a phenomenon called inbreeding depression.
In populations with very small Ne, this inbreeding depression can be significant enough to threaten the population with extinction. If a population remains small for many generations, mean fitness will continue to decline as new mutations become fixed by drift.
When fitness declines to the point where offspring are no longer overproduced, population size will decrease further. Drift then becomes stronger, mutations are fixed faster, and the population heads down an accelerating trajectory toward extinction. This is called mutational meltdown.
By itself, drift cannot lead to adaptation. However, drift can enhance the ability of selection to do so. Because of diploidy and sexual recombination, some types of mutations, either singly or in combinations, will increase fitness when common but not when rare. Genetic drift can cause such genetic variants to become sufficiently common for selection to promote their fixation.
A likely example is the fixation of new structural arrangements of chromosomes that occurred frequently during the diversification of flowering plants. New chromosome arrangements are usually selected against when they are rare because they disrupt meiosis and reduce fertility.
The initial spread of such a mutation can therefore only be caused by strong genetic drift, either in an isolated population of small effective size or in a larger population divided into small neighborhoods.