6.2.1: Loss of genetic diversity: genetic drift

Alleles can be lost from the population by coincidence. One reason for allele loss can be that not all animals mate and produce offspring, irrespective of selection decisions. Because animals that we selected for breeding may not all manage to produce offspring. Some may die unexpectedly or some are just never mated (e.g. in dogs and horses where owners of superior animals are not always interested to breed with them). Consequence of this not producing offspring despite being selection candidates does influence the allele frequencies in the offspring generation and alleles that were present at low frequency may be lost.

Example of genetic drift

In figure below is an example of how genetic drift could work. It shows how in a small duck population an allele could be lost without any directional selection. Purely by coincidence not all animals managed to reproduce, and also purely by coincidence, the carriers of the red allele not always passed it on to their offspring. Within four generations the red allele was lost from the population. Of course this is an example. The blue allele could also have been lost, or the frequencies may have just fluctuated a bit. The principle that allele frequencies change and homozygosity can increase purely by coincidence is very realistic and is called genetic drift.

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Figure 2. An example of genetic drift in four generations (columns) of a duck population. In generation 1 the red and blue alleles occur at equal frequencies. Duck 1 and 5 do not reproduce (e.g. they may have died earlier or did not find a mate). Duck 2 had the most offspring. Duck 2 could only pass allele blue on to the offspring, ducks 3 and 4 could pass on both colours, and duck 6 could only pass on the red allele. However, of the heterozygous ducks only duck 4 passed on the red allele and only once. In generation 2, the frequency of the blue allele has increased to 8/12 = 2/3. Again not all ducks managed to reproduce, and in generation 3 the frequency of the red allele decreased to 2/12 = 1/6. The red allele, by coincidence, was not passed on to generation 4. The population became homozygous blue.

Consequences of genetic drift

Genetic drift causes changes in allele frequencies, resulting in increase in one frequency at the expense of another. Because of that, it is more likely that animals become homozygous, especially for the most frequent allele. So a loss in genetic diversity at population level has consequences for genetic diversity at individual level. Animals become more alike. Even though they are not closely related through their pedigree, they become more closely related genetically. Genetic drift thus increases relatedness between animals and leads to fixation of alleles in a population.