Small Population Size - Genetic Consequences

Genetic Consequences

Conservationists are often worried about a loss of genetic variation in small populations. There are two types of genetic variation that are important when dealing with small populations.

• The degree of homozygosity within individuals in a population; i.e. the proportion of an individual's loci that contain homozygous rather than heterozygous alleles. Many deleterious alleles are only harmful in the homozygous form.
• The degree of monomorphism/polymorphism within a population; i.e. how many different alleles of the same gene exist in the gene pool of a population. Polymorphism may be particularly important at loci involved in the immune response.

There are two mechanisms operating in small populations that influence these two types of genetic variation.

• Genetic drift — Genetic variation is determined by the joint action of natural selection and genetic drift (chance). In small populations the relative importance of genetic drift (chance) is higher; deleterious alleles can become more frequent and 'fixed' in a population due to chance. Any allele, deleterious, beneficial or neutral is more likely to be lost from a small population (gene pool) than a large one. This results in a reduction in the number of forms of alleles in a small population and in extreme cases to monomorphism where there is only one form of the allele. Continued fixation of deleterious alleles in small populations is called Muller's ratchet, and can lead to mutational meltdown.
• Inbreeding — In a small population, related individuals are more likely to breed together. The offspring of related parents have a far higher number of homozygous loci than the offspring of unrelated parents.

There are two types of potential consequence of loss of genetic variation:

• Inbreeding depression — Inbreeding depression is usually taken to mean any immediate harmful effect, on individuals or the population, of a decrease in either type of genetic variation. Inbreeding depression can almost never be found in declining populations that were not very large to begin with; it is somewhat common in large populations becoming small though. The reason is purging selection, most efficient in populations that are strongly but not dangerously inbred.
• The ability of the population to adapt/evolve to changing conditions, “without variability evolution is impossible”. It is obvious that the absolute size of a population limits the absolute degree of allelic diversity. On the other hand, should an advantageous mutation arise, it is likely to show its effect sooner and more thoroughly.

The effective population size is commonly lower than the actual population size.