CHAPTER 5.
GENETIC DRIFT

Genetic drift is the second important genetic concept that is a function of N_e. Genetic drift is random changes in gene frequency; it is a major factor in evolution and population biology, but most aquaculturists have not heard of it. However, like inbreeding, it too must be considered if hatchery populations are to be properly managed.

Genetic drift can be as important as selection in altering a population's gene pool. The effects of genetic drift can be devastating. Genetic drift can irreversibly alter gene frequencies and eliminate alleles, which can decrease a population's ability to survive or to adapt to an altered environment, and it can preclude future selection. The effect that genetic drift can have on a population's gene pool can make many management goals impossible to achieve.

Those who do not wish to learn about genetic drift, how it is measured, and the effects it has on hatchery populations can skip this chapter and go to: Chapter 6, which describes how inbreeding programmes can be used to improve populations; Chapter 7, which describes techniques that can be used to manage N_e to prevent inbreeding from reaching detrimental levels and which can prevent genetic drift from eliminating rare alleles; Chapter 8, which prescribes N_e's that are needed to prevent inbreeding - and genetic drift-related problems.

EFFECTIVE BREEDING NUMBER AND GENETIC DRIFT

Genetic drift is random changes in gene frequency that occur because of sampling error. Sampling error can be natural, or it can be manmade. Natural sampling errors are those which occur when earthquakes, floods, landslides, or other natural disasters subdivide a population and isolate small groups of organisms. This process is a major force in the evolution of new species. Manmade sampling errors are inaccurate collections: sampling only a portion of the population, sampling only a single age class, sampling only fish that possess a certain phenotype or that spawn on a particular day, etc.

When a population is sampled, there is a chance that the sample does not accurately reflect the make-up of the population. This inaccuracy can include length, sex ratio, body colour, and gene frequencies. The smaller the sample, the greater the likelihood that inaccuracies in the sample will occur. Changes in gene frequency that occur as a result of sampling error are called “genetic drift.”

When culturing fish, the important changes that can occur in gene frequency as a result of genetic drift occur during the creation of the next generation (during spawning season) or during the acquisition of the population. This is when the genes are transferred from parents to offspring (the transfer of genes across time; from one generation to the next) or when they are transferred from one hatchery to another (the transfer of genes across space). As was the case with inbreeding, N_e is the factor that determines the magnitude of genetic drift. The relationship between N_e and genetic drift is:

where: σ²_Δq is the variance in the change of gene frequency, and p and q are the frequencies of alleles p and q for a given gene. The variance of the change in gene frequency is the way genetic drift is measured.

Like average inbreeding value, genetic drift is also inversely related to N_e. A comparison of the two formulae shows that they are similar. This means that large N_e's produce small changes in gene frequency, while small N_e's produce large changes in gene frequency.

Many factors can cause changes in gene frequency; among them are: selection by the farmer or hatchery manager; domestication (selection caused by the hatchery and the techniques used to culture the fish); genetic drift. The changes in gene frequency caused by selection and domestication usually produce genetic improvements, in that the fish grow faster, are more disease resistant, are calmer, accept pelleted feed more readily, have more efficient feed conversions, and are easier to spawn. In contrast, the changes in gene frequency that are caused by genetic drift are random, which means they can be counterproductive.

If the frequency of an allele changes from, say, 0.5 to 0.45 or from 0.4 to 0.3 as a result of genetic drift, the genetic effects on the population might not be that great. The major damage that is caused by genetic drift occurs when the frequency of an allele goes to 0.0; i.e., the allele is lost and no longer exists in the population. The odds of losing alleles via genetic drift are related to their frequencies; i.e., rare alleles (the frequency is low; usually ≤ 0.01) are lost more easily than common ones.

The probability of losing an allele by genetic drift is determined by using the following formula:

P = (1.0 - q)^2N_e

where: P is the probability of losing an allele, and q is the frequency of the allele.

For example, if N_e is 50, the probability of losing an allele whose frequency is 0.1 (q = 0.1) is:

P = (1.0 - 0.1)²⁽⁵⁰⁾

P = (0.9)¹⁰⁰

P = 0.0000265

If N_e is 10, the probability of losing the same allele is:

P = (1.0 - 0.1)²⁽¹⁰⁾

P = (0.9)²⁰

P = 0.12158

These examples clearly show that the probability of losing an allele is inversely related to N_e; the probability of losing the allele was 4,588 times greater when N_e decreased from 50 to 10.

Table 2 lists the probabilities of losing alleles of various frequencies via genetic drift for N_e's that range from 2 to 6,880. For example, an N_e of 10 produces a probability of losing an allele of P = 0.000001 (1 × 10^-6) for an allele whose frequency is 0.5. This means that there is a virtual guarantee (99.9999% chance) of keeping an allele whose frequency is 0.5 (guarantee of keeping the allele = 1.0 - the probability of losing the allele). On the other hand, the same N_e produces a probability of losing an allele of P = 0.81791 for an allele whose frequency is 0.01; the guarantee of keeping such an allele with an N_e of 10 is only 18.2%. These values demonstrate that rare alleles are more likely to be lost than common ones when N_e is small.

Table 2. Probabilities of losing an allele via genetic drift for eight allelic frequencies at various effective breeding numbers (N_e). These probabilities are for a single event (spawning season or acquisition of brood stock). The guarantee of keeping the allele is: 1.0 - the probability of losing the allele. Once the probability of losing an allele reaches 1 × 10^-6 (a 99.9999% guarantee of keeping it), no further probabilities are listed.

From: Tave, D. 1993. Genetics for Fish Hatchery Managers, 2nd ed. Van Nostrand Reinhold, New York, New York, USA.

The probabilities listed in Table 2 are for a single generation (single spawning season with complete brood stock replacement or the acquisition of brood stock). Additionally, they are the probability for losing a single allele of the frequencies that are listed. Consequently, if there is a 20% chance of losing an allele of a given frequency, 20% of all alleles with that frequency will probably be lost while 80% will remain, although the frequencies will probably be different as a result of genetic drift.

The loss of alleles via genetic drift has two effects: First, it increases homozygosity; consequently, it has an effect similar to that seen for inbreeding. The simultaneous effect of an increase in inbreeding and the loss of alleles via genetic drift as a result of a decrease in N_e can cause severe genetic problems. Secondly, the loss of alleles reduces genetic variance. Genetic variance is the raw material with which selection works. A reduction in genetic variance can make selection difficult, if not impractical. If there is no genetic variance, there will be no heritable differences, which means that selection cannot improve a phenotype. Equally important, if the population is being cultured for stocking lakes and rivers, the loss of genetic variance may doom the project to failure. Natural populations need broad gene pools (i.e., they need as much genetic variance as possible), because it is impossible to predict what genotypes and what alleles will be needed to ensure survival. Populations with narrow genetic bases are less likely to survive in the long term.

The problems caused by genetic drift are measurable, and genetic drift has been shown to have damaged the gene pool of several hatchery populations. Research has shown that brood stock acquisition drastically altered gene frequencies, even when measures were taken to prevent genetic drift. Genetic drift has been shown to have robbed an important aquacultured population of Nile tilapia of so many alleles that there was no detectable heterozygosity and there was virtually no heritable variance for growth. Finally, genetic drift-induced changes in gene frequency may be a major reason why many stocking programmes have been unsuccessful.

Techniques that can be used to manage N_e to prevent the loss of alleles are described in Chapter 7.

CONCLUSION

The genetic stability of a closed hatchery population depends on the population's N_e. As was the case with average inbreeding, genetic drift is inversely related to N_e. Consequently, small hatchery populations can cause random changes in gene frequency. The ultimate effect of a small N_e is the loss of alleles via genetic drift. Rare alleles will be lost more easily, but common alleles can also be lost. The loss of genetic variance can produce irreversible damage to a population's gene pool. This loss can prevent future improvements via selection, and it can reduce viability in populations that are stocked in lakes and rivers.