Section 6 Maintenance of Genetic Diversity Levels of genetic diversity result from the joint impacts of: Mutation & migration adding variation Chance & directional selection removing variation Balancing selection impeding its loss The balance between these factors depends strongly on population size and differs across characters.

Conservation biologists need to understand how genetic diversity is maintained through natural processes if conservation programs are to be designed for its maintenance in managed populations. Maintenance of extensive genetic diversity in natural populations is one of the most important, largely unresolved, questions of evolutionary genetics.

The balance of forces maintaining genetic diversity differs between large and small populations. Selection has a major impact in large populations. However, its impacts are greatly reduced in small populations where genetic drift has an increasingly important role.

Five major points about genetic diversity in small populations: Genetic drift fixes alleles more rapidly in smaller populations. Loci subjected to weak selection in larger populations approach effectively neutral in small populations. Mutation-selection equilibria are lower in smaller than larger populations.

The effects of balanced polymorphisms depends upon the equilibrium frequency; the frequency of fixation of intermediate frequency alleles is retarded, but balancing selection accelerates fixation of low frequency alleles. Balancing selection can retard loss of genetic diversity, but it does not prevent it in small populations. The consequence of these effects is that genetic diversity in small populations is lower for both neutral alleles and those subjected to balancing selection

Thus far, we have examined the origin, extent, and fate of genetic variation and explored the evolutionary forces that influence genetic diversity and contrasted the importance in small versus large populations.

Major Conclusions thus far Major Conclusions thus far: Genetic diversity provides the raw material for evolutionary adaptive change. Mutation is the ultimate source of ALL genetic variation. Mutation and migration from conspecific populations or closely related species are the only mechanisms for restoring lost genetic diversity.

Genetic diversity can be estimated by a number of laboratory techniques. Heterozygosity is the most useful parameter to estimate as it can be compared across species for single locus variation and is directly correlated with additive genetic variance for quantitative traits. Some additive genetic variation is maintained within populations by balancing selection

The influence of the deterministic forces of natural selection is directly related to population size. The fate of alleles in most small populations of endangered species is predominated by random factors. Inbreeding, with consequent loss of fitness, becomes inevitable in small populations. Effective population size (N e ) as opposed to census size, determines loss of genetic diversity and inbreeding.

Effects of Sustained Population Size Reduction on Genetic Diversity Five mechanisms by which genetic diversity is lost: Extinction of species & populations relatively uncommon (relatively uncommon) Fixation of favorable alleles by selection relatively uncommon (relatively uncommon) Selective removal of deleterious alleles

Random loss of alleles by inter-generational sampling in small populations. Inbreeding within populations reducing heterozygosity.

Severe population bottlenecks are uncommon. More modest population size reductions are a regular feature of threatened species. The major significance of small population size to genetic diversity is the constant loss of genetic diversity over many generations.

Illinois population of greater prairie chickens dwindled from several million to fewer than 50 individuals of a 130-year period which led to reduced genetic diversity.

1/2N e In each generation, a proportion (1/2N e ) of neutral genetic diversity is lost. Such effects occur every generation and losses accumulate with time. The predicted heterozygosity at generation t is: H t = [1 - 1/2N e ] t H 0

This is usually expressed as the predicted heterozygosity as a proportion of the initial heterozygosity as follows: H t /H 0 = [1 - 1/2N e ] t ≈ e -t/2Ne Predicted declines in heterozygosity with time in different sized populations are shown in Fig. 10.2

The important points of this relationship are: Loss of genetic diversity depends upon the effective population size rather than the census size. Heterozygosity is lost at a greater rate in smaller than larger populations.

For example the proportion of heterozygosity retained over 50 generations in a population with N e = 500 is: H t /H 0 = [1 - 1/(2 X 500)] 50 = (999/1000) 50 = Whereas for a population with N e = 25, it is: H t /H 0 = [1 - 1/(2 X 25)] 50 = (49/50) 50 = 0.364

Loss of genetic diversity depends upon generations NOT NOT years. The shorter the generation length, the more rapid in absolute time will be the loss. Loss of heterozygosity continues with generations, in an exponential decay process. Half of the initial heterozygosity is lost in 1.4N e 1.4N e generations.

Most real populations fluctuate in size from generation to generation. Such fluctuations have profound influences on heterozygosity, N e, and therefore on inbreeding. What is the expected proportion of heterozygosity retained in a population with effective sizes of 10, 100, 1000, and 10,000 over four generations?

H t /H 0 =  [1 - 1/2N e ] H t /H 0 = [1 - 1/20]X[1/200]X[1/2000]X[1/20000] H t /H 0 = 0.95 X X X = Thus, the population loses 5.5% of its heterozygosity over the four generations with the majority (5%) being lost due to population size of 10!

Effective population size Effective population size -- All of the adverse genetic consequences of small populations depends on the N e. Most theoretical predictions in conservation genetics are couched in terms of N e. Thus, it is important to have a clear understanding of the concept of N e.

N e is the number of individuals that would give rise to the calculated loss of heterozygosity, inbreeding, or variance in allele frequencies if Idealized they behaved in the manner of the “Idealized Population Population”. The primary factors responsible for N e to be smaller than census size are: sex-ratio, high variance in family size, and fluctuating population sizes over generations.

N e /N ratios The census population size (N) is usually the only information available form most threatened species. Consequently, it is critical to know the ratio of N e /N so that effective sizes can be inferred. Values of N e /N average only 11%. Thus, long-term effective population sizes are substantially lower than census sizes.

The threatened winter run of Chinook salmon in the Sacramento River of California has about 2,000 adults. However, its effective size was estimated to be only 85 (N e /N = 0.04). Genetic concerns are much more immediate with and effective size of 85 than 2,000.

Long-term effective sizes are, on average, approximately 1/10 th of actual size. Thus, endangered species with 250 adults have an effective size of about 25 and will lose half of their current heterozygosity for neutral loci in 34 generations. By this time, the population will become inbred to the point where inbreeding will increase the extinction risk.

The most important factor reducing N e /N is fluctuations in population size followed by variation in family size, with variation in sex-ratio having a smaller effect. Overlapping versus non-overlapping generations has no significant effect, nor do life history attributes.

Unequal Sex-Ratios In many wild populations the number of breeding males and breeding females is not the same. polygamy Many mammals have harems (polygamy) where one male mates with many females, while many males make no genetic contribution to the next generation. In a few species, this situation is reversed polyandry (polyandry).

The equation for accounting for unequal sex-ratios is: N e = (4N m N f )/(N m + N f ) As the sex ratio deviates from 1:1, the N e /N declines. For example, an elephant seal harem with 1 male and 100 females has an N e of 4. However, it is the life-time sex-ratio over generations that is important.

In practice, harem masters often have limited tenure so that the average sex-ratio over a complete generation is usually much less skewed than that occurring during a single breeding season. Overall, unequal sex-ratios have modest effects in reducing effective population sizes below actual sizes, resulting in average reductions of 36%.

Variation in Family Size Variation in Family Size -- The higher the variance in family size, the lower the effective population size. If family sizes are equalized, V k = 0 then N e ≈ 2N. This is critical to captive breeding programs. Equalization of family sizes potentially allows the limited captive breeding space for endangered species to be effectively doubled.

Because of this, equalization of family sizes forms part of the recommended management regime for captive breeding of endangered species.

Fluctuations in Population Size Fluctuations in Population Size -- the effective size of a fluctuating population is not the average but the harmonic mean of the effective population sizes of t generations. This is the long-term, overall effective population size. Fluctuations in population size are the most important factor reducing N e, on average reducing it by 65%.

Inbred populations Inbred populations -- Inbreeding reduces effective population size as follows: N e = N/(1 + F) Overlapping Generations Overlapping Generations -- Most natural populations have overlapping rather than discrete generations. The effect on N e of overlapping generations are not clearly in one direction however, they are more likely to reduce N e relative to N.