Section 6Maintenance of Genetic Diversity

Levels of genetic diversity result from the jointimpacts of:

Mutation & migration adding variationChance & directional selection removing variationBalancing selection impeding its loss

The balance between these factors dependsstrongly on population size and differs acrosscharacters.

Conservation biologists need to understand howgenetic diversity is maintained through naturalprocesses if conservation programs are to bedesigned for its maintenance in managed populations.

Maintenance of extensive genetic diversity innatural populations is one of the most important,largely unresolved, questions of evolutionarygenetics.

The balance of forces maintaining genetic diversity differs between large and smallpopulations.

Selection has a major impact in large populations.However, its impacts are greatly reduced in smallpopulations where genetic drift has anincreasingly important role.

Five major points about genetic diversity insmall populations:

Genetic drift fixes alleles more rapidly insmaller populations.

Loci subjected to weak selection in largerpopulations approach effectively neutral in smallpopulations.

Mutation-selection equilibria are lower in smallerthan larger populations.

The effects of balanced polymorphisms dependsupon the equilibrium frequency; the frequency offixation of intermediate frequency alleles is retarded, but balancing selection acceleratesfixation of low frequency alleles.

Balancing selection can retard loss of geneticdiversity, but it does not prevent it in smallpopulations.

The consequence of these effects is that geneticThe consequence of these effects is that geneticdiversity in small populations is lower for bothdiversity in small populations is lower for bothneutral alleles and those subjected to balancingneutral alleles and those subjected to balancingselectionselection

Thus far, we have examined the origin, extent,and fate of genetic variation and explored theevolutionary forces that influence geneticdiversity and contrasted the importance in smallversus large populations.

Major Conclusions thus farMajor Conclusions thus far:

Genetic diversity provides the raw material forevolutionary adaptive change.

Mutation is the ultimate source of ALL geneticvariation.

Mutation and migration from conspecificpopulations or closely related species are the only mechanisms for restoring lost geneticdiversity.

Genetic diversity can be estimated by a number oflaboratory techniques. Heterozygosity is the most useful parameter to estimate as it can becompared across species for single locus variationand is directly correlated with additive geneticvariance for quantitative traits.

Some additive genetic variation is maintainedwithin populations by balancing selection

The influence of the deterministic forces ofnatural selection is directly related to populationsize. The fate of alleles in most small populations of endangered species is predominated by randomfactors.

Inbreeding, with consequent loss of fitness,becomes inevitable in small populations.

Effective population size (Ne) as opposed to census size, determines loss of genetic diversityand inbreeding.

Effects of Sustained Population Size Reductionon Genetic Diversity

Five mechanisms by which genetic diversity is lost:

Extinction of species & populations (relatively uncommonrelatively uncommon)

Fixation of favorable alleles by selection(relatively uncommonrelatively uncommon)

Selective removal of deleterious alleles

Random loss of alleles by inter-generationalsampling in small populations.

Inbreeding within populations reducingheterozygosity.

Severe population bottlenecks are uncommon.

More modest population size reductions are aregular feature of threatened species.

The major significance of small population sizeto genetic diversity is the constant loss ofgenetic diversity over many generations.

Illinois population ofgreater prairie chickensdwindled from severalmillion to fewer than50 individuals of a130-year period whichled to reducedgenetic diversity.

In each generation, a proportion (1/2N1/2Nee) ofneutral genetic diversity is lost.

Such effects occur every generation and lossesaccumulate with time.

The predicted heterozygosity at generation t is:

HHtt = [1 - 1/2N = [1 - 1/2Nee]]ttHH00

This is usually expressed as the predictedheterozygosity as a proportion of the initialheterozygosity as follows:

HHtt/H/H00 = [1 - 1/2N = [1 - 1/2Nee]]tt ≈ e ≈ e-t/2Ne-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 theeffective population size rather than the census size.

Heterozygosity is lost at a greater rate in smallerthan larger populations.

For example the proportion of heterozygosityretained over 50 generations in a population withNe = 500 is:

Ht/H0 = [1 - 1/(2 X 500)]50 = (999/1000)50 = 0.951

Whereas for a population with Ne = 25, it is:

Ht/H0 = [1 - 1/(2 X 25)]50 = (49/50)50 = 0.364

Loss of genetic diversity depends upon generationsNOTNOT years.

The shorter the generation length, the morerapid 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 1.4N1.4Nee generations.

Most real populations fluctuate in size fromgeneration to generation.

Such fluctuations have profound influences onheterozygosity, Ne, and therefore on inbreeding.

What is the expected proportion of heterozygosityretained in a population with effective sizes of10, 100, 1000, and 10,000 over four generations?

Ht/H0 = [1 - 1/2Ne]

Ht/H0 = [1 - 1/20]X[1/200]X[1/2000]X[1/20000]

Ht/H0 = 0.95 X 0.995 X 0.9995 X 0.99995 = 0.945

Thus, the population loses 5.5% of itsheterozygosity over the four generations with themajority (5%) being lost due to population size of10!

Effective population sizeEffective population size -- All of the adversegenetic consequences of small populations dependson the Ne.

Most theoretical predictions in conservationgenetics are couched in terms of Ne.

Thus, it is important to have a clear understandingof the concept of Ne.

Ne is the number of individuals that would giverise to the calculated loss of heterozygosity,inbreeding, or variance in allele frequencies ifthey behaved in the manner of the “IdealizedIdealizedPopulationPopulation”.

The primary factors responsible for Ne to besmaller than census size are: sex-ratio, highvariance in family size, and fluctuatingpopulation sizes over generations.

NNee/N ratios/N ratios

The census population size (N) is usually the onlyinformation available form most threatened species.

Consequently, it is critical to know the ratio ofNe/N so that effective sizes can be inferred.

Values of Ne/N average only 11%.

Thus, long-term effective population sizes aresubstantially lower than census sizes.

The threatened winter run of Chinook salmon inthe Sacramento River of California has about2,000 adults.

However, its effective size was estimated to beonly 85 (Ne/N = 0.04).

Genetic concerns are much more immediate withand effective size of 85 than 2,000.

Long-term effective sizes are, on average, approximately 1/10th of actual size.

Thus, endangered species with 250 adults havean effective size of about 25 and will lose halfof their current heterozygosity for neutral lociin 34 generations.

By this time, the population will become inbred to the point where inbreeding will increase theextinction risk.

The most important factor reducing Ne/N isfluctuations in population size followed byvariation in family size, with variation in sex-ratiohaving a smaller effect.

Overlapping versus non-overlapping generationshas no significant effect, nor do life historyattributes.

Unequal Sex-RatiosUnequal Sex-Ratios

In many wild populations the number of breedingmales and breeding females is not the same.

Many mammals have harems (polygamypolygamy) where onemale mates with many females, while many malesmake no genetic contribution to the nextgeneration.

In a few species, this situation is reversed (polyandrypolyandry).

The equation for accounting for unequal sex-ratiosis: Ne = (4NmNf)/(Nm + Nf)

As the sex ratio deviates from 1:1, the Ne/N declines.

For example, an elephant seal harem with 1 maleand 100 females has an Ne of 4.

However, it is the life-time sex-ratio over generations that is important.

In practice, harem masters often have limitedtenure so that the average sex-ratio over acomplete generation is usually much less skewedthan that occurring during a single breeding season.

Overall, unequal sex-ratios have modest effects inreducing effective population sizes below actualsizes, resulting in average reductions of 36%.

Variation in Family SizeVariation in Family Size -- The higher the variancein family size, the lower the effective populationsize.

If family sizes are equalized, Vk = 0 then Ne ≈ 2N.

This is critical to captive breeding programs. Equalization of family sizes potentially allows thelimited captive breeding space for endangeredspecies to be effectively doubled.

Because of this, equalization of family sizesforms part of the recommended managementregime for captive breeding of endangeredspecies.

Fluctuations in Population SizeFluctuations in Population Size -- the effectivesize of a fluctuating population is not the average but the harmonic mean of the effectivepopulation sizes of t generations.

This is the long-term, overall effective populationsize.

Fluctuations in population size are the mostimportant factor reducing Ne, on average reducingit by 65%.

Inbred populationsInbred populations -- Inbreeding reduceseffective population size as follows:

Ne = N/(1 + F)

Overlapping GenerationsOverlapping Generations -- Most natural populationshave overlapping rather than discrete generations.

The effect on Ne of overlapping generations arenot clearly in one direction however, they are morelikely to reduce Ne relative to N.