Transcript Section 6

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:
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 (Ne) 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)
Fixation of favorable alleles by selection
(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.
In each generation, a proportion (1/2Ne) of
neutral genetic diversity is lost.
Such effects occur every generation and losses
accumulate with time.
The predicted heterozygosity at generation t is:
Ht = [1 - 1/2Ne]tH0
This is usually expressed as the predicted
heterozygosity as a proportion of the initial
heterozygosity as follows:
Ht/H0 = [1 - 1/2Ne]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
Ne = 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 generations
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
1.4Ne generations.
Most real populations fluctuate in size from
generation to generation.
Such fluctuations have profound influences on
heterozygosity, Ne, 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?
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 its
heterozygosity over the four generations with the
majority (5%) being lost due to population size of
10!
Effective population size -- All of the adverse
genetic consequences of small populations depends
on the Ne.
Most theoretical predictions in conservation
genetics are couched in terms of Ne.
Thus, it is important to have a clear understanding
of the concept of Ne.
Ne is the number of individuals that would give
rise to the calculated loss of heterozygosity,
inbreeding, or variance in allele frequencies if
they behaved in the manner of the “Idealized
Population”.
The primary factors responsible for Ne to be
smaller than census size are: sex-ratio, high
variance in family size, and fluctuating
population sizes over generations.
Ne/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
Ne/N so that effective sizes can be inferred.
Values of Ne/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 (Ne/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 1/10th 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 Ne/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.
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).
The equation for accounting for unequal sex-ratios
is: Ne = (4NmNf)/(Nm + Nf)
As the sex ratio deviates from 1:1, the Ne/N
declines.
For example, an elephant seal harem with 1 male
and 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 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 -- The higher the variance
in family size, the lower the effective population
size.
If family sizes are equalized, Vk = 0 then Ne ≈ 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 -- 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 Ne, on average reducing
it by 65%.
Inbred populations -- Inbreeding reduces
effective population size as follows:
Ne = N/(1 + F)
Overlapping Generations -- Most natural populations
have overlapping rather than discrete generations.
The effect on Ne of overlapping generations are
not clearly in one direction however, they are more
likely to reduce Ne relative to N.