Get documents about "Population Genetics and Conservation
W
Document Sample
Population Genetics and
Conservation
Is there a relationship between
genetic diversity and population
size in nature?
Frankham, R. 1996. Relationship of genetic
variation to population size in wildlife.
Conservation Biology 10:1500-1508
For 77 animal, plant, and bacterial species
with a minimum of 20 loci
Relationship between Genetic
Diversity and Population Size
r = 0.81
p < 0.001
r = 0.73 with
He E. coli
omitted
log N
Is there a relationship between
reproductive success and
genetic diversity in nature?
Reed, D.H. and R. Frankham. 2003.
Correlation between Fitness and Genetic
Diversity. Conservation Biology 17:230-
237
There are several reasons why there may be a
weak or nonexistent relationship between
fitness and levels of genetic diversity
1) Because molecular markers are neutral, or nearly so, they
may lose genetic variation more rapidly than loci concerned
with fitness
2) Quantitative traits associated with fitness typically have a
larger proportion of their total genetic variance in the form of
epistatic and dominance variance than in the form of traits
less closely associated with fitness – heritabilities can remain
high in spite of reductions in population size
3) Selection tends to purge the population of deleterious
recessive alleles and in theory can create inbred populations
with a higher fitness than their outbred progenitor
Positive Relationship between
Fitness and Genetic Diversity
Overall - X r = 0.432 + 0.058 or 19% of the variation explained
28 of 34 comparisons were positive
Correlates of Fitness
Population Size and Fitness
X = 0.354 + 0.111, N=11
Heritability and Fitness
X = 0.509 + 0.134, N=6
Molecular Heterozygosity and Fitness
X = 0.447 + 0.081, N=17
Their Conclusion
Not only does the level of
heterozygosity relate to
evolutionary potential, but
validates its correlation with
current population fitness
Most models of the rate of loss of genetic
variation in small populations assume that
all genotypes have equal fitness
(i.e., selective neutrality) based on our
neutral model of genetic drift
(1-1/2Ne)t
where an Ne=50 is necessary to avoid
harmful loss of genetic diversity in the short
term
However, some empirical studies
have shown that selection against
homozygotes occurs during early
stages of growth in natural
populations of plants
Consequently, models that assume
selective neutrality may be
misleading
Lesica, P. and F.W. Allendorf.
1991. Are small populations of
plants worth preserving?
Conservation Biology 6:135-
139.
They searched the literature for
multilocus studies of plants in which
genotypic frequencies at two or more
stages in the life cycle were
reported; found 8
Data suggest that heterozygotes often
have a survival advantage that could
affect the rate of loss of heterozygosity
Three possible explanations
for heterozygote advantage:
1) Inbreeding depression - unmasking
deleterious recessives
2) Overdominance - greater fitness of
heterozygotes at the loci examined
3) Associative Overdominance -
Selection at loci that are linked or
nonrandomly associated with the loci
examined
Whatever the cause, heterozygous
advantage slows the loss of variation
due to drift over what neutral models
predict
1 t
Ht = (1 - 2N ) Ho
Heterozygosity remaining after 25
generations; Heterozygote fitness is
1.0, homozygotes 1-s
100
90
80
70 100
60
% 50
50
25
N
Ht 40
10
30
20
10
0
0.00 .02 .05 .10 0.25
Selection Coefficient
Population size that would lose
genetic variation at a selectively
neutral locus at the same rate as
observed in the simulations
____Population Size_______
s 10 25 50 100
0.00 9.7 25.9 49.6 100.4
0.02 10.4 29.5 63.4 129.6
0.05 11.4 34.1 81.0 162.6
0.10 14.8 52.7 116.2 242.9
0.25 31.3 145.1 361.6 718.7
Can conclude that small
populations may be much more
valuable for conservation than
predicted by models that assume
selective neutrality
Effective Population Size
Made the assumption that the
number of males and females
contributing to each subsequent
generation is the same
If the sex ratio is not 1:1 for each
generation then the population
loses genetic variability more
rapidly
This is because the “effective number” of
individuals is smaller than the actual
number of individuals in the population
The Effective Number (Ne) is the
number of individuals in an ideal
population that would lose genetic
variability at the same rate as a
non-ideal population with N
individuals
Effective Number can be
calculated as follows:
Effective Number
# breeding
females in pop.
Ne = 4Nm Nf
Nm + Nf
# breeding males in pop.
For a sex ratio of 1 male:9 females
in a population of 100 animals
Ne = 4(10 X 90)
= 36
10 + 90
Which means that a population of 100
individuals, consisting of 10 breeding
males and 90 breeding females would
lose genetic variability as rapidly as a
population consisting of only 18
males and 18 females or 36
individuals
Influence of fluctuating
population size on the effective
number
e.g., if a population of 100
individuals drops to only 25 in
the tenth generation the
effective number during these
10 generations would be 77
Recall : Harmonic Mean
individuals in each generation
1 1
( 1/N1 + 1/N2 +
N =
.....1/Nt)
e t
Effective # generations
Number
From this example it’s clear
that a single generation with a
low population size has a
large negative influence on
the effective number
Influence of family size on the
effective number
Actual # of breeding individuals
4N - 4
Ne = Vk + 2
Effective Variance in number of
number Offspring
Rearrange equation
Ne/N ~ 4/(Vk + 2)
Ne/N ~ 4/(2+2) = 1.0 ~ N
Ne/N ~ 4/(4+2) = 0.67N
Ne/N ~ 4/(0+2 ) = 2.0N
Over a range of species,
variation in family sizes reduced
effective population sizes to an
average of 54% of census sizes
Frankham, R. 1995. Effective
population size/adult population
size ratios in wildlife: a review.
Genetic Research 66:95-107.
The influence of generation time
on loss of genetic variability -
loss doesn’t occur per year, per
decade or per century but per
generation!
Loss of Genetic Management
Variability Influenced Measures
by to Reduce Loss
1. Size of founder Maximize # of
population founders
2. Rate of growth of Maximize growth rate,
population after especially in the
foundation first generation
3. Sex ratio Equalize # males
females
4. Generation length Maximize
5. Family size Equalize # offspring/
breeding individual
6. Fluctuation of Minimize fluctuations
population size over
generations
7. Size of the stable Maximize carrying
population (@ carrying capacity
capacity)
