Population Genetics and Conservation

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Population Genetics and Conservation
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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 101500-1508
• For 77 animal, plant, and bacterial species with
  a minimum of 20 loci

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Relationship between Genetic Diversity and
Population Size
r 0.81 p lt 0.001 r 0.73 with E. coli
omitted
He
log N
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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 17230-237

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There are several reasons why there may be a weak
or nonexistent relationship between fitness and
levels of genetic diversity

• Because molecular markers are neutral, or nearly
  so, they may lose genetic variation more rapidly
  than loci concerned with fitness
• 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
• 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

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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
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Correlates of Fitness

• Population Size and Fitness
• X 0.354 0.111, N11
• Heritability and Fitness
• X 0.509 0.134, N6
• Molecular Heterozygosity and Fitness
• X 0.447 0.081, N17

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Their Conclusion

• Not only does the level of heterozygosity relate
  to evolutionary potential, but validates its
  correlation with current population fitness

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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 Ne50 is necessary
to avoid harmful loss of genetic diversity in the
short term
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However, some empirical studies have shown that
selection against homozygotes occurs during early
stages of growth in natural populations of plants
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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 6135-139.

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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

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Three possible explanations for heterozygote
advantage

• Inbreeding depression - unmasking deleterious
  recessives
• Overdominance - greater fitness of heterozygotes
  at the loci examined
• Associative Overdominance -Selection at loci that
  are linked or nonrandomly associated with the
  loci examined

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Whatever the cause, heterozygous advantage slows
the loss of variation due to drift over what
neutral models predict
t
1 2N
Ht (1 - ) Ho
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Heterozygosity remaining after 25 generations
Heterozygote fitness is 1.0, homozygotes 1-s

N
Ht
0.00 .02 .05 .10 0.25
Selection Coefficient
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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
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Can conclude that small populations may be much
more valuable for conservation than predicted by
models that assume selective neutrality
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Effective Population Size

• Made the assumption that the number of males and
  females contributing to each subsequent
  generation is the same

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If the sex ratio is not 11 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

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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
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Effective Number can be calculated as follows
Effective Number
breeding females in pop.
Ne 4Nm Nf Nm Nf
breeding males in pop.
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For a sex ratio of 1 male9 females in a
population of 100 animals
4(10 X 90) 10 90
Ne
36
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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
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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

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Recall Harmonic Mean
individuals in each generation
1 Ne
1 t
( 1/N1 1/N2 .....1/Nt)

generations
Effective Number
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From this example its clear that a single
generation with a low population size has a large
negative influence on the effective number
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Influence of family size on the effective number
Actual of breeding individuals
4N - 4 Vk 2
Ne
Effective number
Variance in number of Offspring
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Rearrange equation
Ne/N 4/(Vk 2)
Ne/N 4/(22) 1.0 N Ne/N 4/(42)
0.67N Ne/N 4/(02 ) 2.0N
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The influence of generation time on loss of
genetic variability - loss doesnt occur per
year, per decade or per century but per
generation!
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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 6695-107.