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Molecular Versus Quantitative Genetic Approaches

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Level of genetic variation is of concern in evolutionary ecology because of the ... of some species, e.g., cheetahs, elephant seals, and golden-lion tamarins, ... – PowerPoint PPT presentation

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Title: Molecular Versus Quantitative Genetic Approaches


1
Molecular Versus Quantitative Genetic Approaches
2
Level of genetic variation is of concern in
evolutionary ecology because of the ability of a
population to adapt to novel and changing
environments
3
Genetic surveys of some species, e.g., cheetahs,
elephant seals, and golden-lion tamarins, have
found low levels of diversity
4
How do low levels of genetic variation relate to
adaptive variation?
5
Monitoring genetic variation has been primarily
through the use of molecular markers (proteins,
DNA-based markers), where heterozygosity has been
the preferred measure of genetic variation
6
Attributes that have led to their widespread use
in evolutionary biology
  • - Sampling broadly across the genome
  • - Techniques that use PCR can be applied to live
    organisms with little disturbance and even to
    museum specimens, providing a historical
    perspective
  • - Has an unambiguous genetic basis
  • - Behave in an essentially neutral fashion

7
Useful for
  • - Ascertaining pedigrees/relatedness
  • - Reconstructing phylogenies
  • - Identifying phylogeographic patterns
  • - Estimating gene flow patterns

8
Molecular studies often take the data a step
further, using them to infer adaptive features of
population-genetic structure
9
Often, phenotypes in populations are
quantitative in nature, such as life-history
characters
10
Continuously Distributed Traits
  • - Most ecologically important traits are
    continuous
  • - Phenotypes are inherited through many genes,
    each typically of small effect and affected by
    the environment
  • - Cant identify all genes responsible, so use
    the phenotype
  • - Genetic variation in quantitative characters
    measured by the additive genetic variance, or
    heritability, using quantitative genetic
    approaches

11
Main Point
  • There are several good theoretical reasons to
    doubt that a strong connection will normally be
    found between levels of molecular and
    quantitative-genetic diversity within populations

12
Reasons
  • - Variation at the molecular level
    (heterozygosity) is introduced to a population at
    the per locus rate of mutation of 10-8 to 10-5
    per year
  • - Variation for quantitative traits
    (heritability) is introduced at a rate of
    approximately 10-3 to 10-2 per generation

13
The effective size necessary for maintaining
reasonable amounts of genetic variation is much
higher for single locus variants than for
quantitative characters and recovery time is much
longer for single locus variants than for
quantitative characters
14
An Example Cotton-top Tamarin
  • Cheverud et al. 1994. Quantitative and molecular
    genetic variation in captive cotton-top tamarins

15
Has a low level of molecular heterozygosity
(H1)Exhibits a rather high level of
heritability for body weight (h235)
16
Illustrates that genetic variation required for
adaptation of species to future challenges can
exist despite low levels of molecular
heterozygosity
17
Reasons
  • 1. Differences in behavior of single locus
    variants and quantitative genetic variation at a
    population bottleneck
  • 2. Additive genetic variation, in contrast to
    heterozygosity, has actually been observed to
    increase after a population bottleneck for
    morphological characters in house flies due to
    non-additive gene action (epistasis and
    dominance i.e., due to the average effects of
    genes that occur as the frequencies of
    interacting genes are altered by genetic drift)
  • 3. The lack of a relationship between molecular
    and quantitative measures of genetic diversity is
    also borne out (Reed and Frankham 2001) r
    -0.08 /- 0.11

18
Bottom -Line
  • - Molecular-marker loci will provide little
    insight into conditions at loci underlying
    adaptive variation unless a fraction of the
    former are tightly linked to the relevant
    quantitative-trait loci
  • - This seems unlikely except in species with very
    small chromosome numbers

19
Evolutionary potential will be reduced in
threatened and endangered species due to lowered
reproductive success that will affect the slope
of the selection differential
Selection Differential
Estimate Slope
Fitness
Trait
20
How large should populations be to retain their
evolutionary potential?
  • Franklin (1980) and Soule (1980) proposed that an
    Ne of 500 is sufficient to maintain adequate
    genetic variance for adaptive evolution in
    quantitative traits
  • At equilibrium between mutation and genetic drift
    the expected genetic variance is
  • Vg 2NeVm,
  • where Vg is additive genetic variance, Ne
    effective size, Vm is the mutational variance
    assuming a heritability of 0.5 where VgVe and
    Vm 10-3Ve and solving for effective size gives
    an Ne of 500.

21
Lande (1995) Mutation and Conservation.
Conservation Biology
  • Argues that this number should be revised
  • upwards, given that 90 of mutational variance
    is deleterious
  • If we incorporate the finding that only about 10
    of the spontaneous mutational variance is
    quasineutral (standing variation in quantitative
    traits) we should substitute Vm10-4Ve and the
    Franklin/Soule number would be increased by a
    factor of 10 to 5000.

22
Franklin and Frankham (1998) Animal Conservation
  • Argues heritabilities are often less than 0.5 for
    life history, behavioral and physiological traits
  • Usually 0.1- 0.2
  • This would bring the number back down to 500-1000

23
Lynch and Lande (1998) Animal Conservation
  • Argue that effective sizes of 1000 or less would
    be subject to substantial genetic drift putting
    populations at significant risk of extinction
    when challenged by changing environments
  • In addition, the mutation rates for single-locus
    traits, such a disease-resistance are
    three-orders of magnitude lower than for
    polygenic traits meaning we need larger
    effective sizes to maintain adequate diversity at
    such loci
  • Also given that Ne is often one-third to
    one-tenth of the actual size actual size should
    be several thousand to maintain genetic diversity
  • So back to 1000-5000
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