Molecular Versus Quantitative Genetic Approaches

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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 ability of a
population to adapt to novel and changing
environments
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Genetic surveys of some species, e.g., cheetahs,
elephant seals, and golden-lion tamarins, have
found low levels of diversity
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How do low levels of genetic variation relate to
adaptive variation?
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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
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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

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Useful for

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

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Molecular studies often take the data a step
further, using them to infer adaptive features of
population-genetic structure
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Often, phenotypes in populations are
quantitative in nature, such as life-history
characters
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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

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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

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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

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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
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An Example Cotton-top Tamarin

• Cheverud et al. 1994. Quantitative and molecular
  genetic variation in captive cotton-top tamarins

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Has a low level of molecular heterozygosity
(H1)Exhibits a rather high level of
heritability for body weight (h235)
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Illustrates that genetic variation required for
adaptation of species to future challenges can
exist despite low levels of molecular
heterozygosity
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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

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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

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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
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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.

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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.

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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

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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