Title: Molecular Versus Quantitative Genetic Approaches
1Molecular Versus Quantitative Genetic Approaches
2Level of genetic variation is of concern in
evolutionary ecology because of the ability of a
population to adapt to novel and changing
environments
3Genetic surveys of some species, e.g., cheetahs,
elephant seals, and golden-lion tamarins, have
found low levels of diversity
4How do low levels of genetic variation relate to
adaptive variation?
5Monitoring 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
6Attributes 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
7Useful for
- - Ascertaining pedigrees/relatedness
- - Reconstructing phylogenies
- - Identifying phylogeographic patterns
- - Estimating gene flow patterns
8Molecular studies often take the data a step
further, using them to infer adaptive features of
population-genetic structure
9Often, phenotypes in populations are
quantitative in nature, such as life-history
characters
10Continuously 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
11Main 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
12Reasons
- - 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
13The 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
14An Example Cotton-top Tamarin
- Cheverud et al. 1994. Quantitative and molecular
genetic variation in captive cotton-top tamarins
15Has a low level of molecular heterozygosity
(H1)Exhibits a rather high level of
heritability for body weight (h235)
16Illustrates that genetic variation required for
adaptation of species to future challenges can
exist despite low levels of molecular
heterozygosity
17Reasons
- 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
18Bottom -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
19Evolutionary 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
20How 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.
21Lande (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.
22Franklin 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
23Lynch 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