mtDNA
Piganeau G & Eyre-Walker A 2009 Evidence for variation in the effective population size of animal mitochondrial DNA. PLoS ONE 4:e4396.
- it has recently been shown that levels of diversity in mitochondrial DNA are remarkably constant across animals of diverse census population sizes and ecologies
- here we present several lines of evidence that suggest, to the contrary, that the effective population size of mtDNA does vary, and that the variation can be substantial
- we estimate the effective population size of mitochondrial DNA in selected mammalian groups and show that it varies by at least an order of magnitude
- the relative constancy of DNA diversity may be due to a negative correlation between the effective population size and the mutation rate per generation
- neutral diversity might be held in check by the effects of adaptive substitutions, which purge diversity as they sweep through the population
- if the rate of adaptive evolution is limited by the supply of mutations then the level of neutral diversity is a product of two conflicting processes
- as the population size increases so neutral diversity tends to increase
- at the same time the number of adaptive substitutions increases and this decreases diversity
- Gillespie [5] has shown that these processes tend to cancel each other out to yield a constant level of neutral diversity across species with very different census population sizes
- Bazin et al. [6] show, in support of this "genetic draft" hypothesis, that the neutrality index is significantly lower in invertebrates, which are likely to have higher census population sizes, than vertebrates
- the neutrality index largely depends upon two factors
- the proportion of substitutions that are adaptive, which reduces the neutrality index
- the proportion of polymorphisms that are slightly deleterious, which increases the index
- invertebrates have lower neutrality indices because they have a smaller proportion of slightly deleterious mutations, not because the rate of adaptive evolution is higher
- the degree to which a genome is affected by genetic hitch-hiking depends on the number of adaptive substitutions per generation, not the proportion of substitutions that are adaptive
- the NI may not be strongly correlated to the rate of genetic draft