ScienceWeek

EVOLUTION: GENETIC DIVERSITY AND MITOCHONDRIAL DNA

The following points are made by Adam Eyre-Walker (Science 2006 312:537):

1) New work [1] tests one of the most basic predictions of population genetics: that species with large population sizes should have more genetic diversity than species with small population sizes. The new work finds that this prediction, as expected, is upheld for diversity in nuclear genes, but that there is no correspondence between population size and genetic diversity for mitochondrial genes. Bazin et al [1] conducted their analysis by first compiling an impressive DNA diversity data set for both nuclear and mitochondrial DNA. Using an automated system, they searched the GenBank and EMBL databases for instances in which the same gene had been sequenced in multiple individuals of a species. This yielded, after some restrictions to improve data quality, 417 species for which they had diversity data for nuclear DNA and 1683 species for mitochondrial DNA. They also analyzed a data set of 912 species for which allozyme diversity data were available.

2) Unfortunately, the census population size is not known for the vast majority of organisms, so Bazin et al [1] used a number of phylogenetic and ecological factors to test whether population size and diversity were correlated. For example, they tested whether invertebrates had higher diversities than vertebrates, and whether (within each group) marine organisms had higher diversities than terrestrial or freshwater organisms. In all comparisons, nuclear genes followed the expected pattern -- the group that was expected to have the bigger population size had higher diversity. However, this pattern was not observed for mitochondrial DNA -- there was remarkably little difference in diversity between any of the groups.

3) Why do nuclear DNA and mitochondrial DNA behave so differently? There are a number of possibilities, but the most conspicuous difference between the two genomes is the lack, or very low level, of recombination in mitochondrial DNA. Bazin et al [1] suggest that because of this low level of recombination, mitochondrial DNA might be particularly prone to a process called "genetic hitchhiking". When an advantageous mutation spreads through a population, it reduces the genetic variation at loci that are linked to it. This is most easily seen if we consider the case in which a single advantageous mutation arises in a chromosome when there is no recombination. Once the advantageous mutation has spread through the population, all individuals share the same copy of that chromosome and there is no diversity within the chromosome until new mutations arise.

4) Genetic hitchhiking does not by itself explain why genetic diversity in mitochondrial DNA is independent of population size. To complete the explanation, we must further assume that the rate of adaptive evolution is correlated to population size. There are two reasons why this might be the case. First, more adaptive mutations will occur in big populations because there are more individuals to mutate. Second, selection is more effective, so even adaptive mutations that are very weakly selected may become fixed in big populations. If this is the case, then we might have the following situation: As the population size increases, genetic diversity will tend to increase. But at the same time, the number of adaptive substitutions (and hence genetic hitchhiking events) increases, thus reducing the level of genetic diversity. Gillespie [2] has shown that these two forces tend to cancel each other out, leaving the genetic diversity largely independent of population size. In contrast, for nuclear DNA, recombination reduces the effects of genetic hitchhiking, and diversity increases with population size.[3-5]

References:

1. E. Bazin, S. Glémin, N. Galtier, Science 312, 570 (2006)

2. J. H. Gillespie, Genetics 155, 909 (2000)

3. L. B. Jorde, M. Bamshad, A. R. Rogers, Bioessays 20, 126 (1997)

4. R. C. Lewontin, The Genetic Basis of Evolutionary Change (Columbia Univ. Press, New York, 1974)

5. J. W. O. Ballard, M. C. Whitlock, Mol. Ecol. 13, 729 (2004)

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