Molecular evolution

Molecular evolution refers to incremental changes in the nucleotide sequence of DNA--and usually to changes in the DNA of chromosomes--which take place over the history of a species and distinguish the species from its forebears. Because biologists and geneticists view most heritable change in trait or phenotype as a result of changes in the DNA (there are some exceptions such as genomic imprinting which is "epigenetic"), the evolution of molecular sequences relates to evolution more generally. But the two do not relate in a simple way, and exactly how is a subject of more than one scientific controversy. One obvious inequivalence is that molecular evolution takes place not only in the gene sequences which code for structural, enzymatic or other gene products, but takes place also in DNA with no known function (so-called "junk DNA"). In principle then the DNA of a lineage might "evolve molecularly," even while the phenotype of descendants remains constant.

Table of contents

1 Principles of molecular evolution

1.1 The neutral theory
1.2 Infinite alleles model
1.3 Infinite sites model

2 Related fields
3 See also
4 References
5 External links

Principles of molecular evolution

The neutral theory

Main article: Neutral theory of molecular evolution

One of the central questions surrounding molecular evolution is what proportion of mutations are neutral with respect to natural selection, meaning mutations that do not convey a selective advantage or disadvantage to the individual that inherits them. Answering such questions is an aim of population genetics.

Rare spontaneous errors in DNA replication cause the mutations that drive molecular evolution. The molecular clock technique, which researchers use to date when two species diverged by comparing their DNA, deduces elapsed time from the number of differences. The technique was inspired by the once common assumption that the DNA error rate is constant--not just over time, but across all species and every part of a genome that you might want to compare. Because the enzymes that replicate DNA differ only very slightly between species, the assumption seemed reasonable a priori. But as molecular evidence has accumulated, the constant-rate assumption has proven false--or at least overly general. Molecular clock users are developing workaround solutions.

Infinite alleles model

The Japanese geneticist Motoo Kimura and American geneticist James Crow (1964) introduced the infinite alleles model, an attempt to determine for a finite population what proportion of loci would be homozygous. This was, in part, motivated by assertions by other geneticists that more than 50 percent of Drosophila loci were heterozygous, a claim they initially doubted. In order to answer this question they assumed first, that there were a large enough number of alleles so that any mutation would lead to a different allele (that is the probability of back mutation to the original allele would be low enough to be negligible); and second, that the mutations would result a number of different outcomes from neutral to deleterious.

They determined that in the neutral case, the probability that an individual would be homozygous, F, was:

F = 1/(4N_eu + 1)

where u is the mutation rate, and N_e is the effective population size. From this it is possible to determine an upper limit to number of possible alleles in a population, n as the inverse of the homozygosity:

n = 1/F = 4N_eu + 1

If the effective population is large, then a large number of alleles can be maintained. However, this result only holds for the neutral case, and is not necessarily true for the case when some alleles are more or less fitter than others, for example when the fittest genotype is a heterozygote (a situation often referred to as overdominance or heterosis).

In the case of overdominance, because Mendel's second law (the law of segregation) necessarily results in the production of homozygotes (which are by definition in this case, less fit), this means that population will always harbor a number of less fit individuals, which leads to a decrease in the average fitness of the population. This is sometimes referred to as genetic load, in this case it is a special kind of load known as segregational load. Crow and Kimura showed that at equilibrium conditions, for a given strength of selection (s), that there would be an upper limit to the number of fitter alleles (polymorphisms) that a population could harbor for a particular locus. Beyond this number of alleles, the selective advantage of presence of those alleles in heterozygous genotypes would be cancelled out by continual generation of less fit homozygous genotypes.

These results became important in the formation of the neutral theory, because neutral (or nearly neutral) alleles create no such segregational load, and allow for the accumulation of a great deal of polymorphism. When Richard Lewontin and J. Hubby published their groundbreaking results in 1966 which showed high levels of genetic variation in Drosophila via protein electrophoresis, the theoretical results from the infinite alleles model were used by Kimura and others to support the idea that this variation would have to be neutral (or result in excess segregational load).

Infinite sites model

References

Wen-Hsiung Li Molecular Evolution, Sinauer, 1997 ISBN 0-87893-463-4

Motoo Kimura and James Crow (1964), "The Number of Alleles that Can Be Maintained in a Finite Population," Genetics 49:725-738.
Roderic D.M. Page and Edward C. Holmes Molecular Evoluion: A Phylogenetic Approach, Blackwell Science, 1998 ISBN 0-86542-889-1

External links

MIT History of Science

Basic topics in evolutionary biology
Processes of evolution: macroevolution - microevolution - speciation
Mechanisms: selection - genetic drift - gene flow - mutation
History: Charles Darwin; - The Origin of Species - modern evolutionary synthesis
Subfields: population genetics - ecological genetics - molecular evolution - phylogenetics - systematics - evo-devo
List of evolutionary biology topics \| Timeline of evolution