Molecular Evolution: Patterns and Rates


The rates and patterns of evolution of deoxyribonucleic acid (DNA) sequences vary among species, across the genome and between sites in a gene. These patterns can be considered in the light of the rate and type of mutations, and the forces that determine which of these mutations become substituted in the population. Mutations can rise in frequency if the individuals carrying those mutations leave proportionally more offspring. If a mutation increases in frequency until it replaces all other alternatives in the population, it becomes a substitution, carried by all members of that population. Substitution is driven by selection and chance, and the relative power of each of these processes is affected by population dynamics. The complexity of patterns and rates of DNA sequence evolution create a rich record in the genome of evolutionary mechanisms and history.

Key Concepts:

  • Mutations are heritable changes to the DNA sequence, created by damage to DNA or by errors made when DNA is copied.

  • Mutations occur in individual genomes, and are inherited in copies made from those genomes.

  • A beneficial mutation that increases the chances of its carrier successfully reproducing will tend to increase in frequency over generations (positive selection).

  • A deleterious mutation that decreases the chances of its carrier successfully reproducing will tend to decrease in frequency over generations (negative selection).

  • A neutral mutation that has no effect on fitness can increase in frequency through random sampling, and may become fixed in the population by chance (drift).

  • In small populations, drift can overwhelm selection for mutations with a small impact on fitness, so the rate of fixation of slightly deleterious mutations should be greatest in small populations.

  • The interaction between selection and drift creates complex patterns of molecular evolution across the genome, over time and between species.

Keywords: mutation; substitution; DNA repair; DNA replication; selection; drift; neutral; population size; nearly neutral

Figure 1.

Genetic drift causes random fluctuation in allele frequencies. This can be demonstrated by computer simulations of allele frequencies: (a) In small populations (N=10), this ‘random walk’ in gene frequencies is more likely to result in fixation of neutral or nearly neutral mutations. (b) In large populations (N=100), random sampling has less effect on gene frequencies, so the rate of fixation of alleles by drift will be lower.



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Bromham, Lindell(Sep 2013) Molecular Evolution: Patterns and Rates. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001799.pub4]