Mutation and variation are required for evolution
Evolution is dependent on genetic variation in a population. With evolution, the overall frequency of variants in a population changes over time. Without genetic variation, there’s no change.
Where does genetic variation come from? As discussed in the chapters on Mutation and DNA repair, genetic variation arises from mutation. Mutation arises naturally through unrepaired DNA damage. The rate at which this occurs in an individual depends on species (for example, viruses tend to have very high rates of mutation due to error-prone polymerases) and environmental influences (like exposure to high levels of radiation). Remember that mutations can be advantageous (“good”), disadvantageous (“bad”), or neutral. Variants can include single nucleotide polymorphisms (SNPs), larger insertions or deletions, copy-number variants, and structural variants that rearrange larger parts of the genome.
Most new variants disappear quickly from a large population, regardless of whether they are advantageous, disadvantageous, or neutral. This is for several reasons. A new mutation will typically only be present in one copy of a diploid genome, so only one-half of offspring from a mutant individual will harbor the mutation; there’s no guarantee that any one individual will produce offspring, and most reproducing individuals do not harbor the mutation.
But sometimes, a new allele may begin to accumulate in a population. Eventually, the new allele may become fixed in a population. In the section on population genetics, we discuss Hardy-Weinberg equilibrium. At equilibrium, allele frequency does not change from one generation to the next (so no evolution). But for allele frequency to be consistent from generation to generation, some conditions must be met:
- The alleles are not accumulating mutations (converting one allele to the other, for example)
- There must be a large population
- No migration in or out of the population
- No selective pressure
- Individuals choose mates randomly
When those conditions are not met, mutation, genetic drift, gene flow, natural selection, and assortative mating can influence changes in allele frequencies in the population. Each of these is a factor in the evolution of a population. They are discussed in the next section.
