Evolutionary Forces: Definitions

The forces of evolution include founder effects, genetic drift, mutation, migration and selection.

Founder effects. Founder effects arise when the subgroup colonizing previously uninhabited territory is not fully representative, in a genetic sense, of the parental population from which the subgroup derived. Since fission of groups is more likely to occur along family lines than at random (Neel 1968), founder effects may be expected in any colonizing group. The intensity of the founder effect is directly related to the size of the colonizing group — the smaller the number of colonizers, the greater the chance that genes from the parental group will be under- or over-represented, or lost altogether.

Genetic drift. Genetic drift occurs when the distribution of genes in a given generation differs from the distribution in the previous generation. This can occur by chance. For instance, all offspring in a given mating may be female, by chance, so that the paternal Y chromosome is lost to that and future generations, by chance. If all offspring in a given mating are male, the maternal mitochondrial DNA lineage will terminate. The impact of genetic drift is directly related to population size — the smaller the population, the greater the fluctuations in gene frequencies from one generation to the next and the greater the chance that rare or infrequent genes will be lost from the gene pool. The effects of genetic drift can be brought on by catastrophes such as drought, cyclones, epidemics and boating accidents that can dramatically reduce population size, causing in genetic terminology a “bottleneck”. If the numbers of males and females in a population are unequal, then the effective population size is closer to the smaller number.

It should be noted that chance can also determine the particular individuals selected for study, so that they may not faithfully represent the larger population from which they are drawn. The smaller the sample size, the greater the chance of sampling effects. Analysis of contemporary populations cannot discriminate readily between founder effects and genetic drift. Bottlenecks may have been more important than founder effects in a population where mtDNA is less diverse than nuclear DNA (Birky et al. 1989).

Mutation. When cells replicate, DNA also replicates and is not always a faithful copy of the original; the error is called a mutation. If the mutation occurs in the germ-line it may be passed to the next generation. Mutations may be a single DNA base substitution, a deletion, a gene duplication and so on. Some mutations, present initially as a single copy in the population, will be lost in genetic drift, but other new mutations will become established in the population, especially if the new mutation has a selective advantage. A point mutation in a functional gene is silent if it occurs in a redundant nucleotide (explained below); otherwise, it results in an amino acid change in a molecule and is then possibly subject to selection.

Selection. Selection operates when a particular gene has a survival advantage, through differential fertility or in survival to reproductive age. Post-reproductive survival differentials are not selected because genes have already been passed to the next generation. Amino acids are encoded by a sequence of three nucleotides called a codon, but the third nucleotide (and sometimes the second) is often redundant. For example, the codons CCA, CCC, CCG and CCT all encode the amino acid residue proline, so that the nucleotide in the third position is redundant. For a given gene, comparison of the rate of nucleotide substitutions in the first and third nucleotides of the codon can indicate whether there is positive selection for variability, negative selection for variability, or whether variability is no greater than expected by chance. The extreme polymorphism at the HLA loci has been attributed to selection for diversity per se, due to frequency-dependent selection for high genetic diversity where individuals with a rare allele have some selective advantage, or due to overdominant selection where individuals carrying different alleles at a single locus (i.e. heterozygotes) have a selective advantage. Frequency-dependent and overdominant selection can save rare or new alleles from extinction.