Genetic drift

Genetic drift

Random change in allele frequencies from one generation to the next due to chance.

Random genetic drift

Another name for genetic drift; emphasizes randomness of allele frequency change.

Key features of drift

Random and unbiased; stronger in small populations; leads to loss of variation; can fix alleles; causes population differences.

Effect of population size

Fluctuations are larger in smaller populations; drift is stronger when population size is small.

Loss of variation

Over time, drift reduces genetic diversity as alleles are randomly lost.

Population bottleneck

Sudden reduction in population size causing loss of allelic variation; genetic diversity declines.

Founder effect

Small founding population starts with different allele frequencies than parent population; diversity reduced.

Allele fixation

Random chance can cause one allele to become fixed and the other lost, especially in small populations.

Population differences

Even identical starting populations can diverge genetically through drift alone.

Effective population size (Ne)

Represents the number of breeding individuals contributing genes to next generation; determines drift strength.

Effective population size equation

Ne = 4NmNf / (Nm + Nf), where Nm = males and Nf = females.

Relationship between Ne and drift

Small Ne → strong drift; large Ne → weak drift.

Time to coalescence

For diploids, average number of generations to common ancestor of two alleles ≈ 2Ne.

Neutral mutations

Occur in noncoding or synonymous regions; no effect on fitness; evolve by drift.

Beneficial mutations

Occur in coding regions; selection acts, but drift adds random effects that can speed or slow fixation.

reproductive isolating barrier

temporal, behavioral, geographic

species concepts

Biological, ecological, phylogenetic, and morphological