Microevolution: Genetic Drift

Define neutral evolution

Changes in allele frequencies that occur without the influence of selection. Different phenotypes are neutral with respect to fitness.

Define genetic drift

caused by sampling error. When different alleles have little or no effect on fitness, frequencies will “drift” around. Even when they do have some effect on fitness, alleles may change frequency by chance.

Describe why the effects of drift are more pronounced in small populations

In other words, the assumption of “infinite population size” in Hardy-Weinberg is violated.

Modeling results (figure 6.12, text)

Describe the bottleneck and founder effects, which also result in genetic drift

sampling small subpopulations of main population inevitably causes variability between migrants and main population

Describe the consequences of genetic drift

Inevitable loss of heterozygosity (faster in small populations, but happens in all given enough time)

Which leads to:

Lack of ability to adapt

Increase in homozygosity (and inbreeding depression)

Importance of genetic drift in biological conservation (lecture)

Describe the debate over the relative importance of drift versus selection in causing evolution

The rate of allele substitution in populations if genetic drift is more important = mutation rate at that allele

If most mutations are either neutral or deleterious, (deleterious are eliminated rapidly), rate of substitution = rate of neutral mutations

However, if advantageous mutations are “common enough,” these alleles should reach fixation much faster than drift would carry them

Neutral evolution is the “null model” to compare real-world rates of evolution to.