Genetic Drift
Genetic drift is a mechanism of evolution referring to random changes in allele frequencies in a population due to chance sampling events rather than natural selection, most pronounced in small populations. Unlike natural selection, genetic drift is non-directional and can lead to the fixation (frequency = 1) or loss (frequency = 0) of alleles regardless of their adaptive value. Two important forms are the bottleneck effect (sudden population reduction) and the founder effect (small group establishes a new population), both of which reduce genetic diversity.
Key Formula
Variance in allele frequency change = p(1-p) / (2Ne)
LaTeX: V_{\Delta p} = \frac{p(1-p)}{2N_e}
| Symbol | Meaning | Unit |
|---|---|---|
| V_{\Delta p} | Variance (random change) in allele frequency per generation | dimensionless |
| p | Current frequency of the allele | dimensionless (0–1) |
| N_e | Effective population size | number of individuals |
Worked Example
Problem
An allele A has a frequency of p = 0.5 in a small island population with an effective size of Ne = 50. What is the expected variance in allele frequency change per generation due to genetic drift?
Solution
Step 1: Identify values. p = 0.5, Ne = 50. Step 2: Calculate p(1-p). p(1-p) = 0.5 × 0.5 = 0.25. Step 3: Apply the formula. V = p(1-p) / (2Ne) = 0.25 / (2 × 50) = 0.25 / 100 = 0.0025. Step 4: Interpret. A standard deviation of √0.0025 = 0.05 means the allele frequency typically shifts by ±5% per generation by chance alone.
Answer
Variance = 0.0025; expected allele frequency change per generation ≈ ±0.05 (±5%)
Comparison of Genetic Drift Scenarios
| Type | Description | Population Size Effect | Example |
|---|---|---|---|
| Bottleneck Effect | Drastic reduction in population size due to catastrophic event | Severe loss of genetic diversity | Cheetah population after ice age (very low diversity) |
| Founder Effect | Small group establishes a new population | High frequency of founder alleles | Amish community: high Ellis-van Creveld syndrome frequency |
| General Drift (large) | Random sampling in large populations | Minimal effect; slow change | Most continental species populations |
| General Drift (small) | Random sampling in small populations | Rapid fixation or loss of alleles | Island or isolated populations |
| Allele Fixation | One allele reaches frequency of 1.0 | All individuals homozygous for that allele | Loss of alternative allele from gene pool |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Hardy-Weinberg Equilibrium
The Hardy-Weinberg Equilibrium (HWE) is a principle stating that allele and genotype frequencies in an ideal, infinitely large, randomly mating population will remain constant from generation to generation in the absence of evolutionary influences such as mutation, selection, gene flow, and genetic drift. It provides a mathematical null hypothesis against which real populations can be compared to detect evolutionary change. The principle was independently formulated by Godfrey Hardy and Wilhelm Weinberg in 1908.
Genetic Linkage
Genetic linkage refers to the tendency of genes located close together on the same chromosome to be inherited together during meiosis, rather than independently assort as predicted by Mendel's Law of Independent Assortment. The degree of linkage between two genes is inversely proportional to the physical distance between them; closely linked genes rarely recombine, while distantly located genes recombine more frequently and approach independent assortment. Linkage mapping uses recombination frequencies to construct genetic maps that estimate distances between genes in centimorgans (cM).
Non-disjunction
Non-disjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis I, meiosis II, or mitosis, resulting in daughter cells with an abnormal number of chromosomes (aneuploidy). When non-disjunction occurs during meiosis, the resulting gametes may have one extra chromosome (n+1, called trisomy after fertilisation) or one fewer chromosome (n-1, called monosomy after fertilisation). Non-disjunction is the most common cause of chromosomal abnormalities in humans, with its frequency increasing with maternal age.
The term "genetic drift" was introduced by Sewall Wright in the 1930s as part of his shifting balance theory of evolution. "Drift" conveys the random, directionless wandering of allele frequencies over time, analogous to the physical drift of an object carried by random currents. Wright also developed the concept of effective population size (Ne).