Gene Flow
Gene flow, also called gene migration, is the transfer of alleles or genes from one population to another through the movement and interbreeding of individuals. It tends to homogenise allele frequencies between populations, reducing genetic differentiation, and can introduce new alleles into a population or change the frequencies of existing ones. Gene flow counteracts the genetic divergence produced by natural selection, drift, and mutation, and is a critical factor in whether populations will diverge enough to speciate.
Key Formula
F_ST = (H_T - H_S) / H_T
LaTeX: F_{ST} = \frac{H_T - H_S}{H_T}
| Symbol | Meaning | Unit |
|---|---|---|
| F_ST | Fixation index — measure of genetic differentiation between populations (0 = no differentiation, 1 = complete differentiation) | dimensionless |
| H_T | Expected heterozygosity in the total (combined) population | proportion |
| H_S | Average expected heterozygosity within subpopulations | proportion |
Worked Example
Problem
Two butterfly populations are separated by a river. Population A has allele frequency p = 0.8 for wing colour gene; Population B has p = 0.3. After several years of migration, both populations mix equally. What is the new combined allele frequency, and what was the change in each population?
Solution
Step 1: Combined allele frequency after equal mixing: p_combined = (0.8 + 0.3) / 2 = 0.55 Step 2: Change in Population A: Δp_A = 0.55 - 0.80 = -0.25 (decreased) Step 3: Change in Population B: Δp_B = 0.55 - 0.30 = +0.25 (increased)
Answer
New combined frequency p = 0.55. Gene flow reduced difference between populations from 0.50 to 0.00, homogenising their allele frequencies.
Effects of Gene Flow on Population Genetics
| Gene Flow Level | F_ST Range | Genetic Differentiation | Speciation Likelihood | Example |
|---|---|---|---|---|
| Very high | 0.00 – 0.05 | Little or none | Very unlikely | Continuously distributed large species |
| High | 0.05 – 0.10 | Moderate | Unlikely | Migratory birds |
| Moderate | 0.10 – 0.25 | Moderate to great | Possible | Freshwater fish in connected rivers |
| Low | 0.25 – 0.50 | Great | Likely over time | Island populations |
| Very low / none | 0.50 – 1.00 | Very great | High | Isolated island endemics |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Speciation
Speciation is the evolutionary process by which populations evolve to become distinct species, typically through the accumulation of reproductive barriers that prevent gene flow between diverging lineages. It is the source of biodiversity and occurs through several modes, most commonly allopatric speciation (geographic isolation) and sympatric speciation (divergence without geographic barrier). The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring, making reproductive isolation the key criterion.
Genetic Bottleneck
A genetic bottleneck is a sharp reduction in the size of a population due to an environmental event (such as a famine, disease, or habitat destruction), resulting in a dramatic loss of genetic diversity in the surviving population. The surviving individuals carry only a small, random subset of the original genetic variation, and subsequent generations inherit this reduced genetic repertoire regardless of population size recovery. Bottlenecks increase homozygosity, reduce adaptive potential, and can cause rare alleles to be lost or increase in frequency by chance.
Evolution (biology)
Biological evolution is the change in heritable characteristics of biological populations over successive generations, driven by mechanisms such as natural selection, genetic drift, gene flow, and mutation. It unifies all of biology by explaining the diversity of life on Earth through descent with modification from common ancestors. Evolution operates at multiple levels, from changes in allele frequencies within populations (microevolution) to the origin of new species and higher taxa (macroevolution).
The word "gene" was coined by Wilhelm Johannsen in 1909 from Greek "genos" (birth, origin). "Flow" derives from Old English "flowan" (to flow). The combined term "gene flow" became standard in population genetics literature during the mid-20th century.