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Wright–Fisher model

The Wright-Fisher model is a stochastic model in population genetics describing how allele frequencies change due to random genetic drift in finite populations.
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Description

In the 1930s, Sewall Wright and Ronald Fisher independently constructed a mathematical model explaining how gene frequencies behave in small populations. In a large group, the ratio of gene variants remains stable if there is no selection. But in a small one, randomness takes over. Each generation is like a new deck of cards shuffled from the previous one. The model shows that genetic diversity inevitably declines: one variant eventually becomes fixed, and the others are lost. This is key to understanding genetic drift—an evolutionary mechanism that works more strongly the smaller the group size.

History

The model was proposed independently by Sewall Wright in 1931 and Ronald Fisher in 1930. Wright used it to study the genetic structure of populations and the effects of inbreeding, while Fisher used it to analyze the loss of genetic variability. Both scientists laid the foundations of modern population genetics by uniting Mendelian genetics with Darwinian evolution. Later, the model was extended to account for mutations, selection, and migration, becoming a cornerstone for many areas of evolutionary biology.

How it works

The model is used to estimate the time to fixation or loss of an allele, calculate the effective population size, and analyze genetic diversity. It underlies coalescent theory, which reconstructs the genealogy of genes. Limits of applicability: the population must be panmictic (random mating), generations must not overlap, and the size must be constant. In reality, these conditions are often violated, so the model serves more as a theoretical benchmark than a precise predictive tool.

In practice

The model helps predict how quickly genetic diversity is lost in isolated populations—from rare animal species to cell lines in the laboratory.

Fun fact

If a population consists of only ten individuals, then on average after 20 generations all of them will be descendants of a single ancestor—and will carry only its genes.

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