In evolution not all change is determined by selection, sometimes change happens by chance.
Genetic drift is the random change in allele frequencies in a population, arising from the randomness of reproduction in finite populations.
Reproduction is ultimately a sampling process, where each generation inherits only a subset of alleles from the previous generation. Which alleles are passed to the next generation is not deterministic as reproduction follows probabilistic rules.
Each generation represents a random sample of the genetic variation present in the previous generation.
Drift and Populations
In a small population, an allele can become more common in a population purely by chance, despite the selective forces acting upon it.
This is because sampling introduces noise, and in a small population each individual represents a large fraction of the total gene pool.
This means that if a few individuals fail to reproduce, or reproduce more successfully, allele frequencies can shift substantially.
The smaller the population, the larger the stochastic fluctuations and as a result, the stronger the effect of drift will be. In larger populations these random fluctuations have a much smaller effect and usually average out.
Genetic drift ‘doesn’t care’ if a trait is optimised or highly beneficial, it does not push towards optimisation, in some senses drift is ‘blind’.
Through drift alone slightly beneficial mutations can be lost and similarly, slightly harmful mutations can spread.
Chance alone can determine which variants will persist in a population.
Much of the genetic variation we observe in nature is shaped by drift, rather than selection. This idea was formalised in 1968 by Kimura as the Neutral theory of molecular evolution, and further developed by King, Jukes and Ohta.
In the next few posts we’ll explore the different evolutionary forces that shape variation in a population 🙂