Genetic Drift



Genetic Drift

Introduction

Genetic drift is the term used in population genetics to refer to the statistical drift over time of allele frequencies in a finite population due to random sampling effects in the formation of successive generations. In a narrower sense, genetic drift refers to the expected population dynamics of neutral alleles (Peter Gildenhuys, 2009), which are predicted to eventually become fixed at zero or 100% frequency in the absence of other mechanisms affecting allele distributions. Whereas natural selection describes the tendency of beneficial alleles to become more common over time (and detrimental ones less common), genetic drift refers to the fundamental tendency of any allele to vary randomly in frequency over time due to statistical variation alone, so long as it does not comprise all or none of the distribution.

Discussion

Genetic drift may be modeled as a stochastic process that arises from the role of random sampling in the production of offspring. The genes of each new generation are not a simple copy of the genes of the successful members of the previous one, but rather a sampling, which includes some statistical error. Drift is the cumulative effect over time of this sampling error on the allele frequencies in the population. By definition, genetic drift has no preferred direction. A neutral allele may be expected to increase or decrease in any given generation with equal probability. Given sufficiently long time, however, the mathematics of genetic drift (cf. random walk) predict the allele will either die out or be present in 100% of the population, after which time there is no random variation in the associated gene. In this regard, genetic drift tends to sweep gene variants out of a population over time, such that all members of a species would eventually be homozygous for this gene. Genetic drift is opposed in this regard by genetic mutation which introduces novel variants into the population according to its own random processes (Günter, 2007).

Allele or Genetic Variant

An allele, or genetic variant, is a component of a gene that produces a certain trait. Imagine there are both red worms and white worms in the same population. If a red worm mates with a white worm, each will pass one allele to its offspring, red or white, to form a gene. The dominant, or strong, allele will determine which trait the baby worm presents. If white is dominant, the baby worm will be white, if red is dominant, the baby worm will be red, and if the baby worm receives two of the same recessive alleles, it will exhibit that recessive feature. Genetics is far more complex than this example allows, but this is the general concept (Lindell Bromham, 2009).

Now magine these worms live in a swamp filled with red mud and are surrounded by birds that want to eat them. The red worms are more likely to survive because they are camouflaged by the mud and won't be as easily seen by predators. Therefore, more red worms will live to reproduce and more ...