2
Grouping: Pairs or groups for discussion, but most work done individually
Teacher Background:
Evolution is the change of heritable traits in populations over generations. Evolution can be
caused by one or more of the following mechanisms: natural selection, sexual selection,
genetic drift, mutations, migration, and horizontal gene transfer. Because it is 1 of the 6
mechanisms of evolution, genetic drift by itself can cause evolution in a trait. In many
populations, two or more mechanisms can act simultaneously to facilitate evolution.
Genetic drift is evolution in a trait caused by random sampling error. In this case, sampling
error is simply the fact that in a population, the parents that create the next generation are not
an exact representation of the population. This is true in any finite population as long as not
every individual breeds and produces the same number of offspring. A simple example of
sampling error is when flipping a coin 10 times, you do not always get 5 heads and 5 tails,
which is the expected probability.
In the case of genetic drift, sampling error leads to differences between allele frequencies from
one generation to the next. That is, if for a particular gene there were two alleles, a1 and a2,
and the population in one generation started with 50% a1 and 50% a2, in the next generation
the allele frequencies might shift to 40% a1 and 60% a2. The shift in allele frequencies could
occur because just by chance, more individuals carrying the a2 allele might have successfully
reproduced than those carrying a1. In genetic drift, the change in allele frequency is due to
chance, not the differences in fitness between a1 and a2.
Genetic drift can cause fixation of an allele at a locus, which means that only one allele is left
in the entire population—reversibly, all other alleles are lost completely. The only way for a
new allele to enter the population is through mutations or migration.
There is always less sampling error in bigger populations and more error in smaller
populations. This means that the effect of genetic drift is strongest in small populations.
However, it’s important to remember that genetic drift happens in every population, no matter
the size. The only difference is the effect of drift on the trait mean in the population. Similarly,
genetic drift occurs most strongly for traits that are controlled by few loci (strongest for 1 locus
traits). Sampling error is more pronounced with small sample sizes. In the coin example, if you
flip a coin 100 times, you are more likely to get 50% heads and 50% tails than if you only flip a
coin 10 times
The result of genetic drift is less genetic variance within one population and greater variance
among different populations.
Lenski’s experimental evolution studies of bacteria are a good introduction to the idea of
genetic drift. You can read more about them here:
https://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment
Vocabulary: genetic drift, evolution, population, trait, sampling error, locus, allele, fixation, variation