Chance and Movement: Genetic Drift, Bottlenecks, Founders and Gene Flow

Evolution by luck

Natural selection is not the only way a population's genes can change. Imagine a small population of wildflowers in which a few plants with white flowers happen to be under a fallen tree. The tree crushes them before they make seeds. White flowers were not worse; they were just unlucky. Next year, there are fewer white alleles in the population.

This is genetic drift: evolution by chance. In this lesson you will measure allele frequencies, watch drift in action, and learn how it combines with gene flow to shape populations, especially small ones.

Measuring the gene pool: allele frequency

To see evolution happen, biologists track allele frequencies. Most organisms have two copies of each gene (one from each parent). If a gene has two alleles, A and a:

frequency of A = (number of A alleles) ÷ (total number of alleles)

The frequencies of all the alleles of a gene add up to 1. A change in allele frequency over generations is microevolution. (You will learn more about alleles, genotypes and how they are inherited in the Genetic Processes unit.)

Genetic drift: chance changes

In every generation, which individuals happen to reproduce, and which of each parent's alleles happen to end up in its offspring, involves chance. Like flipping a coin, the results can wander away from what you would expect.

• In a large population, chance effects mostly cancel out (flip a coin 1000 times and you get close to 50% heads), so allele frequencies change little by drift.
• In a small population, chance has big effects (flip 10 times and you could easily get 7 or 8 heads). Allele frequencies can swing widely, and an allele can be lost completely (frequency 0) or become fixed (frequency 1) in just a few generations.

Drift is random. Unlike natural selection, it does not make a population better adapted. It can even remove helpful alleles. Drift reduces genetic variation, which makes small populations less able to adapt to future changes.

The bottleneck effect

A bottleneck happens when a disaster, disease, or over-hunting suddenly reduces a population to a small number of survivors. The survivors carry only a random sample of the original alleles. Even if the population later grows large again, the lost variation does not come back (except slowly, by new mutations).

• Northern elephant seals were hunted for their oil until, by the 1890s, probably fewer than 100 were left (some estimates say about 20). Protected since then, they have recovered to well over 100 000, but they have very low genetic variation.
• Cheetahs have unusually low genetic variation, which scientists think is the result of one or more ancient bottlenecks.

Low variation can make a species more vulnerable to new diseases or environmental changes, since few individuals may carry alleles that help.

The founder effect

The founder effect happens when a few individuals leave a large population and start a new, isolated population, for example on an island, or a small human community that marries mostly within itself. The founders' alleles are a random sample, so the new population can have very different allele frequencies from the original.

• Some isolated human communities have high frequencies of otherwise rare inherited conditions because one or a few founders happened to carry the allele. For example, certain inherited disorders are more common in the Saguenay–Lac-Saint-Jean region of Quebec, than elsewhere. The region was settled in the 1800s largely by families from nearby Charlevoix, so much of today's population descends from a relatively small number of founders.
• Many island species descend from a few founders that arrived by flying, swimming or drifting on floating vegetation, like the ancestors of Darwin's finches.

Gene flow

Gene flow is the movement of alleles between populations, when individuals migrate and breed, or when gametes travel (pollen blown by wind or carried by bees).

• Gene flow adds variation to a population (alleles new to that population can arrive).
• Gene flow makes neighbouring populations more similar to each other. It works against the differences that selection and drift create in separate populations.
• When gene flow between two populations stops, they can evolve independently. That is an important step toward forming new species (the next lesson).

Human activity changes gene flow. Highways and dams can cut gene flow between populations of turtles, salamanders or fish; wildlife crossings and fish ladders help restore it. Pollen from crops can carry alleles into wild relatives growing nearby.