Population Genetics

Darwin Meets Mendel—Not Literally

When Darwin came up with his theories of evolution and natural selection, he knew that the processes he was describing depended on heritable variation in populations. That is, they relied on differences in the features of the organisms in a population and on the ability of these different features to be passed on to offspring.

Darwin did not, however, know how traits were inherited. Like other scientists of his time, he thought that traits were passed on via blending inheritance. In this model, parents’ traits are supposed to permanently blend in their offspring. The blending model was disproven by Austrian monk Gregor Mendel, who found that traits are specified by non-blending heritable units called genes.

Although Mendel published his work on genetics just a few years after Darwin published his ideas on evolution, Darwin probably never read Mendel’s work. Today, we can combine Darwin’s and Mendel’s ideas to arrive at a clearer understanding of what evolution is and how it takes place.

Microevolution and Population Genetics

Microevolution, or evolution on a small scale, is defined as a change in the frequency of gene variants, alleles, in a population over generations. The field of biology that studies allele frequencies in populations and how they change over time is called population genetics.

Microevolution is sometimes contrasted with macroevolution, evolution that involves large changes, such as formation of new groups or species, and happens over long time periods. However, most biologists view microevolution and macroevolution as the same process happening on different timescales. Microevolution adds up gradually, over long periods of time to produce macroevolutionary changes.

Let’s look at three concepts that are core to the definition of microevolution: populations, alleles, and allele frequency.

Populations

A population is a group of organisms of the same species that are found in the same area and can interbreed. A population is the smallest unit that can evolve—in other words, an individual can’t evolve.

Alleles

An allele is a version of a gene, a heritable unit that controls a particular feature of an organism.

For instance, Mendel studied a gene that controls flower color in pea plants. This gene comes in a white allele, w, and a purple allele, W. Each pea plant has two gene copies, which may be the same or different alleles. When the alleles are different, one—the dominant allele, W—may hide the other—the recessive allele, w. A plant’s set of alleles, called its genotype, determines its phenotype, or observable features, in this case flower color.

The phenotype is the flower color. The genotype is a pair of alleles. A capital W indicates the dominant purple allele. A lowercase w indicates the recessive white allele. A flower with two capital Ws in its genotype will be purple. A flower with a capital W and lowercase w genotype will be purple. A flower with two lowercase ws in its genotype will be white.

Allele Frequency

Allele frequency refers to how frequently a particular allele appears in a population. For instance, if all the alleles in a population of pea plants were purple alleles, W, the allele frequency of W would be 100%, or 1.0. However, if half the alleles were W and half were w, each allele would have an allele frequency of 50%, or 0.5.

In general, we can define allele frequency as

[latex]\text{Frequency of allele }A=\frac{\text{Number of copies of allele }A\text{ in population}}{\text{Total number of }A\text{/}a\text{ gene copies in population}}[/latex]

Sometimes there are more than two alleles in a population (e.g., there might be A, a, and A​i alleles of a gene). In that case, you would want to add up all of the different alleles to get your denominator.

It’s also possible to calculate genotype frequencies—the fraction of individuals with a given genotype—and phenotype frequencies—the fraction of individuals with a given phenotype. Keep in mind, though, that these are different concepts from allele frequency. We’ll see an example of this difference next.

Video Summary

This video talks about population genetics, which helps to explain the evolution of populations over time.