Population Genetics

What does population genetics study?

We mainly assess genetic variation in population genetics by determining the frequency of genes and alleles within populations and if these change over time and/or space. The proportion of a specific genotype within a population is called its genotype frequency. However, the proportion of a specific phenotype within a population is called its phenotype frequency. An allele frequency is the proportion of a specific allele within a population.

The field of population genetics represents an extension of Gregor Mendel's heredity principles, integrated with Darwin's theory of evolution by natural selection. Let's first have a quick review of the main terms from mendelian genetics that we will use in population genetics:

• The trait color of the eyes in humans is determined by the genes inherited from our parents. A gene has a specific location on a chromosome, called the locus (plural loci). The gene for eye color has different variants or alleles, which determine the different colors like brown, blue, green, etc. The brown color in eyes is dominant over the other colors. When a person has one allele for the brown color and the other for the blue color, their eyes will be brown because it is the dominant allele (the blue allele would be recessive). Thus their genotype will be constituted by two different alleles (heterozygous genotype), but their phenotype will be brown eyes.

Figure 1. The phenotype for eye color in humans is determined by specific alleles. A person with at least one copy of the dominant allele has brown eyes.

Population Genetics Example

Let's see an example to understand how these definitions and concepts relate to population genetics. Think of a hypothetical plant population of 1000 individuals. These plants are now in the flowering season, and there are some individuals with purple flowers and others with white flowers.

Researchers found that this population has two alleles for this locus: one dominant and one recessive allele. They also studied the genetic composition of all individuals in this population, so we know the genotype and phenotype for each individual: AA (purple flower) = 460, Aa (purple) = 430, aa (white) = 110.

The allele for the purple color is dominant over the white one. We know that because, as we saw above, two genotypes have the same phenotype. Can you identify these genotypes? (you can check the table below). Now let's fill this table with the information we have:

Table 1. Example of how genotype frequency is calculated in a hypothetical plant population.

The total frequency for any trait we study is always 1, which means that the frequencies of all genotypes in a population add up to 1 (AA + Aa + aa = 1), the same for alleles (A + a = 1). Frequencies always go from 0 to 1 (as percentages always go from 0 to 100%). In population genetics, a frequency of 0 would mean that an allele is not present in the population, while a frequency of 1 means the allele is the only one present in the population; in this case, the allele is fixed in the population.

Genetic differences between populations

The allele and genotype frequencies we calculated in the previous example are like a snapshot of that population at a certain point in time or space. Do you think these frequencies will remain the same over time? It is common for allele frequencies in a population to change over time, which means a population has evolved. It is important to remember that while natural selection acts on individuals, it is the population that evolves. When allele frequencies change in a relatively short time or a few generations, it is called microevolution (which usually refers to changes at the population level).

Factors Affecting Population Genetics

In addition to natural selection, other evolutionary processes that affect allele frequency are:

• mutation
• nonrandom mating
• genetic drift
• genetic flow.

These mechanisms can affect a population at any time, and several can act upon an allele or gene simultaneously. Each mechanism's effect on a population varies and can increase or decrease genetic variation, allele frequency, or adaptation.

Mutations

Mutations are unpredictable changes in the nucleotides that form a DNA sequence. These are the base for genetic variation since mutations can produce new alleles. A mutation occurs in individual organisms, and for a mutation to have a long-term effect on the population, it must be heritable to be transmitted to subsequent generations. When a mutation happens in somatic cells (body cells), it affects the organism, but it is not transmitted to the next generation (offspring). For a mutation to be heritable, it must occur in a reproductive cell (gametes = sperm or egg).

Although mutations are important for creating genetic variation, they do not happen often enough to affect allele frequency. However, the effect can be significant when natural selection or other evolutionary mechanisms act on these mutations.

Nonrandom Mating

In wild populations, individuals usually choose another individual for breeding based on the phenotype (indirectly choosing the corresponding genotype). For example, female birds might prefer to mate with male birds that are similar in coloration. In other words, not all individuals will have the same probability to breed, so it is not random. Nonrandom mating can change the genotype frequencies, but not the allele frequencies, within a population (thus, its effect on evolution is debatable).

Genetic Drift

Genetic drift causes a reduction in the genetic variability in the population, and the changes caused by genetic drift are usually not adaptive (because they are random, caused by chance). Random natural disasters such as hurricanes, flooding, or landslides can affect animal and plant populations. Many individuals may die due to these random events, even if they are well adapted to their environment.

A key factor here is that these drift effects are stronger in small populations because a dramatic reduction in an adaptive allele or genotype can decrease the overall fitness of that population. It is less likely that a large population will lose a significant percentage of these adaptive alleles or genotypes. A sudden reduction in population size (and its genetic variability) caused by adverse environmental conditions is a bottleneck. When a small part of a population colonizes a new area, it is called the founder effect.

Gene Flow

Many animals move from their birth population to a different one during the breeding season; this is a type of migration. Migrants can introduce a new allele to a population, or if it carries the same alleles already present in the population, they can change the frequency of the allele.

Gene flow is a movement of alleles between populations and can cause changes in allele frequency. Interchanging alleles between two populations tend to counteract the effects of natural selection and genetic drift; hence, genetic flow usually decreases the differences or variations between these populations.

Natural Selection

The genotypes (and corresponding phenotypes) with greater survival and reproduction probabilities for a specific environment will contribute more offspring to the next generation through natural selection. Natural selection causes an adaptive change (higher survival and reproduction probabilities) in allele frequency. Natural selection acts on the phenotype of an individual, but it is the population that adapts to a particular environment.

Figure 2. This illustration shows how natural selection occurs in a population.