2.1 Evolution of Populations

The mechanisms of inheritance, or genetics, were not understood at the time Charles Darwin and Alfred Russel Wallace were developing their idea of natural selection. This lack of understanding was a stumbling block to understanding many aspects of evolution. In fact, the predominant (and incorrect) genetic theory of the time, blending inheritance, made it difficult to understand how natural selection might operate. Darwin and Wallace were unaware of the genetics paper by Gregor Mendel, called Experiments on Plant Hybridization, which was published in 1866, not long after the publication of Darwin’s book On the Origin of Species (Mendel, 1866). Mendel’s work was rediscovered in the early twentieth century, at which time geneticists were rapidly coming to an understanding of the basics of inheritance. Over the next few decades, genetics and evolution were integrated into what became known as the modern synthesis—the coherent understanding of the relationship between natural selection and genetics that took shape by the 1940s and is generally accepted today. In sum, the modern synthesis describes how evolutionary processes, such as natural selection, can affect a population’s genetic makeup, and, in turn, how this can result in the gradual evolution of populations and species.

Population Genetics

A gene for a particular trait can have different versions, called alleles. Each individual in a population of diploid organisms carries two alleles for each gene—one from each parent. However, in the whole population, there can be many different alleles for the same gene.

Gregor Mendel studied how alleles are passed from parents to offspring (Mendel, 1866). Later, scientists in a field called population genetics began to study how these alleles change in a population over time.

The allele frequency is how common a certain allele is in a population. Until now, we’ve talked about evolution as changes in traits we can see. But behind those visible changes are changes in genes. In population genetics, evolution means a change in allele frequency over time.

For example, in the ABO blood type system, a study in Jordan found that the I ^A allele made up 26.1% of the population’s ABO alleles. The I ^B allele was 13.4%, and i was 60.5% (Hanania, Hassawi, & Irshaid, 2007). If these numbers change over time, that’s evolution. These small, gradual genetic changes are called microevolution.

Microevolution happens when certain alleles help individuals survive or reproduce better. These helpful alleles become more common because they get passed on more often. Over time, this changes the gene pool, which is the total set of alleles in a population.

Variation and Genetic Diversity

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have a genetic basis; otherwise, selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons, such as an individual being taller because of better nutrition rather than different genes.

Genetic diversity, the total variety of genetic traits within a population, comes from two main sources: mutation and sexual reproduction. As discussed in Section 1.3, mutations are random changes in DNA and are the ultimate source of new alleles in a population. These new alleles introduce novel traits that may be acted upon by natural selection.

In addition to mutation, sexual reproduction increases genetic variation by reshuffling existing alleles. During meiosis, processes like crossing over and independent assortment create new combinations of genes. When two parents reproduce, their offspring inherit a unique mix of alleles, resulting in diverse genotypes and, consequently, a wide range of phenotypes. This genetic variation is essential for populations to adapt to changing environments over time.

Hardy-Weinberg Principle of Equilibrium

In the early twentieth century, English mathematician Godfrey Hardy and German physician Wilhelm Weinberg stated the principle of equilibrium to describe the genetic makeup of a population. The theory, which later became known as the Hardy-Weinberg principle of equilibrium, states that a population’s allele and genotype frequencies are inherently stable— unless some kind of evolutionary force is acting upon the population, neither the allele nor the genotypic frequencies would change. The Hardy-Weinberg principle assumes conditions with no mutations, migration, or selective pressure for or against genotype, plus an infinite population. While no population can satisfy those conditions, the principle offers a useful model against which to compare real population changes.

Working under this theory, population geneticists represent different alleles as different variables in their mathematical models. The variable p represents the dominant allele in the population, while the variable q represents the recessive allele. For example, when looking at Mendel’s peas, the variable p represents the frequency of Y alleles that confer the colour yellow, and the variable q represents the frequency of y alleles that confer the colour green. If these are the only two possible alleles for a given locus in the population, . In other words, all the p alleles and all the q alleles make up all of the alleles for that locus that are found in the population.

However, what ultimately interests most biologists is not the frequencies of different alleles but the frequencies of the resulting genotypes, known as the population’s genetic structure, from which scientists can surmise the distribution of phenotypes. If the phenotype is observed, only the genotype of the homozygous recessive alleles can be known; the calculations provide an estimate of the remaining genotypes.

Since each individual carries two alleles per gene, if the allele frequencies (p and q) are known, predicting the frequencies of these genotypes is a simple mathematical calculation to determine the probability of getting these genotypes if two alleles are drawn at random from the gene pool. So in the above scenario, an individual pea plant could be pp (YY), and thus produce yellow peas; pq (Yy), also yellow; or qq (yy), and thus produce green peas. In other words, the frequency of pp individuals is simply the frequency of pq individuals is ; and the frequency of qq individuals is . And, again, if and are the only two possible alleles for a given trait in the population, these genotype frequencies will sum to one:

Let us assume that there are two alleles for a particular gene in a population, typically represented by:

Then, according to the Hardy-Weinberg equation, the sum of the allele frequencies must equal 1

Using these allele frequencies, we can predict the genotype frequencies in the population with the formula:

Read the following examples and try to solve them. Click on “View Solution” to check your work.

“Population Evolution” from Principles of Biology by Lisa Bartee, Walter Shriner & Catherine Creech is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

“Natural Selection & Population Genetics” from Introductory Biology: Ecology, Evolution, and Biodiversity by Erica Kosal is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.