What are the two main causes of variation in a population?

Genetic variation is the difference in DNA sequences between individuals within a population. Variation occurs in germ cells i.e. sperm and egg, and also in somatic (all other) cells. Only variation that arises in germ cells can be inherited from one individual to another and so affect population dynamics, and ultimately evolution. Mutations and recombination are major sources of variation.

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What are mutations?
What is recombination?

What are mutations?

Mutations are the original source of genetic variation. A mutation is a permanent alteration to a DNA sequence. De novo (new) mutations occur when there is an error during DNA replication that is not corrected by DNA repair enzymes. It is only once the error is copied by DNA replication, and fixed in the DNA that it is considered to be a mutation (Figure 1). Mutations may be beneficial to the organism; deleterious (harmful) to the organism; or neutral (have no effect on the fitness of the organism).

Somatic mutations can accumulate in our cells and are mostly harmless. They can lead to local changes in tissues such as moles appearing on the skin, and can also have more serious effects – for example leading to cancer. To learn more about the role of somatic mutations in cancer have a look at this paper by Martincorena and Campbell1. In this course we focus on heritable genetic variation, i.e. variation that occurs in germ cells.

What is recombination?

Recombination is another major source of genetic variation Each of us has a mixture of genetic material from our parents. The mixing of this genetic material occurs during recombination when homologous DNA strands align and cross over. Recombination effectively ‘shuffles’ maternal and paternal DNA, creating new combinations of variants in the daughter germ-cells (Figure 2).

Figure 2 Recombination contributes to human genetic variation by shuffling parental DNA and creating new combinations of variants. Image source: Creation Wiki.

Genetic variation is a measure of the variation that exists in the genetic makeup of individuals within population.

Learning Objectives

Assess the ways in which genetic variance affects the evolution of populations

Key Points

Genetic variation is an important force in evolution as it allows natural selection to increase or decrease frequency of alleles already in the population.
Genetic variation can be caused by mutation (which can create entirely new alleles in a population), random mating, random fertilization, and recombination between homologous chromosomes during meiosis (which reshuffles alleles within an organism’s offspring).
Genetic variation is advantageous to a population because it enables some individuals to adapt to the environment while maintaining the survival of the population.

Key Terms

genetic diversity: the level of biodiversity, refers to the total number of genetic characteristics in the genetic makeup of a species
crossing over: the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes
phenotypic variation: variation (due to underlying heritable genetic variation); a fundamental prerequisite for evolution by natural selection
genetic variation: variation in alleles of genes that occurs both within and among populations

Genetic variation is a measure of the genetic differences that exist within a population. The genetic variation of an entire species is often called genetic diversity. Genetic variations are the differences in DNA segments or genes between individuals and each variation of a gene is called an allele.For example, a population with many different alleles at a single chromosome locus has a high amount of genetic variation. Genetic variation is essential for natural selection because natural selection can only increase or decrease frequency of alleles that already exist in the population.

Genetic variation is caused by:

mutation
random mating between organisms
random fertilization
crossing over (or recombination) between chromatids of homologous chromosomes during meiosis

The last three of these factors reshuffle alleles within a population, giving offspring combinations which differ from their parents and from others.

Figure \(\PageIndex{1}\): Genetic variation in the shells of Donax variabilis: An enormous amount of phenotypic variation exists in the shells of Donax varabilis, otherwise known as the coquina mollusc. This phenotypic variation is due at least partly to genetic variation within the coquina population.

Variation allows some individuals within a population to adapt to the changing environment. Because natural selection acts directly only on phenotypes, more genetic variation within a population usually enables more phenotypic variation. Some new alleles increase an organism’s ability to survive and reproduce, which then ensures the survival of the allele in the population. Other new alleles may be immediately detrimental (such as a malformed oxygen-carrying protein) and organisms carrying these new mutations will die out. Neutral alleles are neither selected for nor against and usually remain in the population. Genetic variation is advantageous because it enables some individuals and, therefore, a population, to survive despite a changing environment.

Figure \(\PageIndex{1}\): Low genetic diversity in the wild cheetah population: Populations of wild cheetahs have very low genetic variation. Because wild cheetahs are threatened, their species has a very low genetic diversity. This low genetic diversity means they are often susceptible to disease and often pass on lethal recessive mutations; only about 5% of cheetahs survive to adulthood.

Some species display geographic variation as well as variation within a population. Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or when they are under selection pressures from a different environment. One example of geographic variation are clines: graded changes in a character down a geographic axis.

Gene duplication, mutation, or other processes can produce new genes and alleles and increase genetic variation. New genetic variation can be created within generations in a population, so a population with rapid reproduction rates will probably have high genetic variation. However, existing genes can be arranged in new ways from chromosomal crossing over and recombination in sexual reproduction. Overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction.

Genetic variation refers to differences among the genomes of members of the same species. A genome is all the hereditary information—all the genes—of an organism. For instance, the human genome contains somewhere between twenty and twenty-five thousand genes.

Genes are units of hereditary information, and they carry instructions for building proteins. The genes that are encoded within these proteins are what enable cells to function. Most organisms that reproduce sexually have two copies of each gene, because each parent cell or organism donates a single copy of its genes to its offspring. Additionally, genes can exist in slightly different forms, called alleles, which further adds to genetic variation.

The combination of alleles of a gene that an individual receives from both parents determines what biologists call the genotype for a particular trait, such as hair texture. The genotype that an individual possesses for a trait, in turn, determines the phenotype—the observable characteristics—such as whether that individual actually ends up with straight, wavy, or curly hair.

Genetic variation within a species can result from a few different sources. Mutations, the changes in the sequences of genes in DNA, are one source of genetic variation. Another source is gene flow, or the movement of genes between different groups of organisms. Finally, genetic variation can be a result of sexual reproduction, which leads to the creation of new combinations of genes.

Genetic variation in a group of organisms enables some organisms to survive better than others in the environment in which they live. Organisms of even a small population can differ strikingly in terms of how well suited they are for life in a certain environment. An example would be moths of the same species with different color wings. Moths with wings similar to the color of tree bark are better able to camouflage themselves than moths of a different color. As a result, the tree-colored moths are more likely to survive, reproduce, and pass on their genes. This process is called natural selection, and it is the main force that drives evolution.