Genetic variation is necessary for evolution because it allows for natural selection to alter the frequency of alleles that already exist in the population.
Looking around at individuals in any population, a variation in traits is noticeable. Within a species many traits may be displayed among individuals of the population. Differences in size, shade, and shape are abundant. Some of the differences seen are caused by the environment and experiences of each individual. For example, hormonal changes brought on by cooler temperatures result in the fur of an arctic fox turning from brown to white. But much of the variation seen in populations is caused by differences in genes, which are the portion of a DNA molecule that control the characteristics that an offspring will have. With the exception of clones (e.g., identical twins), each individual within a population carries a unique set of genes, half of which are received from one parent and half from the other. The total set of genes of all individuals in a population is called the gene pool.
A nucleotide is an organic compound consisting of a sugar, a phosphate, and a nitrogenous base; this compound forms the basis of a genetic sequence. Differences between individuals can be measured all the way down to the level of individual nucleotides. However, measuring differences within a gene pool at this level is not particularly useful, because much of that variation lies in components of an organism's DNA that do not encode the order by which proteins occur and, therefore, do not result in variations. It is often better to measure variation at the gene level because it is at this level that both quantitative and discrete traits are coded. An allele is an alternative version of a gene. The physical location of a gene on a chromosome is the locus (plural, loci). Variation at the gene level can be estimated by calculating the proportion of loci within a gene pool that are heterozygous, meaning they have two different alleles for a gene. This means finding the average percentage of chromosome locations that have two different alleles.
Genetic variation can arise as a change in DNA sequence, called a mutation. Mutations can occur as mistakes during DNA replication. However, if the mutation does not happen in a cell that is passed down to offspring, such as an egg or a sperm cell, the change cannot lead to a new allele. Variation can also arise at the chromosome level, which affects many loci at once. These changes can occur as mistakes during meiosis. Meiosis is a process in cell division during which the number of chromosomes decreases to half the original number by two divisions of the nucleus, resulting in the production of gametes. Gametes are the cells used during sexual reproduction to produce a new individual organism. Gametes have half the number of chromosomes as the parent cell.
The random distribution of the pairs of genes on different chromosomes to the gametes during meiosis is known as independent assortment. Independent assortment increases genetic variation. As the chromosomes line up in preparation for separation during the first round of cell division, each homologous pair is randomly oriented. Homologues are those chromosomes with near identical structures. In humans, for example, each of the 23 pairs has equal probability of arranging either way. This means that there are millions of possible combinations of chromosomes that could end up in a gamete. Prior to the first round of cell division, another process occurs that increases genetic variation. Crossing-over is the exchange of DNA between homologous chromosomes during meiosis. This process creates recombinant chromosomes—chromosomes that have a mix of DNA from the individual's parents. This added layer of genetic mixing occurs before independent assortment. After the gametes are formed at the end of meiosis, some will participate in fertilization with a random gamete from another individual. This final step is yet another opportunity for genetic variation to be introduced. At this point, possible combinations of genes in the offspring exceed 70 trillion. Each of these processes is critical, because genetic variation is required for evolution.