Genetic Variation Genetic Variation in Populations A diversity of phenotypes helps the species adapt to a changing environment. Species with limited diversity run the risk of not having the traits necessary for adapting to new environmental stresses. In many cases, diversity is the result of genetic variation, defined as genetic differences between individuals within a species, which are heritable. If a trait is not passed on to the next generation, it does not play a major role in evolution. Some variation does not change the DNA sequence itself, but rather regulates which genes are turned on or off, e.g., adding methyl groups. Extra methyl groups on the outside of DNA are epigenetic tags and make up part of an organism's epigenome. An individual's lifestyle and environment can change her or his epigenome, and epigenetic information passes on to the next generation or two. Principles of Biology

Evolution depends on genetic variation. Genetic Variation in Populations Evolution depends on genetic variation. Genetic variation is the basis for evolution because the adaptive value of traits determines which traits pass on to the next generation. Principles of Biology

Genetic Variation in Populations Figure 1 Mimulus guttatus (monkey flower). This population of Mimulus has some genetic variation, at least in flower color. Mimulus populations with greater genetic diversity tend to have greater resistance to pests such as Philaenus spumaris (spittlebug). Principles of Biology

Evolution depends on genetic variation. Genetic Variation in Populations Evolution depends on genetic variation. One way to measure genetic variation is to measure levels of heterozygosity. If a population has low genetic diversity, many of the loci will be homozygous because there are fewer options for each locus. Average heterozygosity is the percentage of the gene loci that is heterozygous within a single population. Principles of Biology

Genetic Variation in Populations Figure 2 Hybrid vigor. Three tomato plants demonstrate hybrid vigor. Three plants and the fruit they produced are all lined up. The plant in the middle is a hybrid of the two on either side; its yield is much greater. The tomato plants on the left and right are highly inbred. This difference in fruit yield (number of tomatoes) demonstrates hybrid vigor. Principles of Biology

Geographic location affects genetic variation. Genetic Variation in Populations Geographic location affects genetic variation. Genetic variation exists between different populations of the same species. Because geographic features often separate the different populations of a species, this type of genetic variation is called geographic variation. Principles of Biology

Genetic Variation in Populations Figure 3 Geographic variation. Mice from geographically isolated areas have different traits as a result of differences in genotypes. The pie charts show the proportion of light and dark fur alleles present in each population (Hoekstra et al. 2006). Principles of Biology

Geographic location affects genetic variation. Genetic Variation in Populations Geographic location affects genetic variation. A cline exists when phenotypes change gradually on a continuum as a result of geographic and environmental differences. Principles of Biology

Genetic Variation in Populations Figure 4 A cline. Stotz and his colleagues compared alpha-hemoglobin amino acid variation among three different deer mouse populations in Colorado and Kansas in the U.S. The areas of the hemoglobin protein that vary in amino acid sequence are numbered in the lower left molecular model. The images at the top show the locations of the deer mouse populations. The pie charts on the bottom right shows the frequency of the two alleles that determine the amino acid sequence in the numbered regions of the protein. Principles of Biology

Genetic Variation in Populations Figure 5 Genotype distribution of Vriesea gigantea. The colors on the pie charts show the allele frequencies present in 13 different populations of Vriesea gigantea. More colors in a pie chart mean more alleles with the area of color indicating the frequency of that allele. Principles of Biology

What factors affect genetic variation? Genetic Variation in Populations What factors affect genetic variation? Changes to the nucleotide sequence of DNA, called mutations, are one way to increase genetic diversity. Mutations do not always change an organism's individual phenotype; however, silent mutations occur when the changes in the DNA result in exactly the same proteins being produced. Silent mutations do not change the fitness of an organism and can build up in populations over time. Silent mutations can be used for DNA fingerprinting or paternity testing. Mutations in DNA occur by two mechanisms: mutagens and errors in DNA replication. If these mutations occur in reproductive cells, they can be passed on to the next generation. Mutagens are environmental factors, e.g., ultraviolet light, some chemicals, or even byproducts of aerobic respiration. Whenever a cell divides, the DNA must be replicated so that each new daughter cell gets a full copy of the genome. Mistakes in this process lead to mutations. Sexual reproduction is a source of genetic diversity. The overall process of sexual reproduction has three main steps and can result in unique combinations of genes that have never been together before. The process of crossing over (or recombination) that occurs during meiosis can result in new combinations of genes on the same chromosome. The process of independent assortment of chromosomes during meiosis can result in new combinations of genes on different chromosomes. During fertilization, a cell that has two unique sets of chromosomes is made. Smaller-scale changes to DNA tend to be less deleterious, although not in all cases (e.g., Huntington's disease). Copy number variation is the variation in the number of copies a cell has of a particular DNA region. If errors occur during crossing over, the new cell can have a chromosome that has a duplicated piece or is missing a piece. If errors occur during independent assortment, the new cell can have extra or missing chromosomes. In humans, large-scale chromosomal changes tend to lead to disease. When DNA is being copied in preparation for meiosis, the DNA polymerase can "slip" and copy certain regions twice. This is particularly likely in regions that have repetitive sequences. Principles of Biology