What is the term used to describe genetic changes in a species population?
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The synthetic theory of evolution as described by Sewell Wright attempts to explain evolution in terms of changes in gene frequencies. This theory states that a species evolves when gene frequencies changes and the species moves it to a higher level of adaptation for a specific ecological niche. Several factors such as mutation of alleles and migration of individuals with those new alleles will create variation in the population. Selection will then chose the better adapted individuals, and the population will have evolved. Show
The classic example which supports this theory is that of the peppered moth in England. The moth can be either dark or light colored. Prior to the industrialization of central England, the light-colored allele was most prevalent. The light-colored moths would hide on the white-barked trees and avoid bird predation. But the pollution generated by the new industries stained the light-colored trees dark. Gradually the light-colored moth was attacked and that allele became much less prevalent. In its place, the dark-colored allele became the most predominant allele because moths that carried that allele could camouflage themselves on the stained trees and avoid being eaten by their bird predators. Clearly the population had evolved to a higher adaptive condition. Because population changes require changes in gene frequencies, it is important to understand how these frequencies can change. The three primary methods of change are mutation, migration and selection. Each will be considered individually. MutationMutations are classified as beneficial, harmful or neutral. Harmful mutations will be lost if they reduce the fitness of the individual. If fitness is improved by a mutation, then frequencies of that allele will increase from generation to generation. The mutation could be a change in one allele to resemble one currently in the population, for example from a dominant to a recessive allele. Alternatively, the mutation could generate an entirely new allele. Most of these mutations though will be detrimental and lost. But if the environment changes, then the new mutant allele may be favored and eventually become the dominant alelle in that population. If the mutation is beneficial to the species as a whole, migration from the population in which it initially arose must occur for it to spread to other populations of the species.The most basic type of mutation is the change in a single nucleotide in the gene. Mutations are generally deleterious and are selected against. But the genome of a species can undergo another type of change, gene duplication, which actually favors mutational events. If a single gene that is important undergoes a duplication, mutation in the duplicated copy would not necessarily reduce the fitness of the individual because it still would have a functioning copy of the original gene. With this adaptive constraint removed, further changes can occur that generate a new gene that has a similar function in the organism, but may function at a specific time in development, or in a unique location in the individual. This type of evolution generates multigene families. Many important genes such as hemoglobin and muscle genes in humans, and seed storage and photosynthetic genes in plants are organized as multigene families. Evolution is the process by which populations of organisms change over generations. Genetic variations underlie these changes. Genetic variations can arise from gene variants (also called mutations) or from a normal process in which genetic material is rearranged as a cell is getting ready to divide (known as genetic recombination). Genetic variations that alter gene activity or protein function can introduce different traits in an organism. If a trait is advantageous and helps the individual survive and reproduce, the genetic variation is more likely to be passed to the next generation (a process known as natural selection). Over time, as generations of individuals with the trait continue to reproduce, the advantageous trait becomes increasingly common in a population, making the population different than an ancestral one. Sometimes the population becomes so different that it is considered a new species. Not all variants influence evolution. Only hereditary variants, which occur in egg or sperm cells, can be passed to future generations and potentially contribute to evolution. Some variants occur during a person’s lifetime in only some of the body’s cells and are not hereditary, so natural selection cannot play a role. Also, many genetic changes have no impact on the function of a gene or protein and are not helpful or harmful. In addition, the environment in which a population of organisms lives is integral to the selection of traits. Some differences introduced by variants may help an organism survive in one setting but not in another—for example, resistance to a certain bacteria is only advantageous if that bacteria is found in a particular location and harms those who live there. So why do some harmful traits, like genetic diseases, persist in populations instead of being removed by natural selection? There are several possible explanations, but in many cases, the answer is not clear. For some conditions, such as the neurological condition Huntington disease, signs and symptoms occur later in life, typically after a person has children, so the gene variant can be passed on despite being harmful. For other harmful traits, a phenomenon called reduced penetrance, in which some individuals with a disease-associated variant do not show signs and symptoms of the condition, can also allow harmful genetic variations to be passed to future generations. For some conditions, having one altered copy of a gene in each cell is advantageous, while having two altered copies causes disease. The best-studied example of this phenomenon is sickle cell disease: Having two altered copies of the HBB gene in each cell results in the disease, but having only one copy provides some resistance to malaria. This disease resistance helps explain why the variants that cause sickle cell disease are still found in many populations, especially in areas where malaria is prevalent.
National Institute of General Medical Sciences: The New Genetics Chapter 3: Life’s Genetic Tree Learn.Genetics from the University of Utah: Evolution: DNA and the Unity of Life Cold Spring Harbor Lab: Genetic Origins Understanding Evolution from the University of California Museum of Paleontology: Huntington’s Chorea: Evolution and Genetic Disease Understanding Evolution from the University of California Museum of Paleontology: Misconceptions About Natural Selection Other chapters in Help Me Understand Genetics The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health. |
