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Mutation, Migration, Drift, and Selection Are Ongoing Natural Processes That Genetic diversity is always changing — both across Migration is the movement of genetic diversity, usually space and through time. Typically, the amount and type within a species. In plants, this occurs through pollen of genetic diversity within a species vary across its dispersal, seed dispersal, and movement of vegetative natural range. Additionally, its genetic diversity changes propagules, such as suckers or rhizomes, in species that over time — at least in the longterm, and sometimes can reproduce asexually. Migration, also called gene even over shorter timeframes such as a few generations flow, occurs both with the advancing front of a population of the species. These natural changes in the genetic when it is colonizing new areas, and when genes of two diversity of a species create a dynamic landscape upon or more populations mix through pollen and seed dispersal. which any influences that we exert are superimposed. The rate of migration is obviously related to the frequency To better understand our impacts, and to decide if of reproduction and the distances over which pollen and management actions are warranted, it is useful to seeds typically disperse. understand the natural dynamics of genetic diversity. Genetic Drift, or random genetic drift, is simply the change in genetic diversity, or, more specifically, the [ mutation, migration, change in frequencies of different alleles, over genera- drift, and selection are ongoing tions because of chance. For example, every pollen grain natural processes that affect contains a different combination of alleles. Which pollen grains — whether carried by wind, insects, or genetic diversity ] some other medium — actually succeed in arriving at a compatible flower and producing a seed — are largely Genetic diversity is affected by several ongoing natural determined by chance events. Thus, some genetic diver- processes. These processes are: mutation, migration, sity is usually lost at every generation through these genetic drift, and selection. chance events. Mutation is the origin of all new genetic diversity, Selection is perhaps the best known of the processes occurring when there are occasional errors in the affecting genetic diversity and is the only process that replication of DNA or other elements of the production directly results in populations becoming better adapted and packaging of genetic information within the cells. to their environment. For natural selection to occur, Although it implies something negative, mutations can there must be differences in fitness and survival among have positive, neutral, or deleterious impacts. Mutations individuals and a genetic basis for those differences. occur rather slowly but continuously. Mutations at one Over time (generations), those individuals that are level, for example, in the nucleotides that are the basis better suited to the environment live, or live longer, and of DNA, may not all be expressed at other levels — such produce more offspring — those offspring having as protein differences or observable changes in the inherited the more adaptive traits (or rather, have a appearance of a plant. The rate of mutation is useful in higher frequency of the alleles that confer better determining evolutionary relationships. adaptation). National Forest Genetics Laboratory (NFGEL) Genetic Resources Conservation Program Pacific Southwest Research Station University of California USDA Forest Service One Shields Avenue 2480 Carson Road Davis, CA USA 95616 Placerville, CA USA 95667 http://www.grcp.ucdavis.edu http://www.fs.fed.us/psw/programs/nfgel/ red oak (Quercus rubra) had a fairly may take a long time to show large and continuous distribution in noticeable changes in genetic diversity eastern North America during the last of a population. Furthermore, the glacial maximum (21,000 to 18,000 processes generally do not act with the years before present). The northern red same force or in the same direction. oaks grew close to the ice sheets as the Typically, mutations increase genetic glaciers retreated, and there is relatively diversity; the other three processes little (compared with many other, reduce it. Natural selection and genetic including European, drift tend to enhance oak species) genetic genetic differences Red Oak [ genetic diversity differentiation among populations; changes over time These processes continue over the among populations, migration tends to lifetimes of individuals, populations, and as one goes and has important homogenize genetic and species. Using estimates of the north in its range implications for difference, decreasing historic mutation rates for a species, or the populations the differences among genetic principles related to the other become a bit more species management ] populations. processes, one can derive clues about genetically distinct. The slippery, dynamic the geographic history and past demo- New techniques for extracting DNA nature of genetic diversity has impor- graphic changes. Genetic diversity from ancient trees (fossilized wood or tant implications for species manage- maintains a footprint of historic cones) allows another type of insight — ment and for questions concerning the influences, allowing one to look back a direct comparison of the genetic ‘normal’ or ‘healthy’ status of genetic in time and see how the species diversity of historic plant populations diversity. Because taking a diversity expanded and contracted its range in with current plant populations. measurement at different times, places, response to glacial and other climatic The timeframes over which these or generations of a species would events, whether it was much reduced in processes show significant impacts can naturally give different values, this size at some time (a bottleneck), and vary widely. Because changes in genetic context must be carefully considered where it commenced to rerestablish or diversity happen over generations, the when designing a genetic study and radiate when environmental conditions length of a generation for the species interpreting such values. For example, became more favorable (founder will greatly influence the absolute time differences in genetic diversity between effects). For example, from genetic over which genetic changes occur. the parent plants and their seeds, studies it is apparent that the northern Typically, for example, natural selection between samples taken ten years apart, or between plants in wildland and more managed areas, could indicate Plants that are better adapted survive, grow problems that might be addressed with more quickly, and produce Some errors during appropriate management practices or more young than others DNA production/copying (natural selection) might lie within a normal range of become incorporated into gametes (mutation) variation. The ability to distinguish these two interpretations lies in appropriately designed studies. (See Volume 8 for more information.) The United States Department of Agriculture (USDA) prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation and marital or family status. (Not all prohibited bases apply to all programs.) Persons with disabilities who require alternative Loss of pollen means for communication of program information (Braille, through chance large print, audiotape, etc.) should contact USDA’s TARGET events (genetic drift) Loss of some seed Center at: (202) 720-2600 (voice and TDD). To file a complaint through chance of discrimination, write: USDA Director, Office of Civil events (genetic drift) Rights, Room 326-W, Whitten Building, 14th and Independent Avenue, SW, Washington, DC 20250-9410, or call: (202) 720-5964 (voice or TDD). Migration/Gene flow USDA is an equal opportunity provider and employer. Figure. Various processes that effect genetic diversity. 2006.
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