Genetic diversity is always changing — both acrossspace and through time. Typically, the amount and typeof genetic diversity within a species vary across its natural range. Additionally, its genetic diversity changesover time — at least in the longterm, and sometimeseven over shorter timeframes such as a few generationsof the species. These natural changes in the geneticdiversity of a species create a dynamic landscape uponwhich any influences that we exert are superimposed. To better understand our impacts, and to decide if management actions are warranted, it is useful tounderstand the natural dynamics of genetic diversity.

Genetic diversity is affected by several ongoing naturalprocesses. These processes are: mutation, migration,genetic drift, and selection.

Mutation is the origin of all new genetic diversity,occurring when there are occasional errors in the replication of DNA or other elements of the productionand packaging of genetic information within the cells.Although it implies something negative, mutations canhave positive, neutral, or deleterious impacts. Mutationsoccur rather slowly but continuously. Mutations at onelevel, for example, in the nucleotides that are the basisof DNA, may not all be expressed at other levels — suchas protein differences or observable changes in theappearance of a plant. The rate of mutation is useful indetermining evolutionary relationships.

Migration is the movement of genetic diversity, usuallywithin a species. In plants, this occurs through pollendispersal, seed dispersal, and movement of vegetativepropagules, such as suckers or rhizomes, in species thatcan reproduce asexually. Migration, also called geneflow, occurs both with the advancing front of a populationwhen it is colonizing new areas, and when genes of twoor more populations mix through pollen and seed dispersal.The rate of migration is obviously related to the frequencyof reproduction and the distances over which pollen andseeds typically disperse.

Genetic Drift, or random genetic drift, is simply thechange in genetic diversity, or, more specifically, thechange in frequencies of different alleles, over genera-tions because of chance. For example, every pollen graincontains a different combination of alleles. Whichpollen grains — whether carried by wind, insects, orsome other medium — actually succeed in arriving at acompatible flower and producing a seed — are largelydetermined by chance events. Thus, some genetic diver-sity is usually lost at every generation through thesechance events.

Selection is perhaps the best known of the processesaffecting genetic diversity and is the only process thatdirectly results in populations becoming better adaptedto their environment. For natural selection to occur,there must be differences in fitness and survival amongindividuals and a genetic basis for those differences.Over time (generations), those individuals that are better suited to the environment live, or live longer, andproduce more offspring — those offspring having inherited the more adaptive traits (or rather, have ahigher frequency of the alleles that confer better adaptation).

[ mutation, migration,drift, and selection are ongoingnatural processes that affect

genetic diversity ]

National Forest Genetics Laboratory (NFGEL)Pacific Southwest Research StationUSDA Forest Service2480 Carson RoadPlacerville, CA USA 95667http://www.fs.fed.us/psw/programs/nfgel/

Genetic Resources Conservation ProgramUniversity of California

One Shields AvenueDavis, CA USA 95616http://www.grcp.ucdavis.edu

These processes continue over the lifetimes of individuals, populations,and species. Using estimates of the historic mutation rates for a species, orgenetic principles related to the otherprocesses, one can derive clues aboutthe geographic history and past demo-graphic changes. Genetic diversitymaintains a footprint of historic influences, allowing one to look backin time and see how the speciesexpanded and contracted its range inresponse to glacial and other climaticevents, whether it was much reduced insize at some time (a bottleneck), andwhere it commenced to rerestablish orradiate when environmental conditionsbecame more favorable (foundereffects). For example, from geneticstudies it is apparent that the northern

red oak (Quercus rubra) had a fairlylarge and continuous distribution ineastern North America during the lastglacial maximum (21,000 to 18,000years before present). The northern redoaks grew close to the ice sheets as theglaciers retreated, and there is relativelylittle (compared with many other,including European,oak species) geneticd i f f e r e n t i a t i o namong populations,and as one goesnorth in its rangethe populationsbecome a bit moregenetically distinct.New techniques for extracting DNAfrom ancient trees (fossilized wood orcones) allows another type of insight —a direct comparison of the geneticdiversity of historic plant populationswith current plant populations.

The timeframes over which theseprocesses show significant impacts canvary widely. Because changes in geneticdiversity happen over generations, thelength of a generation for the specieswill greatly influence the absolute timeover which genetic changes occur.Typically, for example, natural selection

may take a long time to show noticeable changes in genetic diversityof a population. Furthermore, theprocesses generally do not act with thesame force or in the same direction.Typically, mutations increase geneticdiversity; the other three processesreduce it. Natural selection and genetic

drift tend to enhancegenetic differencesamong populations;migration tends tohomogenize geneticdifference, decreasingthe differences amongpopulations.

The slippery, dynamicnature of genetic diversity has impor-tant implications for species manage-ment and for questions concerning the ‘normal’ or ‘healthy’ status of geneticdiversity. Because taking a diversitymeasurement at different times, places,or generations of a species would naturally give different values, thiscontext must be carefully consideredwhen designing a genetic study andinterpreting such values. For example,differences in genetic diversity betweenthe parent plants and their seeds,between samples taken ten years apart,or between plants in wildland andmore managed areas, could indicateproblems that might be addressed withappropriate management practices ormight lie within a normal range ofvariation. The ability to distinguishthese two interpretations lies in appropriately designed studies. (SeeVolume 8 for more information.)

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Red Oak

Some errors during DNA production/copying become incorporatedinto gametes (mutation)

Plants that are betteradapted survive, growmore quickly, and producemore young than others(natural selection)

Loss of pollen through chanceevents (genetic drift) Loss of some seed

through chanceevents (genetic drift)

Migration/Gene flow

[ genetic diversitychanges over time and has important

implications forspecies management ]

Figure. Various processes that effect genetic diversity.