Perceiving patterns in nature is a beginning to and seed dispersal, its mating system, and its natural understanding the basis of biological diversity, sensing range) can be used to establish a reasonable idea of its some order in what may otherwise seem chaotic or random spatial genetic pattern. Sometimes environmental spatial arrays. Spatial patterns in genetic diversity, also features are indicators of the spatial genetic patterns if a called ‘genetic structure’ or ‘spatial genetic structure,’ species is adapted to changes in these features. For often reflect biologically meaningful processes. example, a species that occurs at a range of elevations Recognizing these patterns, giving genes a ‘physical may show a gradient in genetic diversity that address,’ and understanding their basis can provide a corresponds with adaptations to various elevations. stronger scientific basis for conservation and restoration However, these natural processes may not all be consistent decisions by making use of biologically meaningful or pushing in the same direction. For example, a units. These patterns are the result of natural processes, population may be locally adapted to microclimate and the characteristics of the species, and historical events. moisture availability (which would suggest local genetic structure) but also have long-distance seed and pollen [ patterns of genetic dispersal (that would tend to mix up the genes with diversity are the result of other populations and undermine local adaptations). So natural processes, the direct genetic studies are needed for confirmation of the genetic structure. characteristics of the species, A traditional approach to describing spatial patterns in and historical events ] genetic diversity of plants is to sample individuals widely Spatial patterns reflect the natural genetic processes across the species’ range and present a picture of the (described in Volume 3) — migration, natural selection, overall genetic structure based on various kinds of data. and genetic drift. For example, if pollen and seeds are However, with further or more detailed studies, it is distributed widely for a particular species, one might common to discover that spatial genetic structure often expect little evidence of genetic structure within a local varies within a species — both over the range of the geographic area. Conversely, if the plant is capable of species and over time. The spatial scale at which the self-fertilization or has heavy seeds that tend to germinate genetic diversity is studied — over its entire range, in within short distances of the parents, one might expect relation to large-scale physical features such as to find more local or fine-scale patterns in genetic diversity. mountain ranges or drainages, or in a specific locale — In fact, in the absence of any direct genetic data, certain will affect the genetic pattern that we observe. For features of the plant species (such as its type of pollen example, in some areas of the species’ range there may

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/ be strong selection pressures (for example, climate may be very limiting) resulting in fine-scale genetic structure; in other areas, the same species may show only weak spatial genetic structure (that is, genetic diversity seems randomly distributed) because of little spatial genetic patterns over time. Over local adaptation or countervailing the longer term (evolutionary time influences of gene flow. Similarly, scales), these influences are compounded different kinds of genetic data may and may be supplemented with yield different patterns (see Volumes 5 extreme events (such as glaciations or and 7 for further discussion on this continental drift) that severely reduce populations may be more equal in topic). These differing patterns are not the species (and its genetic diversity), their size and longevity, and exchange inconsistent messages — each is fragment its natural range and genetic material on a more equitable providing a part of the story about how disrupt gene flow, allow hybridizations level. Understanding genetic patterns the genetic diversity of the species has between formerly unconnected popula- at the regional level, and the underlying been influenced by the environment tions (or sometimes species), or shift reasons for them, is important for and its own biological characteristics effective conservation and restoration over time. [ recognizing genetic decisions. For example, the loss of a Spatial patterns also may change over patterns can population through habitat conversion time — for example, in response to lead to better can have a range of impacts for the changes in direction or intensity of conservation and species on a regional level, depending natural processes. Over the short-term restoration decisions ] on the genetic interactions among (generations), the spatial genetic populations. Similarly, this information pattern exhibited by the adult plants assists decisions about whether gene genetic diversity dramatically if only a may differ substantially from that seen flow corridors (re-establishing connec- small fraction is adapted to the in the seed produced in any particular tions between plant populations) are emerging environmental conditions. year, in the resulting seedlings, and in advised, the importance of conserving Thus, current patterns of genetic later life stages. Genetic drift and suitable but currently unoccupied diversity may reflect historic conditions natural selection continue to shape the habitat, and whether the better conser- or events, even if these influences are vation strategy is to conserve one large no longer present. or several smaller populations for a Finally, spatial genetic patterns reflect particular plant species. the regional dynamics of a species — how plant populations interact on a

larger regional or landscape level. The United States Department of Agriculture (USDA) Different plant species display different prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, regional dynamics: they differ in the disability, political beliefs, sexual orientation and marital or family status. (Not all prohibited bases apply to all pro- amount and direction of genetic grams.) Persons with disabilities who require alternative means for communication of program information (Braille, connection (gene flow) among the large print, audiotape, etc.) should contact USDA’s TARGET populations. Sometimes there are larger Center at: (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write: USDA Director, Office of Civil populations that are relatively stable Rights, Room 326-W, Whitten Building, 14th and Independent Avenue, SW, Washington, DC 20250-9410, or over time and act as the source of new call: (202) 720-5964 (voice or TDD). genes for smaller, less stable popula- tions or as foundation stock for new USDA is an equal opportunity provider and employer. Photo credits: (top left) Paul Wray, Iowa State University, (top right) USDA populations. For other species, the APHIS PPQ, (others) Dave Powell, USDA Forest Service, www.forestryimages.org. 2006