Biological diversity is a commonly recognized value in that are geographically close would be more likely to natural resources management. This diversity is often interbreed than those more distant, we recognize plant represented as a hierarchy of discrete units such as ‘populations’ as local breeding units. Across a species’ native range, these populations may differ genetically species, ecosystems, and landscapes. There is a parallel from one another — sometimes a reflection of local hierarchy in taxonomy that orders diversity from species adaptations. Within populations, all the individuals can (e.g., Populus trichocarpa) to genera (Populus) to families be genetically distinct from one another depending on (Salicaceae) and so on, up to kingdoms (Plantae). In both their breeding system. Genetic diversity continues down cases, it is understood that biological diversity is really the line (see Figure) at the gene level, with different more of a continuum — particularly with plant species variations of a gene, called alleles, potentially occurring that tend to hybridize more freely than do animals. The within the same plant (as they have two or more copies of each gene). Genes themselves are constructed of a discrete units are helpful organizing tools but to truly sequence of biological units, called nucleotides: variation understand biological diversity, the continuity should be in these nucleotides can (but doesn’t necessarily) create considered. meaningful changes in the gene. So genetic diversity [ biological diversity is neither ends nor begins with the species: it continues in really more of a both directions. continuum — particularly As suggested above, plants, in particular, frequently blur with plant species that the distinctions among units we recognize as species or tend to hybridize ] populations. In part, this is a consequence of plants

The species is a unit that is common to both the Figure. Hierarchy of genetic diversity in nature biological diversity and taxonomic classification systems. Typically, it is also the focus in management and regulation of natural resources. However, in nature, diversity does not stop at the species level. Within species, there is often considerable diversity related to environmental influences, genetic differences, or a combination of both. For example, a species may contain two or more subspecies that typically have some genetic differences from one another. Because plants

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/ More Information Forest conservation genetics, principles and practice. (2000) A. Young, D. Boshier, and T. Boyle (eds.). CSIRO Publishing, Collingwood VIC, Australia. sometimes having fewer reproductive Genetically appropriate choices for plant barriers to crossing with other popu- materials to maintain biological diversity. lations or species than do animals. (2004) D.L. Rogers and A.M. Montalvo. University of California. Report to the USDA Although technically a ‘hybrid’ could Forest Service, Rocky Mountain Region, describe the progeny that results Lakewood, CO. Online: www.fs.fed.us/r2/publi- from a cross between any two dissim- cations/botany/plantgenetics.pdf. ilar individuals, it is more The role of forest genetics in managing conventionally applied to [ genetic diversity is ecosystems. (1997) L.E. DeWald and M.F. individuals that arise from important to the Mahalovich. Journal of Forestry 95(4):12-16. mating between two differ- species’ ability to Collecting plant genetic diversity, technical ent species, subspecies, or adapt to changing guidelines. (1995) L. Guarino, V. Ramanatha populations. So for species Rao, and R. Reid (eds.). CAB International, environmental condi- Oxon, UK. that do hybridize, this tions over time ] process contributes to more Genetic diversity and the survival of popula- and selection for the desirable trait tions. (2000) G. Booy, R.J.J. Hendricks, M.J.M. of a continuum in genetic diversity. Smulders, J.M. VanGroenendael, and B. can lead to an increase in the Focusing on diversity levels within Vosman. Plant Biology 2(4):379-395. frequency of that trait. These species — such as regional units, Most species are not driven to extinction processes are merely concentrating populations, and hybrid zones (to the before genetic factors impact them. (2004) and repackaging existing genetic extent they occur) — is an important D. Spielman, B.W. Brook, and R. Frankham. diversity. PNAS 101(4):15261-15264. consideration in conserving biological • Important to the species’ ability to diversity. Recognizing and managing The population genetic consequences of habi- colonize new areas and occupy the genetic diversity within a species tat fragmentation for plants. (1996) A. Young, new ecological niches. The upper- T. Boyle, and T. Brown. TREE 11(10):413-418. is valuable because genetic diversity is: elevation populations of plant • Important to the species’ ability to Get a grip on genetics. M. Brookes. (1999) The species (e.g., white fir (Abies concolor)) Ivy Press Limited, East Sussex, England. adapt to changing environmental are often genetically differentiated conditions over time, such as those A primer of conservation genetics. (2004) from the lower-elevation popula- R. Frankham, J.D. Ballou, and D.A. Briscoe. related to climate change. It is not tions. In these cases, genetic diversity Cambridge University Press, Cambridge, UK. the entire species that adapts in allows the species to exist in concert, but particular populations substantially differing environments. over time. Some populations may The United States Department of Agriculture (USDA) • Generally correlated with some become better adapted than others; prohibits discrimination in all its programs and activities measures of fitness. Although the on the basis of race, color, national origin, gender, some may become extinct. religion, age, disability, political beliefs, sexual orientation cause-effect connections aren’t all and marital or family status. (Not all prohibited bases Similarly, when we find something apply to all programs.) Persons with disabilities who understood at present, there is of economic, social, or cultural require alternative means for communication of substantial evidence that levels program information (Braille, large print, audiotape, etc.) value within a particular species should contact USDA’s TARGET Center at: (202) 720-2600 of genetic diversity are positively (such as a component for a useful (voice and TDD). To file a complaint of discrimination, related to a species’ ability to write: USDA Director, Office of Civil Rights, Room 326- drug or an aesthetically pleasing W, Whitten Building, 14th and Independent Avenue, SW, produce substantial and robust Washington, DC 20250-9410, or call: (202) 720-5964 flower color or plant shape), it is (voice or TDD). progeny and persist in the long often a trait that is variably term. USDA is an equal opportu- expressed within the species and nity provider and employer. often has a genetic basis. Breeding 2006