 * Critical Findings ASSESSMENTS * Terrestrial Vertebrates Restricted to the Sierra Nevada * SNEP Significant Areas Inventory
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Genetic Diversity Genes are the fundamental unit of biodiversity, the raw material for evolution, and the source of the enormous variety of plants, animals, communities, and ecosystems that we seek to conserve and use in the Sierra Nevada. Genetic variation shapes and defines individuals, populations, subspecies, and ultimately all plant, animal, fungal, and bacterial life on earth. The gene pool (collection of all genes within a species) of a widespread species such as ponderosa pine consists of many populations; of a rare species, it may be only a single population. From one species to the next, the composition and structure of individual gene pools varies. Some species of plants and animals consist of populations each locally adapted to its environment, while other species appear to be generalists, possessing relatively low overall diversity or showing genetic diversity mostly among individuals rather than among populations. Forces of natural selection and history shape gene pools in the continuous process of short-term adaptation and long-term evolutionary change. The composition and structure of the gene pool, as shaped by natural selection, has a unique relationship to viability and long-term survival of populations and ultimately each species. Many human actions on the landscape have some genetic effect. While certain changes in genetic diversity occur naturally, some human activities in the Sierra Nevada accelerate or alter the direction of evolution in undesired ways. Gradual or rapid loss of genetic diversity (genetic erosion), introduction of ill-adapted genes (genetic contamination), and major shifts in gene pool structure are changes that have been brought about by human actions in the Sierra. With direct information on genetic diversity virtually nonexistent for all taxa except a few well-studied trees, fish, and scattered plants and animals, we are left to make indirect inferences about the potential effects of past human actions on gene pools and the future consequences of those effects. In the Sierra, any human activity that breaks the chain of natural selection, or forces rapid changes in adaptation on populations, is potentially detrimental to gene pools in both the short and the long term. Such effects include habitat alteration (habitat destruction, degradation, and/or fragmentation); silviculture (tree harvest, seedling culture, and planting methods); severe wildfire (artificially large and stand-replacing fires); ecological restoration (planting); fish management (hatchery culture, fish stocking); range habitat management (shrub planting); and accidental introduction of non-native pathogens. While genetically aware programs exist for managing tree stock (tree planting) that likely mitigate most potentially detrimental effects to forests, attention to genetic consequences is mostly lacking in other forest- and range-management activities. Introduction of salmonid fish to Sierran waterways, in addition to its cascading effects on invertebrates and amphibians, has resulted in hybridization with native trout and led to the loss of local distinctiveness of most native Sierran stocks, as well as threatening the very existence of some species, such as the Little Kern golden trout, through genetic swamping. Transmission of disease pathogens from domestic sheep to native bighorn sheep has caused high mortality in the latter species, which had evolved with little resistance to Eurasian diseases. This has probably caused severe losses of genetic diversity in the small populations, as well as the more obvious immediate effect of population extinctions. Direct knowledge of genetic diversity and its implications for adaptation will likely never be well known for most Sierran taxa. In light of this, however, preventive actions can be taken and genetic guidelines followed in many forms of management to mimic natural selection and the evolutionary process in preserving as much genetic diversity as possible. 
 
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