EXTINCTION BY HYBRIDIZATION AND INTROGRESSION

Nonindigenous species can bring about a form of extinction of native f lora and fauna by hybridization and introgression either through purpo seful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be espe cially problematic for rare species coming into contact with more abun dant ones. Increased use of molecular techniques focuses attention on the extent of this underappreciated problem that is not always apparen t from morphological observations alone. Some degree of gene flow is a normal, evolutionarily constructive process, and all constellations o f genes and genotypes cannot be preserved. However, hybridization with or without introgression may, nevertheless, threaten a rare species' existence.