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Summary: When phenotypic traits are characterized at the
molecular level, many have a complex GENETIC
ARCHITECTURE1,2 (see Glossary). By genetic
architecture, we mean the genes as well as the
INTERACTIONS among them (epistasis), and between
genes and environments that affect trait expression.
Epistasis is particularly important in several areas of
current evolutionary research, including speciation,
CANALIZATION, inbreeding depression, the evolution of
sex and interdemic selection in Wright's shifting
balance theory3. In speciation, epistasis plays a key
role in reproductive isolation genes that function
well in conspecific genetic backgrounds function
poorly when combined in interspecific hybrids.
Diminished effects of one locus as a result of
interactions with another are fundamental to
concepts of developmental homeostasis and
canalization4. With inbreeding and other systems of
nonrandom mating, negative synergism between
deleterious alleles can contribute to the decline and
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