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additive genetic variance that component of the genetic variance in a character that is attributable to additive effects of alleles canalization the evolution of internal factors during development that reduce the effect of perturbing environmental and genetic influences, thereby constraining variation and consistently producing a particular (usually wild-type) phenotype correlated selection independent genetic variation in two or more characters, but selection favors some combination of character states over others, usually because the characters are functionally related. environmental correlation 2 features of individuals with the same genotype may vary together because both are affected by environmental factors; such features dispaly rE environmental variance variation among individauls in a phenotypic trait that is caused by variation in the environment rather than by genetic differences epistasis an effect of the interaction between two or more gene loci on the phenotype or fitness whereby their joint effect differs from the sum of the loci taken separately G matrix a table of values of VA for each character and the genetic covariance, closely related to the correlation, between each pair of characters is called the genetic variance-covariance matrix, usally referred to simply as the __ ___ genetic assimilation a character state that initially developed in response to the environment had become genetically determined, Waddington called this ___ ____. genetic correlation rG, in genetically variable populations, the correlated variation may also be caused by genetic differences that affect both characters genetic variance variation in a trait within a poplation, as measured by the variance that is due to genetic differences among individuals genotype x environment (G X E) interaction phenotypic variation arising from the difference in the effect of the environment on the expression of different genotypes heritability the proportion of the variance in a trait among individuals that is attributable to differences in genotype; in the narrow sense is the ratio of additive genetic variance to phenotypic variance intensity of selection if selection is directional norm of reaction the set of phenotypic expressions of a genotype under different environmental conditions phenotypic correlation rP, body size and fecundity is simply what we measure in a random sample from a population; genetic and environmental components phenotypic integration the hypothesis of morphological integration holds that functionally related characteristics should be genetically correlated with one another; characters should remain coordinated even as they vary within spcies and as they evolve. phenotypic plasticity the capacity of an organism to develop any of several phenotypic states, depending on the environment; usually this capacity is assumed to be adaptive phenotypic variance VP, a phenotypic trait is the sum of the variance that is due to differences among genotypes and the variance that is due to direct effects of the environment and developmental noise. QTL mapping a chromosome region containing at least one gene that contributes to variation in a quantitative trait. a procedure for determing the map positions of these loci on chromosomes quantitative genetics field developed to analyse quantitative characters, and its methods are used by biologists who study the evolution of morphology, life history characteriistics, behavior, and other phenotypic traits. realized heritability h2N, the change in mean phenotype in offspring, R that is due to selection of parents, S, can be read directly from the regression line; the equation can be rearranged response to selection the change in the mean value of a character selection differential the mean tail length of the selected parents differs from that of the population from which they were taken by an amount S. selection gradient a measure of selection: the slope of the relation between phenotype values and the fitnesses of those phenotypes selection plateau populations eventually stopped responding ---cessation of response to selection not caused by loss of genetic variation, because when Yoo terminated ("relaxed") selection after 86 generations, mean bristle number declined, proving that genetic variation was still present. threshold trait expressed as discrete alternatives but are controlled by polygenic variation rather than by single loci trade-off the existence of both a fitness benefit and a fitness cost of a mutation or character state, relative to another. variability refers to the ability, or potential, to vary