Simultaneous selection by independent culling levels of two correlated traits in all four combinations of directions was investigated with Tribolium in a replicated experiment extending over nine generations. In addition to the two primary traits, 13-day larval weight and pupal weight, four secondary traits (pupation time, adult emergence time, adult weight and larval number) were observed.The observed responses for both selected and unselected traits agreed with theoretical expectations after the latter were adjusted for changes which occurred in genetic and phenotypic parameters. Phenotypio variances for the selected traits were correlated positively with population means, yet genetic variances and heritabilities declined in all selected populations. No change was detected in the genetic correlation between selected traits even though the divergent two-trait selection was designed especially to ' break' the positive correlation of + 0-55 + 012 present in the base population.Striking changes in growth and developmental patterns resulting from the divergent selection were discussed in terms of metamorphic limits and 'stabilizing' genetic correlations.

The origin of the word heritability remains unknown. Its usage has evolved through three stages, becoming more restrictive in its meaning along the way. In the initial stage, 1832 and possibly earlier, heritability was used to denote the hereditary transmission of characteristics or material things, simply having the capability (legally or biologically) of being inherited. The second stage, beginning around the turn of this century, followed Johannsen's classical definition of nongenetic or environmental fluctuations distinct from genotypic differences, and usage closely approximated "broad sense heritability" and Johannsen's Erblichkeit. Finally, in 1936, we come to the modern day usage of narrow sense heritability, the ratio of additive genetic variance to the total phenotypic variance within a population, and credit Dr. J.L. Lush with its origin.

Fifteen methods of maintaining control populations have been studied over eight generations using the flour beetle, Tribolium castaneum. Populations were reproduced each generation from 50 males and 50 females. The methods were compared as to their ability to establish the level of the environment with respect to the base population and to separate environmental and genetic effects in two directionally selected lines.It was demonstrated that a foundation stock of these beetles produced pupae which weighed about 10% more when grown in 70% relative humidity than when grown in 40% relative humidity. All lines were grown in one environment for two generations and then in the other environment for the next two generations. The mean pupa weights of two master controls were averaged to give an average master value which served as a standard against which the other controls were compared.Inbred lines derived from a separate source of stock responded differently to the environments than did the foundation stock from which selected lines were drawn.Evidence of genetic drift in mass mated lines was presented. No differences among the other methods of maintaining the original population were observed. It was suggested that if a smaller number of families had been used, additional differences might have been detected.Response to selection in both directions over eight generations was symmetrical. Symmetry was also demonstrated for within environment regressions and it appears that greater progress was made in the dry environment than in the wet.Initially the foundation stock weighed more in the wet environment than in the dry environment. After selection, the large line weighed more in the dry than in the wet. It was shown that the original population contained the basis for this interaction of directionally selected lines with environments since some ofthe initial famllies weighed more in dry than in the wet environment. It was suggested that the apparent differential response to selection, dependent upon the environment, may have been due to the larger variance in the dry environment. This variance was shown to be about twice as large as that in wet.Due to the presence of the interaction, the base control populations were ineffective in separating genetic and environmental effects in the later generations of the selected lines. Repeated and relaxed methods of maintaining controls more closely indicated how environmental shifts affected the later generations of the selected lines.It is concluded that a control line must be closely related to the selected line in origin and time in order for its reactions to environmental shifts to be similar in nature to the selected line. If these conditions are not met, undetected genotype by environment interactions may contribute to faulty comparisons. In addition, some method of reproducing the original population must be followed if interactions with respect to the origin are to be detected.It is further concluded that it is unnecessary to place restrictions on the method of mating base populations for the purpose of maintaining control stocks, provided that the size of the breeding population is not less than 50 males and 50 females and that mass mating is not used.

An experimental evaluation of the effect of mating systems and selection upon an additive trait thought to be highly heritable was made. There were two similar replications. Each consisted of a mass selected and randomly selected group, with five mating systems within each group.Realized heritabilities in the mass selected lines were considerably less than was expected prior to the initiation of the experiment, and averaged approximately fourteen percentage points less than heritability estimated from the zero generation. This in turn resulted in smaller correlations between the genotypes of mates than had been previously expected in the assortatively and disassortatively mated lines.The average response of the mass selected, assortatively mated lines was slightly more than the mass selected, randomly mated lines, though not statistically significant. This result seems to conform to theoretical expectations.In the mass selected lines, estimates of phenotypic and genetic variance declined regardless of mating systems. There was a tendency for phenotypic variances to decrease in the randomly selected lines, but this was not the case for estimates of genetic variance.As an aid to selection, it seems that assortative mating would be of little value with traits of low or intermediate heritability but might be useful if the trait is highly heritable.

Selection for large and small 13-day larval weight in Tribolium castaneum was studied for sixteen generations in a replicated experiment to evaluate the effectiveness of various selection methods and the importance of genotype by environment interactions under two levels of nutrition.Direct selection responses generally were larger than correlated ones under both Good and Poor nutritional environments. However, contrary to theoretical expectations, those populations selected on average performance in both environments were not superior for this attribute.Asymmetrical responses were observed to be dependent on the environment of selection. When selection was based on performance in the optimal environment, the asymmetry was observed toward small size. This situation in the sub – optimal environment was completely reversed. This phenomenon was discussed in terms of physiological limits rather than gene frequency and directional dominance.It was proposed that selection of compound traits such as body weight at a fixed age may affect the component characters quite differently. As the latter are differentiated by direction and environment of selection, the compound trait may reflect parametric changes and enhanced genotype by environment interactions. Such changes hamper the precision of current selection theory for predicting response even in the short run.