Response to selection for body weight and egg weight in chickens.

W E L L conducted selection experiments are useful in testing the validity of modern selection theory. Observed genetic changes from selection frequently show poor agreement with expectation. The relationship of selection to the genetic variance composition of a population, to correlated responses, to plateaued populations and to selection limits are problems not yet well understood. This study reports the resul-s of a %way selection experiment for both body weight and egg weight in White Leghorns. Body weight is highly responsive to selection in chickens ( SIEGEL 1962a; MALONEY, GILBREATH and MORRISON 1963) and in turkeys ( ABPLANALP, OGASAWARA, and ASMUNDSON 1963) but evidently not in Cotournix quail (COLLINS and ABPLANALP 1965). Egg weight is also highly heritable in chickens (OLSON and KNOX 1940; SNYDER 1945; SHULTZ 1953) and is correlated with body weight (SIEGEL 1963a, b; MALONEY et al. 1963; SHULTZ 1953). Although correlated responses in reproductive traits are of special interest to us in our over-all selection experiment, we will report on that separately. This report is confined to changes in body weight and egg weight over 7 years from 2-way selection for each of these traits. Changes in genetic parameter estimates over generations are also given. As experimental checks, both a randombred control line and a companion line of common origin, but selected exclusively for high egg production, are included. NORDSKOG and FESTING (1962) gave a preliminary report of the first 4 years of the experiment.