“…The resulting population was randomly mated for 3 generations before the formation of the different selected lines (Liljedahl et al, 1979 (Rose, 1984 (Clayton and Robertson, 1966;Flock, 1977;Liljedahl et al, 1984 in laying hens; Rose and Charlesworth, 1981a;Burla and Taylor, 1982;Engstr6m et al, 1989 in Drosophila medanogaster). This pattern is probably due to the impairment of the individual's ability to cope with environmental stress due to increasing difficulties in maintaining their physiological homeostasis (Lerner, 1954).…”