Selection experiments its effect on inbreeding depression, particularly on

W .G. Hill Livestock Production Science 63 2000 99 –109 103 2 1 N , s , 0, and zero if more deleterious. The almost 100 years and generations, and in which e cumulative consequence of many mutations segregat- responses have continued in the lines selected for ing in the population, however, some of which high oil content or for high protein content in the eventually become fixed, could be a substantial drop kernel plants Dudley, 1977. The populations are in mean performance, particularly in overall fitness bred with only 30 or so selected plants maternal for which most mutations are likely to be deleterious. half-sib families. More male parents are likely to be Thus considerable attention has been given to the involved, but the lines are so superior to the natural size of the natural population necessary for the rate base for the selected traits that selection of offspring of elimination of deleterious mutations to be suffi- got by cross pollination outside the selected lines cient that mean fitness does not drop substantially; seems most unlikely to occur. Response in the low or, in the jargon, that the population is large enough lines has attenuated, but they are approaching levels to prevent it going into non-sustainable mutational of zero oil content in the kernels. ‘meltdown’. Conclusions depend on assumptions of Continued selection has been practised in Dum- the rate of occurrence and size of effects of deleteri- merstorf for increased body weight in mice, with 160 ous mutations: for a given input of variance, the animals selected each generation, a relatively large smaller their mean effect, the harder the mutants are size for a selection experiment in laboratory animals ¨ to eliminate and the greater their cumulative in- Bunger et al., 1990. Selection has now been fluence on mean fitness. Some theoretical arguments continued for over 90 generations. At 6 weeks the indicate that populations as small as 100 could be at high line mice average |65 g and the unselected risk Lynch et al., 1995; whereas experimental controls 30 g, a response of about 12 phenotypic checks suggest that smaller populations are expected standard deviations. Response continues, albeit at a ¨ to be safe for very long periods Gilligan et al., reduced rate L. Bunger, personal communication. 1997. For populations under artificial selection, In a review of mouse selection experiments to that fitness is also likely to fall as a consequence of date, Eisen 1980 showed that long-term responses pleiotropic effects of genes affecting the trait under increased with population size and Jones et al. selection that have increased in frequency or become 1968 showed similar relationships in Drosophila. fixed by artificial selection. Laboratory or domestic Selection experiments in animals with the largest livestock populations are, however, protected against populations have been carried out by Weber 1990, extinction by careful and good management, for who got 30 higher responses over 55 generations example by giving many opportunities to breed and of selection for wing-tip height in Drosophila with by health control. 1000 than with 200 selected parents. The effect of population size on maintaining variation and selection response is confounded with

3. Selection experiments its effect on inbreeding depression, particularly on

fitness associated traits not necessarily under direct 3.1. Effect of population size selection. A contributing factor to the greater re- sponses in populations of larger size may have been The many laboratory selection experiments con- reductions in deleterious inbreeding associated ef- ducted over the years give us much information on fects except in many Drosophila experiments where what can be achieved by continual selection, indeed fixed numbers of animals are sampled to be recorded whether variation and thus response is sustainable. as candidates for selection. Rapid inbreeding can be Although there is not a uniform outcome, it is very hard to sustain in livestock populations in which possible to draw some inferences from them to the it has been attempted for review see e.g. Wright, design of animal improvement programmes. 1977, ch. 3. The effects of inbreeding over the long It is clear that some long-term experiments have term in selected lines cannot be simply predicted given continued responses over very many genera- from data on close matings in non-inbred base tions, without obvious limits. The classical example populations, however. Natural selection or artificial is the Illinois corn experiment that has now run for selection on fitness associated traits may reduce the 104 W .G. Hill Livestock Production Science 63 2000 99 –109 effects, and also see Section 4 because pleiotropic In summary, the selection experiments in labora- or stabilising selection effects confound the picture. tory species show that substantial progress can be maintained for very many generations, even with 3.2. Contribution of mutations populations of effective size well under 100, but that responses increase with population size. Mutation Observations such as regression of performance on contributes to response, but it is clearly demonstrated relaxation or reversal of selection, and statistical only in populations selected from an inbred base. analyses undertaken within selected lines indicate that there is substantial genetic variation present, for example in lines of mice divergently selected for

4. Correlated traits