Principles of Genetics and Breeding

13.1.8.1. Limitations in individual selection

• Individual selection can only be applied to traits that have relatively high heritability (h²' s) and can be recorded directly and accurately on the individual. Such traits could contain very large environmental components of variation. For e.g. differences in age may give a competitive advantage to older fish, maternal effects may give an advantage in some families even if they were produced at the same time.
• Selection of simply the largest individuals in a population may actually choose individuals from a few families which are larger than average for environmental rather than genetic reasons.
• In small hatchery populations without form of pedigree system this could result in rapid inbreeding.
• In large populations and when selecting for a single trait with a high h² (> 0. 3) and were a high S can operate very efficiently but care needs to be exercised to ensure that no negative correlated responses are also involved.
• Inbreeding is inevitable when conducting individual selection. Because family relationships are ignored during the selection process, a majority of the select brood fish can come from only a few families, particularly if one or two mating produce outstanding offspring. When this occurs, the population goes through a severe bottleneck which generates inbreeding.
• When mating are made, relative will be mated; among these mating will be brother-sister and half-sib matings, which produce considerable inbreeding.
• Numerous reports on the use of mass selection to improve a range of traits in many different species have been published. However, it must be said that many of these reports are in someway flawed and few of these "improved" strains are used commercially. The main problems with these selection programmes is the inability to assess the level of genetic change as no control populations have been used.
• The importance of this can be seen in the studies on carp that have used controls, substantial fluctuations (>300%) are observed in controls kept at different farms within a year on the same farm from year to year (>200%) resulting in the need for many correction factors (Moav and Wohlfarth, 1976).
• Increases in the average size of unselected rainbow trout controls over a number of generations of a selection experiment suggest substantial husbandry improvement must also have occurred (Kincaid et al., 1977). It is therefore going to be difficult to observe small percentage improvements against such large environmentally induced variation. One of the most publicised improvement programmes on the Washington rainbow trout strain did not use a control and it appears today that much of the gain was through husbandry improvement.
• Even though inbreeding cannot be prevented when conducting individual selection and when the fish are not marked, the rate of inbreeding might be minimized by maintaining (effective breeding population) N_e at a predetermined size. When a selective breeding programme is being conducted, matings are not random; there are what is called “assortative matings” (the best are mated with the best).
• Additionally, each act of selection creates a mini-bottleneck, which can retroactively lower the N_e of the previous generation.
• One way to prevent or reduce inbreeding is to mark fish and avoid consanguineous matings when select brood fish mate. This might decrease the genetic gain, but it will prevent inbreeding.
• If this mating programme is used, the population must be sufficiently large so that adequate numbers of select brood fish can be spawned without consanguineous matings. If relatives are not allowed to mate, the number of potential matings that can be made among the select brood fish will decreases dramatically.
• After a few generations, it might not be possible to prevent consanguineous matings, but if brother-sister matings or half-sib matings can be avoided, large accumulations of inbreeding can be prevented. If only cousins are allowed to mate, relatively little inbreeding will accumulate in the select population. Five generations of first cousin matings will produce less inbreeding than a single generation of brother-sister matings. If only second cousins are allowed to mate, inbreeding will never excess 2%.