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UBC Theses and Dissertations

Studies on genetic drift and inbreeding in small populations under artificial selection Aggrey, Samuel E.


The effects of population size, mating strategy, and selection on selection response, genetic drift, and variability of response were studied in small Japanese quail populations. The effects of population size on genetic gain, variability of response and inbreeding were examined for a 3 generation phenotypic selection experiment. Effective population sizes of 12 (Large) and 6 (Small) per replicate were evaluated at the same selection intensity. The selection criterion was Week 2 body weight in Japanese quail. Estimates of genetic gain at Generation 3 were 10.67 and 8.89 grams, respectively for the large and small populations. A selection experiment involving 3 mating strategies was conducted in order to evaluate the efficiency of selection and effectiveness of reducing genetic drift and/or inbreeding in small populations. The mating strategies used were random (RM), minimum coancestry (MN), and maximum coancestry (MX) mating. Genetic gain at Generation 3 was 7.03, 7.36 and 6.62 grams and inbreeding levels were 6%, 4% and 12% in the RM, MN and MX lines, respectively. After 3 generations of selection, inbreeding effective population size had declined by 62% in the MX lines. In the MN lines, inbreeding effective population size increased by 16%. The effects of inbreeding on reproductive performance were studied in the large and small populations (Experiment I) and the RM, MN and MX lines (Experiment II). Ten percent increase in the embryo’s inbreeding caused about 10% reduction in fertility in both experiments. Ten percent increase in the embryo’s inbreeding caused 10% reduction in hatchability in Experiment I and RM lines in Experiment II, and about 13% reduction in hatchability in MX lines. The goal of short-term selection has been to maximize genetic gain. Results from the present study suggest that the goal of a breeding program for small populations should shift from maximization of response to optimization of genetic gain and effective population size. Among the factors to be optimized are rate of selection, variability of response, inbreeding depression in the selection trait and fitness. Restriction of family size coupled with the use of either circular pair or maximum avoidance mating is preferred over within-family selection under random mating or mass selection with maximum avoidance in terms of reducing inbreeding in small populations undergoing artificial selection.

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