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Q37 - Long-Term Selection Response and Its Limits in Tribolium Body Size

Theoretical A Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Quantitative Genetics, Heritability & Inbreeding

F.D. Enfield selected for increased body size in the flour beetle Tribolium castaneum. He started with a population of beetles that had a mean weight of 2.4 g and variance of 4.0 g2. For each generation, the selection differential (the difference between the mean of a generation subset selected for further breeding and the overall mean of that generation) was 0.022g. The initial value of h2 (heritability in narrow sense- h2 is the ratio of the additive variance to the total phenotypic variance) for body size in the original population was 0.3. For the first 50 generations, the mean weight of the selected population increased steadily. After 125 generations of selection, the mean weight had increased to 5.8g, more than twice the original mean, and additional selection did not result in a further increase in size. The lightest individuals in the selected population were heavier than the heaviest individuals in the original population. Enfield determined h2 for the selected population after 125 generations and discovered that it was only slightly less than for the original population.

Using the information and data, determine which of the statements are true or which are false.

A. The failure of the population to respond to further selection was because the population genetic variation is exhausted.
B. The reason why the mean of the population could be shifted to a value outside the original range of the population is that the selection for the increased body size is a selection favoring heterozygotes.
C. After 125 generations of selection, additional selection can result in a further increase in size if we increase the selection differential (higher than 0.022g).
D. If selection was stopped after 125 generations of selection then the body size would decrease.
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Question reproduced from IBO 2016, Theoretical Paper A, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)