Eugene Eisen selected for increased 12-day litter weight...

Eugene Eisen selected for increased 12-day litter weight (total weight of a litter of offspring 12 days after birth) in a population of mice (E. J. Eisen. $1972 .$ Genetics $72: 129-142$ ). The 12 -day litter weight of the population steadily increased, but then leveled off after about 17 generations. At generation $17,$ Eisen took one family of mice from the selected population and reversed the selection procedure: in this group, he selected for decreased 12 -day litter weight. This group immediately responded to the reversed selection: the 12 -day litter weight dropped 4.8 $g$ within 1 generation and dropped $7.3 \mathrm{g}$ after 5 generations. On the basis of the results of the reverse selection, what is the most likely explanation for the leveling off of 12 -day litter weight in the original population? (IMAGE CANNOT COPY)

Key Concepts

Selection Plateaus

A selection plateau occurs when a trait no longer responds to ongoing selection despite continued directional efforts. This phenomenon can arise from factors such as the depletion of advantageous alleles, physiological constraints, or opposing selective pressures (for example, natural selection) that maintain an optimal trait value, thereby limiting further changes.

Artificial Selection

Artificial selection is the process by which humans choose individuals with desirable traits to breed, thereby shifting the trait distribution in the population over generations. This process is a cornerstone in quantitative genetics for studying and manipulating traits, and it helps uncover the underlying genetic architecture affecting trait variability and overall improvement or change over time.

Natural Selection as a Counteracting Force

Natural selection refers to the environmental and biological pressures that favor individuals with traits best suited to survival and reproduction. When artificial selection pushes a trait beyond the natural optimum, these natural pressures may counterbalance the artificial gains, resulting in a plateau of the selected trait. This interplay is crucial in understanding limits to trait evolution even under deliberate selection.

Reverse Selection

Reverse selection involves altering the selection direction to favor the opposite extreme of a trait. It is a useful experimental approach in quantitative genetics to test whether a plateau in selection response is due to a true exhaustion of genetic variation or to other factors. A rapid change in the trait following reverse selection indicates that significant genetic variation remains, suggesting that the original plateau was likely due to counteracting forces such as natural selection rather than a lack of genetic diversity.

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