What Is Life? 2nd Edition by Jay Phelan

Exercise 31

Artificial selection was used on corn to produce a single strain of corn with increased growth rates and greater resistance to a fungus. Although farmers have continued to select for these traits, the productivity of this strain is no longer increasing. This suggests that:
A) the population size has been decreasing.
B) all or most of the natural variation for these traits has been eliminated.
C) gene migration is a major evolutionary agent in corn.
D) long-term disruptive selection may lead to speciation.
E) artificial selection is not as strong as natural selection.