Natural Selection (Continued) Practice Test

In a bird population, beak depth varies continuously. Over 15 years, rainfall becomes more consistent and seeds are mostly medium hardness each season. Researchers measure beak depth and track reproductive success. Birds with intermediate beak depth consistently leave more surviving offspring than birds with either very shallow or very deep beaks. Across generations, the population mean beak depth changes little, but the proportion of extreme beak depths declines. Which pattern best illustrates the observed selection on beak depth?

A population of prairie flowers varies in stem height. After grazing mammals become common, very tall stems are eaten frequently, while very short stems are shaded by surrounding vegetation and set fewer seeds. Flowers with intermediate stem height produce the most seeds that survive to adulthood. Across seven generations, the mean height changes little, but the frequency of extreme heights decreases. Which type of selection best explains the change in stem height distribution?