In which of the following scenarios is stabilizing selection most likely to take place?
In which of the following scenarios is stabilizing selection most likely to take place?
- A. Male elk that have larger antlers are better able to fight for dominance. The most dominant elk in an area produces the most offspring.
- B. In a population of hawks, very large hawks are able to carry off large prey such as small deer, but very small hawks are slightly faster and so have more success hunting small mammals.
- C. A population of feral cats begins to hunt the mice in a neighborhood. Mice that are very small are unable to run fast enough to escape the cats, whereas mice that are very large are unable to fit in small nooks and crannies to hide from the cats.
- D. A few individuals in a population of coastal-dwelling moths is blown to an offshore island during a tropical storm
Correct Answer: C. A population of feral cats begins to hunt the mice in a neighborhood. Mice that are very small are unable to run fast enough to escape the cats, whereas mice that are very large are unable to fit in small nooks and crannies to hide from the cats.
Explanation
Stabilizing selection is a type of natural selection in which the extremes of phenotype are selected against. In other words, individuals with traits on either end of the phenotypic spectrum are more likely to die, or less likely to produce offspring, than those in the middle of the spectrum. This selection against both extremes favors individuals with an intermediate phenotype, and therefore reduces the diversity of the population. The only scenario in which the intermediate phenotype is selected for is the situation describing the cats and the mice. The smallest mice and the largest mice are both eaten by the cats, but medium-sized mice can both run fast enough to escape the cats and fit in the nooks and crannies, making them more evolutionarily fit for survival in this particular situation. The scenario describing rattlesnakes presents an example of directional selection. Smaller rattlesnakes benefit from being able to fit into burrows. Based on this information alone, smaller rattlesnakes may have more offspring than larger rattlesnakes, and the rattlesnake population may eventually select for a smaller body size. The scenario describing elk and antler size is also an example of directional selection. Male elk that have larger antlers are more likely to have more offspring, so the average antler size of the male elk in the population is likely to increase over time. In the example describing hawks, the largest and smallest hawks each benefit from their body sizes, but medium-sized hawks do not. In this situation, disruptive selection is likely to take place, dividing the population over time into two predominant phenotypes. Given enough time, the hawk population might eventually split into two distinct species, one larger and one smaller. The situation describing the moths being blown to an island in a storm describes a founder event, not any type of selection. We don't know anything about the makeup of the population of moths, so we can't say that any particular type of selection is taking place.