Severe flooding in an area caused a population of minnows to become split into two separate ponds. Prior to the separation, the male minnows all had a moderate number of spots.
Following the separation, the first pond was filled with larger fish that fed on minnows, so male minnows with fewer spots were better able to avoid detection by predators. The minnows in the second pond, however, had very few predators, and female minnows in this pond preferred to mate with males that had the most spots.
After many generations, the two minnow populations evolved into different species and could no longer interbreed.
Fish Species Average # of Spots
Original Species four to six
New Species 1 zero to two
New Species 2 eight to ten
Which of the following best describes why the two minnow populations evolved into separate species?
A.
Natural selection acted upon the traits of the minnows in the first pond only, causing these fish to evolve into a new species with new adaptations.
B.
Natural selection acted upon the traits of the minnows in the second pond only, causing these fish to evolve into a new species with new adaptations.
C.
The same selective pressures were acting upon both populations, leading to a decrease in genetic variation and the formation of two new species.
D.
Different selective pressures were acting upon the two populations, leading to an increase in genetic variation and the formation of two new species.