Question

after many generations, the two minnow populations evolved into different species and could no longer interbreed.

fish species	average # of spots
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 second pond only, causing these fish to evolve into a new species with new adaptations.
b. different selective pressures were acting upon the two populations, leading to an increase in genetic variation and the formation of two new species.
c. 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.
d. the same selective pressures were acting upon both populations, leading to a decrease in genetic variation and the formation of two new species.

Explanation:

To determine the correct answer, we analyze each option:

• Option A: Natural selection acting only on the second pond is incorrect, as both populations evolved (New Species 1 and 2), so selection must have acted on both.
• Option B: Different selective pressures (e.g., environmental factors) on the two populations would lead to increased genetic variation (as traits diverge) and speciation. This matches the scenario where two new species with different spot traits (adaptations) formed.
• Option C: Similar to A, natural selection acting only on the first pond is wrong since both populations evolved.
• Option D: Same selective pressures would lead to similar traits, not divergence into species with different spot counts (zero - two vs. eight - ten), and would decrease variation (not increase as needed for speciation), so D is incorrect.

Answer:

B. Different selective pressures were acting upon the two populations, leading to an increase in genetic variation and the formation of two new species.