Question: What is the most plausible explanation for the repeated presence of pairs of benthic and limnetic "sister" species of sticklebacks? Question options: Benthic sticklebacks evolved in some lakes, while limnetic sticklebacks evolved in others. Over time birds carried individuals of both types between lakes, thereby establishing two types of stickleback in each lake. Limnetic sticklebacks evolved in some lakes, while benthic sticklebacks evolved in others. From each of these initial forms, there was convergent evolution to produce a complementary "sister" species. As the benthic and limnetic regions of the lakes are quite different ecologically, differential selection acted on pre-existing genetic variation in populations in each region, ultimately resulting in their divergence. In the freshwater lakes, there is was a temporary physical barrier between the benthic and limnetic regions, which permitted, over time, the divergence and ultimately speciation of the populations trapped on each side of the barrier. The presence of extensive chromosomal inversions in the marine stickleback genome, permitted the sympatric speciation of the two sister species.