"Why has n't natural selection weeded out species with their inferior `` optical vision `` and given them all echolocation , which would allow them superior senses in day and night , rather than just daytime ?"
"Because natural selection works on a cost/benefit basis . For bats and whales and cave-dwelling birds echolocation confers huge advantage , allowing them to exploit niches closed to competing species . For ground- or tree-dwelling terrestrial animals , it would be of marginal or zero benefit , given the 'noise ' in their immediate surroundings ."
"Why has n't natural selection weeded out species with their inferior `` optical vision `` and given them all echolocation , ?"
"Because natural selection works on a cost/benefit basis ."