June 21, 2024 — A flounder looks like a hallucination of a fish. Its body is flat as a pancake, its head permanently tilted to one side, and instead of having one eye on each side of its head, both eyes are crowded on one side.
This anatomy, as weird as it may be, is one of evolution’s remarkable success stories. Flounder, like more than 800 other species of flatfish, lie flat on the sea floor, their two eyes gazing up at the water overhead. When a smaller fish swims by, a flatfish shoots up and strikes. One species, the Pacific halibut, can grow to the size of a barn door.
The bizarre bodies of flatfish have long puzzled biologists. In fact, Charles Darwin’s critics used it as evidence against his theory of evolution.
In “The Origin of Species,” Darwin argued that natural selection favors tiny variations. Each little increment benefited an organism. Over countless generations, he said, those increments gradually added up to large-scale transformations.