Nothing in Darwin's mechanism guarantees humans will think rationally. Fitness might actually favor stupidity. For proof, look at the reasoning of people who believe in natural selection.
Despite institutional rejection of intelligent design, researchers are attracted to ID when it can make money, make them famous, or advance understanding of nature.
Darwinian evolution is supposed to innovate new things, not remove them or conserve them. Here are the latest examples of "Darwin Fail" embarrassments.