Have you noticed that some South American tribes have huge heads? The mystery has been solved! Usually, environmental pressures drive evolution (biotic factors like predators and disease or abiotic factors like altitude, UV radiation, or climate). In the case of the Amazonian Xavánte tribe, sexual selection and isolated cultural practices led to significant changes to their morphology (shape). A full quarter of the tribe’s population was made up of sons of a single chief, Apoena, who had five wives. You can see how being more reproductively successful means that any traits of theirs can quickly come to dominate the population.