AbstractThis review examines three critical questions in evolutionary biology: (1) why do virtually allmulticellular and many unicellular organisms reproduce sexually? (2) Why have some animalsevolved extravagant ornaments and complex mating rituals that appear detrimental? (3) How does natural selection maintain high-fidelity genetic replication? I present a simple "Everest hypothesis" to answer these questions. First, I note that the genes that specify the proteinsthat replicate DNA are subject to mutation, so replicative fidelity must vary between individuals. Moreover, many organisms choose their mates by responding to various complex biochemical mechanisms, physical displays, and behaviors, generated by many gene products acting together. I propose that natural selection consistently adds unnecessary complexity to the mechanisms that transfer genetic material between individuals, and since most mutations are either harmful or neutral, mutator mutations are likely to disrupt these mechanisms. Sex may, therefore, provide a filter to reduce the dissemination of mutator mutations. Sex may also have allowed ancient organisms to colonize unstable environments where strong selective pressures would apply. This can explain many puzzling biological phenomena. For example, the migration and spawning of Atlantic salmon and the complex displays of birds of paradise may be bestunderstood as "tests" to show that potential mates possess genes for high-fidelity geneticreplication. Animals that have developed ornaments that appear harmful, such as peacocks,and animals that undertake remarkable migrations, such as monarch butterflies, may provide conspicuous examples of such tests. I also make suggestions for experiments to test the hypothesis.