The Red Queen
Sex is an evolutionary puzzle. In most sexual species, males make up half the population, yet they bear no offspring directly and generally contribute little to the survival of offspring. In birds and mammals that idea seems less true. They often help protect females, and help in some way after the young are born. Therefore these males definitely do help in the survival of their offspring. But sexual reproduction is very common in simpler forms of life, where the males have nothing to do with their offspring.