Why males exist: An inquiry into the evolution of sex

All species reproduce in excess, way past the carrying capacity of their niche. In her lifetime a lioness might have 20 cubs; a pigeon, 150 chicks; a mouse, 1,000 kits; a trout, 20,000 fry, a tuna or cod, a million fry or more; an elm tree, several million seeds; and an oyster, perhaps a hundred million spat. If one assumes that the population of each of these species is, from generation to generation, roughly equal, then on the average only one offspring will survive to replace each parent. All the other thousands and millions will die, one way or another.