"One great mystery is why sexual reproduction became dominant for higher life-forms. Optimization theory says it should be otherwise. Take a fish or lizard, ideally suited to her environment, with just the right internal chemistry, agility, camouflage—whatever it takes to be healthy, fecund, and successful in her world. Despite all this, she cannot pass on her perfect characteristics. After sex, her offspring will be jumbles, getting only half of their program from her and half their re-sorted genes somewhere else. Sex inevitably ruins perfection. Parthenogenesis would seem to work better—at least theoretically. In simple, static environments, well-adapted lizards who produce duplicate daughters are known to have advantages over those using sex. Yet, few complex animals are known to perform self-cloning. And those species exist in ancient, stable deserts, always in close company with a related sexual species. Sex has flourished because environments are seldom static. Climate, competition, parasites—all make for shifting conditions. What was ideal in one generation may be fatal the next. With variability, your offspring get a fighting chance. Even in desperate times, one or more of them may have what it takes to meet new challenges and thrive. Each style has its advantages, then. Cloning offers stability and preservation of excellence. Sex gives adaptability to changing times. In nature it is usually one or the other. Only lowly creatures such as aphids have the option of switching back and forth."