Organisms can reproduce with or without sex. Bacteria, and other single-celled organisms can simply undergo cell division to reproduce. Plants can create offshoots that will eventually separate from the parent and become an independent plant. Perhaps as a child you cut a worm in half and watched in some sort of awestruck horror as the two halves began to move as if each was its own worm. These are all forms of asexual reproduction. Only one parent and one set of genetic information is involved, and each offspring is a clone of the parent. Asexual reproduction is beneficial to propagate the species in times of environmental stress, or when mates are either few or out of reach.
Sexual reproduction, on the other hand, involves the mixing of genetic information from two parents; not blending, as some traits from one parent will dominate over those of the other parent, but combining. Sexual reproduction is evolutionarily favorable as it creates opportunities for genetic variation and new combinations of genes, but comes at a greater cost. For one, the mixing of genetic information from two parents has unpredictable results. Evolution proceeds randomly. Organisms who have genes that make them well-suited for survival will reach sexual maturity and pass those genes on to their offspring. Conversely, organisms who do not have genes that make them well-suited for survival will most likely die before reaching sexual maturity, removing their genes from the pool. When the goal of a species is to survive, and the goal of the individual to reproduce, that sort of genetic unpredictability may seem too risky. It seems, however, that the risk is minimal compared to the reward. After all, in a world that constantly changes, sexual reproduction can help a species keep up.
