Why Have Sex?

      A dumb question, you may think: sex has all sorts of uses, not least the procreation of life. Remember, however, that the first living things were unicellular, almost certainly reproducing a sexually by cell division. Many modern multicellular organisms, particularly plants, also reproduce asexually, at least some of the time. Sex has several disadvantages. For the individual, sexual reproduction requires that another individual of the same species but opposite sex must be found; sexual displays are usually developed, including flowers, bright feathers, or elaborate behaviors; and in all known cases large numbers of gametes are formed but only a relative handful ever give rise to offspring. The energy spent in such endeavors must surely exert a penalty upon sexually reproducing organisms. Besides energy considerations, sexual reproduction also has a genetic handicap. As you know, each parent only contributes half of its genes to each offspring, the other half coming from the other parent. Let us suppose that, due to a mutation, an individual bears a single copy of a favorable allele, conferring a superior adaptation to its environment. Only half of its sexually produced offspring will receive the allele, whereas all of its sexually produced offspring would receive it. Other things being equal, an asexually reproducing variety would produce twice as many well-adapted offspring as its sexually reproducing counterparts and would quickly replace the sexual variety.

      How, then, could sex evolve, and once evolved, why might it be favored over asexual reproduction? Evolutionary biologists hotly debate this question, but most feel the variability is the key. Barring mutation, asexual reproduction produces offspring that are identical to their parents. If the parents are perfectly adapted to their environment and if the environment never changes, then clearly asexual reproduction is the way to go.