Transposable genetic elements.

N atural selection, as Darwin rec­ ognized more than a centu­ ry ago, favors individuals and populations that acquire traits con­ ducive to survival and reproduction. The generation of biological variation, which gives rise to new and potentially advantageous combinations of genetic traits, is therefore a central requirement for the successful evolution of species in diverse and changing environments. Hereditary information is encoded in the sequence of the building blocks, called nucleotides, that constitute a mol­ ecule of DNA, the genetic material. The basic step in the creation of genetic vari­ ation is the mutation, or alteration, of the DNA within a gene of a single in­ dividual. Mutations involve changes in nucleotide sequence, usually the re­ placement of one nucleotide by another. This can lead to a change in the chain of amino acids constituting the protein encoded by the gene, and the resulting change in the properties of the protein can influence the organism's biological characteristics. Spontaneous mutations are too rare, however, for genetic varia­ tion to depend on new mutations that arise in each generation. Instead varia­ tion is generated primarily by the re­ shuffling of large pools of mutations that have been accumulated within a population in the course of many gener­ ations. In higher organisms this reshuffling is done in the process of sexual reproduc­ tion. The genes are arrayed on two sets of chromosomes, one set inherited from the female parent and the other set from the male parent, so that there are two copies of each gene. Sometimes the nu­ cleotides of a genetic sequence differ slightly as a result of earlier mutation, producing alleles, or variant forms of a gene. In the formation of gametes (egg or sperm cells) the breakage of structur­ ally similar pairs of chromosomes can result in the reciprocal exchange of al­ leles between the two members of a pair of chromosomes. Such genetic recom­ bination requires that the segments of DNA undergoing exchange be homolo-