 One source of variation in genes of sexual organisms supplementing mutation is crossover. Pairs of homologous chromosomes line up next to each other at meiosis, crossover, and recombine. For stretches of DNA in which different alleles exist, the result can be a change in information. Quite obviously, the shorter the stretch of DNA involved, the less likely that crossover will occur and the message change. Dawkins appeals to such dismantling of entities to argue against organisms functioning as replicators. In sexual organisms, organisms themselves are torn apart and repeatedly assembled each generation (Caporael 2003). If long stretches of DNA lack the necessary identity by descent to function as replicators, then sexual organisms certainly lack it. However, some other explanation has to be provided for asexual organisms because they pass on their overall structure largely unchanged from generation to generation. For example, R. A. Fisher, in The Genetical Theory of Natural Selection (Fisher 1930) considered the entire genetic complement of asexual organisms to be a single gene, and this view has been repeated from time to time since. According to Dawkins, genes, and only genes, can function as replicators in biological evolution. How large these genetic units are depends on such things as the prevalence of sex, the frequency of crossover or lateral gene transfers, and the intensity of selection.