and thus avoid the evaporation that occurs in the leaves; photosynthesis is performed on the surface of the stem instead. In addition, their stems have evolved into barrel-like structures that store a reserve of water. A second mode of adaptation occurs in desert plants that have no leaves during the dry season, but after it rains, they burst into leaves and flowers and quickly produce seeds. A third mode of adaptation is that of desert ephemeral plants, which germinate from seeds, grow, flower, and produce seeds, all within the few weeks of the year when rainwater is available; at other times, the seeds lie quiescent in the soil.
The fossil record shows that life has evolved in a haphazard fashion. The radiations of some groups of organisms, the numerical and territorial expansions of other groups, the replacement of some kinds of organisms by other kinds, the occasional but irregular occurrence of trends toward increased size or other sorts of change, and the ever-present extinctions are best explained by natural selection of organisms subject to the vagaries of genetic mutation, environmental challenge, and past history. The scientific account of these events does not necessitate recourse to a preordained plan, whether imprinted from the beginning or through successive interventions by an omniscient and almighty Designer. Biological evolution differs from a painting or an artifact in that it is not the outcome of preconceived design. The design of organisms is not intelligent but imperfect and, at times, outright dysfunctional.
Natural selection accounts for the “design” of organisms because adaptive variations tend to increase the probability of survival and reproduction of their carriers at the expense of maladaptive, or less adaptive, variations. The arguments of intelligent design proponents that state the incredible improbability of chance events, such as mutation, to account for the adaptations of organisms are irrelevant because evolution is not governed by random mutations. Rather, there is a natural process (namely, natural selection) that is not random but oriented and able to generate order or “create.” The traits that organisms acquire in their evolutionary histories are not fortuitous but rather determined by their functional utility to the organisms, designed, as it were, to serve their life needs.
Chance is, nevertheless, an integral part of the evolutionary process. The mutations that yield the hereditary variations available to natural selection arise at random. Mutations are random or chance events because (i) they are rare exceptions to the fidelity of the process of DNA replication and because (ii) there is no way of knowing which gene will mutate in a particular cell or in a particular individual. However, the meaning