(Note: the blue horizontal menu bar directly above lists the subsections of "Evolution According to Contemporary Science." Beginning with "How Has Life Evolved?," be sure to read each of these subsections before moving on to the next primary section, "Evil, Ethics, and Human Values in an Evolving World.")

The Darwinian Understanding of Evolution

Given the overall factuality of evolution, we naturally seek meaningful patterns among the lesser facts of life's history that the theory, or "metafact," of evolution organizes and explains. But evolutionary patterns are not the clear, straight-line trends that seekers of an underlying plan or purpose often expect to see. Instead, the family "tree" of life resembles a bush that has branched and spread more or less equally in many directions. Our large brains and self-conscious intelligence, however important in our own eyes, can be seen as just the peculiar specializations of one evolving lineage among countless others, and no more nor less a cause for wonder than the longevity of the giant sequoia or the whale's mastery of the deep.

The dominant impression one gets of the organic world is that, in its production of such myriads of insect and other species, diversity seems to have been an end in itself. When asked what he could infer from his study of nature about the attributes of the Creator, the great geneticist J. B. S. Haldane is said to have replied, "an inordinate fondness for beetles."

To those seeking purpose in nature, the process of evolution has proven even more distasteful than its pattern. Once organic evolution was demonstrated by paleontology and other disciplines, a central problem of biology was to explain why organisms are so well adapted to their surroundings that they appear specifically tailored to those surroundings by an intelligent Designer. Virtually all biologists now agree with Darwin that adaptations arise ultimately out of two basic processes, today called mutation and natural selection.³

Mutation refers to changes in the genetic material of living things: chromosomes and other gene-bearing structures within living cells, and the self-replicating molecules, DNA and RNA, that make up the genes and store and decode the instructions for the assembly and functioning of the complete organism. Mutations are changes in the genotype (the genetic code of an individual) that result from accidental damage (caused by exposure to radiation, chemicals, or other agents) or from mistakes made in copying the code during cell reproduction. They resemble typographical errors, or the sorts of damage that can degrade the information stored on magnetic tapes or computer disks.

Mutations in themselves are thought to occur by "chance." This does not mean they do not have causes (some possible causes were named just above), but that their causes have nothing to do with the organism's needs for adaptation to its environment at that moment: the changes are random with respect to adaptation. A mutation that would benefit a population of organisms is not more likely to occur simply because it would be useful. The vast majority of mutations are in fact harmful or neutral, as would be expected of changes made at random to any finely tuned mechanism. Only rarely will such random changes result in a useful innovation. However, given that the dice are being thrown constantly, among hordes of members of each species in each generation, and continuously over millions of years, winning combinations are bound to occur with some frequency.

Furthermore, in sexually reproducing species, even many combinations that are not immediate winners are kept on hand for possible future use as part of the large hidden store of genetic variation that is held in every naturally-occurring population, but not expressed in the phenotype (the visible makeup of an individual). This stored variation, originally produced by mutation, is constantly (and randomly) being unmasked by the process known as recombination—a sort of reshuffling of the cards in the genetic deck as part of the process of sexual reproduction—and manifested outwardly in members of a new generation, potentially influencing how they live and how well they survive and reproduce.

Mutation and recombination together create the variations that characterize every natural population. These "variations," however, are not mere departures from some ideal norm which constitutes the "essence" of a species. One of the key insights of Darwinism is that there are no such "essences": real species and populations are nothing but collections of variations, embodied in groups of actually or potentially interbreeding individuals, within which the "norm" is merely the statistical average of the members of the group at a given moment. Because this statistical "norm" can shift indefinitely, the species can evolve.

In Darwinian theory, adaptation—useful change to genetic information—results from interplay of the chance factors of mutation and recombination with an anti-chance factor: natural selection. This is analogous to the artificial selection exercised by animal and plant breeders. In the case of natural selection, the organism's environment—both physical conditions and other living things, including members of its own species—plays the role of breeder.

Every species can potentially produce far more offspring than can possibly survive on a finite planet, and therefore these offspring must compete (with each other and with other species) for limited resources. Since the offspring virtually always vary among themselves genetically in ways that are relevant to success in this competition, it is inevitable, in an actuarial sense, that those that by chance are better equipped for survival will tend to leave more offspring in the following generation. Hence, any heritable characteristic that enhances their number and survival will automatically tend to be preserved and to spread in the population with each successive generation.

Like artificial selection, natural selection can thus be described as the nonrandom reproduction of random variants: only the "fittest" survive and reproduce, with "fitness" or competitive ability defined pragmatically in terms of effectiveness of engineering and efficiency of function.

Darwinian evolution involves processes operating simultaneously on at least three levels: the genes mutate, the individuals are selected, and as a result the population evolves. An individual develops from conception to death, but does not evolve; its genetic makeup is fixed throughout its lifetime. Only through turnover of individuals in the population—by birth, death, and migration—does the gene pool change. Eventually, with the passage of much time, the population's new range of variation may have little or no overlap with its old range. This change of gene frequencies within a population, and this alone, is evolution.

The genetic code is constantly being revised, not by an intelligent designer but by that very environment to which the organism must immediately adapt. The environment itself sees to it that only the adequately-adapted live and reproduce. The survivors' level of adaptation thus remains adequate or even improves with time. It never, however, becomes "perfect"; natural selection can be expected to produce only adequate adaptations that are equal to or better than those of an organism's actual competitors. Importantly, adaptation is only to present conditions, since natural selection—narrowly focused on short-term advantage—cannot foresee the future.

In the Darwinian view, the beautifully simple mechanism of natural selection is all that is required to produce, from the raw material of chance genetic variation, all the adaptations of all the living things we observe. Indeed, it functions automatically in every system that involves imperfect copying of information and exposure of the results to competition for survival and reproduction. It is even being used to design industrial products and computer software. There is no doubt that natural selection works, and no way to stop it from working in natural populations—as shown by the rapid evolution of resistance to antibiotics on the part of disease-causing microbes, and of pesticide resistance on the part of insect pests.

According to Darwinian theory, therefore, individuals do whatever they can to ensure their own survival in the broadest sense: not only (and not necessarily) by maximizing their own lifespans, but by maximizing the numbers of copies of themselves in the next generation. (The technical expression for this is maximizing one's "Darwinian fitness".) In most cases, this is best done by producing more offspring, each of which typically embodies half or more of a parent's genetic information. However, even a very incomplete copy of one's own genetic code confers more immortality than no copy at all, so outwardly altruistic behavior that benefits an individual's siblings, their offspring, or even more distant relatives can also help to perpetuate that individual's own genes, in proportion to the degree of genetic relationship (i.e., similarity). By promoting the reproduction of close kin, an individual perpetuates copies of some of its own genes as well. This maximizes what biologists call its inclusive fitness—a more subtle and "farsighted" manifestation of "Darwinian fitness". This refinement of natural selection is known as kin selection.

But not even genetic relatedness preempts self-interest. Siblings compete, notoriously, despite their close kinship. A mother of any species readily sacrifices her life for a single offspring when it is the only one she has any hope of leaving; but in other circumstances the interests of mother and young can be opposed, and the same Darwinian logic can dictate a different strategy. In time of famine, for example, a young female may not only suppress ovulation but may spontaneously abort or abandon (or even eat) an offspring whose nutritional demands jeopardize her own survival, if she can thereby gain the chance to live and breed again another day.

The reproductive strategies of parents can likewise conflict. For example, the father's genetic interests may be served best by large, robust young, but the mother who has to bear and feed them may incur less risk to her health and her own genetic investment if the young are smaller. Both sexes choose mates with an eye to their own reproductive advantage; but even when females are coerced into copulating, they may still have numerous ways to control which sperm of which males fertilize their eggs, or which eggs or offspring develop to maturity.⁴ The result: a dynamic state of competition, compromise, and coevolution of reproductive attributes by each sex in response to adaptations in the other. Competition, in short, is pervasive; an individual cooperates with another only when (and only to the extent that) this promotes its own interests better than any other option.

Cooperation, in consequence, is also pervasive in nature, because it so often benefits all concerned. Many cases, especially in "higher" animals, can be explained by reciprocal altruism: I'll scratch your back because you scratched mine yesterday and may do so again tomorrow, even if we are unrelated. This kind of "doing unto others" is quite in accord with selection theory, since it is obviously motivated by self-interest. But in contrast to kin selection, it involves an essentially economic rather than genetic exchange, and is therefore less direct and more vulnerable to abuse by individuals who accept benefits but don't reciprocate

This insistence on the role of individual advantage is perhaps the most distinctive characteristic of neo-Darwinian theory. Whether selection can ever favor the interests of groups at the expense of the individuals in those groups is still controversial, but to the neo-Darwinian, the individual organism is the most important, if not the only, unit on which selection acts, and individual self-interest accordingly dominates the evolutionary process.

Putting it in the most general terms possible, each living organism, even the simplest, consciously or unconsciously seeks (in competition with others) to maximize its own share and control of the available energy and resources, and to apply these to its own survival, growth, and reproduction. It has been programmed to do this by that influence of its environment called natural selection, which is the "designer" or "programmer" that has gradually "written" the genetic code of its population and continues to maintain and upgrade it as environmental conditions change.

Thus, this powerful force of natural selection has, from the very dawn of life, worked constantly and automatically to create, and then to enforce, biological functions and behaviors that are directed toward self-perpetuation, in the broadest sense, on the part of all things living. The genetically-programmed drive to perpetuate oneself and one's posterity, and to sustain this effort by arrogating to oneself as much energy and resources as possible, is the most basic and necessary of all instincts, and therefore inseparable from the very notion of life. Because this drive is patently self-centered and ultimately (in a world of finite resources) succeeds only at the expense of others, there is no violence to language in calling it—in the simplest, most objective, non-psychological and non-pejorative sense—selfish.

3. The idea of natural selection was Charles Darwin's greatest intellectual contribution, and not the idea of evolution itself, which already had been discussed widely even before his time. However, the idea of evolution only became scientifically respectable after Darwin proposed a plausible mechanism—natural selection—to explain how it might operate.



4. Birkhead (2000) describes these often amazing strategies in detail.

Words highlighted in green appear in the Glossary.