Evolution is the process by which new living forms arise naturally over long periods of time. Such changes are driven by several evolutionary mechanisms. The most important is natural selection, which continually sifts random genetic changes (mutations) to determine which shall be passed on. Although the mutations on which selection operates are random, evolution is not; contrary to some popular belief, evolution is not the theory that life’s complexity arose “by pure chance.” Mutation is random, but selection is not. In natural selection, specific environmental conditions cause some mutations to be favored by allowing some variants to survive and reproduce more successfully than others—a profoundly non-random process. Other evolutionary mechanisms include mass extinctions and random genetic drift.
Evolutionary theory is the organizing principle of modern biology. It unites all the fields of biology under one theoretical umbrella to explain the changes in any given gene pool of a population over time. Evolutionary theory is a “theory” in the scientific usage of the word—that is, it is not tentative or speculative, but is a body of well-tested explanations embracing a wide variety of phenomena. There is a vast abundance of observational and experimental data to support the theory and its subtle variations: all facts known to modern biology are compatible with evolution and explainable in terms knowledge regarding the origin and history of the universe as a whole. There are no currently accepted scientific data that are incompatible with the general postulates of evolutionary theory and with the mechanisms of evolution. Scientific disputes over the roles of various evolutionary mechanisms are due to the fact that all scientific theories, no matter how useful, are subject to modification when data demand such revision. Evolutionary theory is thus a work in progress, open to constant improvement and tinkering. This is the work that evolutionary biologists do. But this does not mean that the overall structure of evolutionary biology—the fact that evolution has happened, and continues to happen—is not in doubt, any more than the various disputes among astronomers cast doubt on whether Earth goes around the sun.
Fundamental to the modern concept of evolutionary mechanism is that characteristics of living things can be inherited, that is, passed on from one generation to the next. Although Darwin did not know the mechanism of heredity, we know now that the fundamental unit of heredity is the gene. A gene is a section of a DNA (deoxyribonucleic acid) molecule. Some genes contain information that can be used to construct proteins via the cellular processes of transcription and translation. A gene pool is the set of all genes in a species or population. Mutation changes genes; natural selection allows some mutations to spread through gene pools; evolution is the result.
Evolution requires genetic variation. These variations or changes (mutations) can be beneficial, neutral or deleterious; deleterious changes will be sifted out by natural selection, slowly or rapidly, depending on how deleterious they are. Mechanisms that increase genetic variation include mutation, recombination, and gene flow.
Mutations generally occur via chromosomal mutations, point mutations, frame shifts, and breakdowns in DNA repair mechanisms. Chromosomal mutations include translocations, inversions, deletions, and chromosome non-disjunction. Point mutations may be nonsense mutations leading to the early termination of protein synthesis, missense mutations (a that results an a substitution of one amino acid for another in a protein), or silent mutations that cause no detectable change.
Recombination involves the re-assortment of genes through new chromosome combinations. Recombination occurs via an exchange of DNA between homologous chromosomes (crossing over) during meiosis. Recombination also includes linkage disequilibrium. With linkage disequilibrium, variations of the same gene (alleles) occur in combinations in the gametes (sexual reproductive cells) than should occur according to the rules of probability.
Gene flow occurs when individuals change their local genetic group by moving from one place to another. These migrations allow the introduction of new variations of the same gene (alleles) when they mate and produce offspring with members of their new group. In effect, gene flow acts to increase the gene pool in the new group. Because genes are usually carried by many members of a large population that has undergone random mating for several generations, random migrations of individuals away from the population or group usually do not significantly decrease the gene pool of the group left behind.
In contrast to mechanisms that operate to increase genetic variation, there are fewer mechanisms that operate to decrease genetic variation. Mechanisms that decrease genetic variation include genetic drift and natural selection.
Genetic drift results form the changes in the numbers of different forms of a gene (allelic frequency) that result from sexual reproduction. Genetic drift can occur as a result of random mating (random genetic drift) or be profoundly affected by geographical barriers, catastrophic events (e.g., natural disasters or wars that significantly affect the reproductive availability of selected members of a population), and other political-social factors.
Natural selection is based upon the differences in the viability and reproductive success of different genotypes within a population. When individuals bearing a distinct genotype are more successful than others at passing on that genotype by having more viable offspring, this is termed differential reproductive success. Natural selection can only act on those differences in genotype that appear as phenotypic differences that affect the ability to produce viable offspring that are, in turn, able to produce viable offspring. Evolutionary fitness is the success of an entity in reproducing (i.e., contributing alleles to the next generation). Stronger individuals are not necessarily more successful in evolution: “survival of the fittest” does not mean survival of the most violent or combative. A meek, inconspicuous individual that leaves more viable offspring may be more successful in evolutionary terms than a large, showy individual.
There are three basic types of natural selection. With directional selection an extreme phenotype is favored (e.g., for height or length of neck in giraffe). Stabilizing selection occurs when intermediate phenotype is fittest (e.g., neither too high or low a body weight) and for this reason it is often referred to a normalizing selection. Disruptive selection occurs when two extreme phenotypes are fitter that an intermediate phenotype.
Natural selection does not act with foresight. Rapidly changing environmental conditions can, and often do, impose challenges on a species that result in extinction. In fact, over 99.9% of all species of animals and plants that have ever lived are extinct; extinction (generally after about 10 million years) is the fate of the typical species.
In human beings, the operation of natural evolutionary mechanisms is complicated by geographic, ethnic, religious, and social factors. Accordingly, the effects of various evolution mechanisms on human populations are not as easy to predict. Increasingly sophisticated statistical studies are carried out by population geneticists to characterize changes in the human genome.
See also Chromosome mapping; Evolution, convergent; Evolution, divergent; Evolution, evidence of; Evolution, parallel; Evolutionary change, rate of; Genetic engineering; Genetic testing; Genotype and phenotype; Molecular biology.
Darwin, Charles. On Natural Selection. New York: Penguin, 2005 (1859 original).
Fullick, Anne. Inheritance and Selection. Oxford, UK: Heinemann, 2006.
Vincent, Thomas L. and Joel S. Brown. Evolutionary Game Theory, Natural Selection, and Darwinian Dynamics. Cambridge, UK: Cambridge University Press, 2005.
Otto, Sarah P. and Scott L. Nuismer. “Species Interactions and the Evolution of Sex.” Science. 304 (2004): 1018-1020.
K. Lee Lerner