Although not the only agent of evolutionary change, natural selection is certainly the most important mechanism of adaptive evolutionary change in populations of organisms. Through the process of natural selection, individual organisms become better adapted to their local environment and thus acquire greater fitness, defined as an increase in reproductive success. If individual organisms vary in their ability to survive and reproduce, and those variations are inherited from parent to offspring, traits that are favored in the current biological conditions will spread in the population.
Natural selection as a means for evolutionary change in living creatures was first proposed, simultaneously, by two British naturalists, Charles Robert Darwin (1809-1882) and Alfred Russel Wallace (1823-1913), in a joint paper presented to the Linnean Society in London in July 1858. It was a case of a simultaneous discovery. Since the early 1840s, Darwin had been at work compiling his book On the Origin of Species by Means of Natural Selection, laboriously presenting his theory and its ramifications; but he was stunned when in 1858 he received a letter from Wallace and a manuscript in which the young scientist detailed a strikingly similar theory. With some anxiety, Darwin contacted his friend, naturalist Sir Charles Lyell (1797-1875), who responded by arranging the joint announcement of Darwin's and Wallace's theory before the audience of the Linnean Society. The following year, Darwin published a condensed version of his book, ultimately establishing himself as the principal creator of the theory of natural selection.
A great deal of the material in the Origin of Species was inspired by Darwin's earlier travels aboard the HMS Beagle, on which he served as ship's naturalist during the years 1831-1836. The vessel circumnavigated the globe. Although the voyage ultimately focused on surveying the coast of South America, including the Galapagos Islands, Darwin's observations spanned the globe. The notebooks he produced during this time contained his detailed documentation of the great variety of living creatures he saw and his efforts to understand their sometimes curious features. He observed that marsupials, mammals who carry their young in a pouch, are almost entirely restricted to the Australian continent (although he could think of no environmental factor unique to Australia that could explain the need for pouches!). Specimens of mockingbirds and finches collected in different islands in the Galapagos were so distinct from one another that Darwin apparently came to doubt the fixity of species; after the Beagle's return, he set about gathering evidence for evolutionary change and a mechanism to account for it.
Darwin did not develop his theory of natural selection, however, until some time after his return to London. Then, in the late 1830s, he engaged in the study of the writings of numerous philosophers and statisticians, including the economist T. R. Malthus. Malthus's Essay on the Principle of Population (1798) contained a critical idea that Darwin used in developing his argument, namely that uncontrolled growth of the human population must lead to famine and the ultimate elimination of a significant portion of the human race. Herein lies the foundation for Darwin's notion of the "struggle for existence" in a world in which all organisms are observed to produce many more offspring than are needed to maintain their numbers, and where superior variations would be preserved at the expense of less favorable alternatives.
Darwin was not the first to propose a theory suggesting that change in the forms of living things might have an environmental cause. One earlier proposal, put forward by the French naturalist Jean-Baptiste Lamarck (1744-1829), argued that organisms could improve themselves by their own efforts, and pass these improvements on to their offspring. This "inheritance of acquired characters" could explain, for example, the giraffe's long neck, which it would presumably develop by trying to reach leaves in high tree branches, stretching its neck to do so. Other early evolutionists expounded on the internal perfecting tendencies of living things, as if each somehow aspired toward an evolutionary goal. Darwin's contribution was different in that he proposed a testable theory that was based on the interactions between individuals, each engaged in the struggle to survive and reproduce in the local environment. Because there would never be enough food, nest sites, or mates for all progeny of all individuals produced, only the best competitors would survive and reproduce. The natural outcome of this struggle is that the features of the superior competitors become better represented in the population--not because of an internal "perfecting tendency," but because they provide greater fitness.
The modern understand of genetics, base upon the transmission of genetic information via genes comprised of DNA, completely disproves the errant Lamarckian theory. Although there are types of selection pressures (e.g. disruptive, stabilizing, directional; See Selection) that can be subsets of natural selection, there is abundant laboratory confirmation of the role of natural selection in evolutionary theory.
The differential survival and reproduction of living organisms is measured as fitness, which is technically defined as the number of offspring produced that survive to maturity. Fitness is a relative concept; measuring the relative rate of increase of one genotype, the genetic constitution of an individual, over alternative genotypes. Evolutionary biologists investigating the process of natural selection generally measure either the relative survival of individuals possessing some trait of interest, or the change in gene frequencies (the rate at which a gene increases in a population), between successive generations. Both measures are difficult to obtain, since they require documenting the survival of individuals and their offspring in the lab or in the field, and establishing a pattern of inheritance for the trait of interest. The process is further complicated since most observable traits--that is, most aspects of an organism's phenotype--are controlled by more than one gene. Individual genes may assort in different ways between generations. Furthermore, they frequently interact to produce complicated effects. Sorting out the connection between phenotype and genotype is not usually an easy task, but it has been done for some traits in some organisms.
The traits that confer the greatest fitness will depend on the local environment, and what proves advantageous in one environment may be quite unsuitable in another. An example is sickle-cell anemia, a disease that, if left untreated, kills the affected individuals, who carry two sickle-cell genes. However, in populations where the disease malaria occurs, the sickle-cell trait will increase because heterozygotes--those with one sickle-cell and one normal gene--have increased resistance to malaria, and only mild anemia. Thus, natural selection produces genetic change in populations that may fluctuate or even reverse themselves, in accordance with local environmental conditions.
The use of the insecticide DDT provides a well-known example of natural selection in operation. When DDT is first sprayed on an area populated by insects, the population declines abruptly; the poison can remain effective on such populations for up to ten years or more. The initial effectiveness then begins to diminish, so that increasingly greater applications are required to have the same impact. The reason is that DDT, a potent nerve poison, places a strong selective force on the insects; any that remain after the application of the pesticide are likely have some resistance to the poison, sometimes due to a single gene enabling the insects to detoxify it. Such a gene would quickly spread under a selective regime featuring repeated applications of the poison. What is more, once insects have evolved resistance to one insecticide, the time required to evolve resistance to others is reduced. It appears that once selection has honed the mechanisms of detoxification, those mechanisms are fairly easily appropriated for handling new toxins, allowing resistance to appear in one or two years instead of ten.
This is the complete article, containing 1,261 words
(approx. 4 pages at 300 words per page).
