Selection
Evolutionary selection pressures act on all living organisms, regardless whether they are prokaryotic or higher eukaryotes. Selection refers to an evolutionary pressure that is the result of a combination of environmental and genetic pressures that affect the ability of an organism to live and, equally importantly, to produce reproductively successful offspring (including prokaryotic strains of cells).
As implied, natural selection involves the natural (but often complex) pressures present in an organism's environment. Artificial selection is the conscious manipulation of mating, manipulation, and fusion of genetic material to produce a desired result.
Evolution requires genetic variation, and these variations or changes (mutations) are usually deleterious because environmental factors already support the extent genetic distribution within a population.
Natural selection is based upon expressed differences in the ability of organisms to thrive and produce biologically successful offspring. Importantly, selection can only act to exert influence (drive) on those differences in genotype that appear as phenotypic differences. In a very real sense, evolutionary pressures act blindly.
There are three basic types of natural selection: directional selection favoring an extreme phenotype; stabilizing selection favoring a phenotype with characteristics intermediate to an extreme phenotype (i.e., normalizing selection); and disruptive selection that favors extreme phenotypes over intermediate genotypes.
The evolution of pesticide resistance provides a vivid example of directional selection, wherein the selective agent (in this case DDT) creates an apparent force in one direction, producing a corresponding change (improved resistance) in the affected organisms. Directional selection is also evident in the efforts of human beings to produce desired traits in many organisms ranging from bacteria to plants and animals.
Not all selective effects are directional, however. Selection can also produce results that are stabilizing or disruptive. Stabilizing selection occurs when significant changes in the traits of organisms are selected against. An example of this is birth weight in humans. Babies that are much heavier or lighter than average do not survive as well as those that are nearer the mean (average) weight.
On the other hand, selection is said to be disruptive if the extremes of some trait become favored over the intermediate values. Although not a factor for microorganisms, sexual selection and sexual dimorphism can influence the immunologic traits and capacity of a population.
Sometimes the fitness of a phenotype in some environment depends on how common (or rare) it is; this is known as frequency-dependent selection. Perhaps an animal enjoys an increased advantage if it conforms to the majority phenotype in the population. Conversely, a phenotype could be favored if it is rare, and its alternatives are in the majority. Frequency-dependent selection provides an interesting case in which the gene frequency itself alters the selective environment in which the genotype exists.
Many people attribute the phrase "survival of the fittest" to Darwin, but in fact, it originated from another naturalist/philosopher, Herbert Spencer (1820–1903). Recently, many recent evolutionary biologists have asked: Survival of the fittest what? At what organismal level is selection most powerful? What is the biological unit of natural selection-the species, the individual, or even the gene?
Selection can provide interesting consequences for bacteria and viruses. For example, reduced virulence in parasites, who depend on the survival of their hosts for their own survival may increase the reproductive success of the invading parasite. The myxoma virus, introduced in Australia to control imported European rabbits (Oryctolagus cuniculus), at first caused the deaths of many individuals. However, within a few years, the mortality rate was much lower, partly because the rabbits became resistant to the pathogen, but also partly because the virus had evolved a lower virulence. The reduction in the virulence is thought to have been aided because the virus is transmitted by a mosquito, from one living rabbit to another. The less deadly viral strain is maintained in the rabbit host population because rabbits afflicted with the more virulent strain would die before passing on the virus. Thus, the viral genes for reduced virulence could spread by group selection. Of course, reduced virulence is also in the interest of every individual virus, if it is to persist in its host. Scientists argue that one would not expect to observe evolution by group selection when individual selection is acting strongly in an opposing direction.
Some biologists, most notably Richard Dawkins (1941–), have argued that the gene itself is the true unit of selection. If one genetic alternative, or allele, provides its bearer with an adaptive advantage over some other individual who carries a different allele then the more beneficial allele will be replicated more times, as its bearer enjoys greater fitness. In his book The Selfish Gene, Dawkins argues that genes help to build the bodies that aid in their transmission; individual organisms are merely the "survival machines" that genes require to make more copies of themselves.
This argument has been criticized because natural selection cannot "see" the individual genes that reside in an organism's genome, but rather selects among phenotypes, the outward manifestation of all the genes that organisms possess. Some genetic combinations may confer very high fitness, but they may reside with genes having negative effects in the same individual. When an individual reproduces, its "bad" genes are replicated along with its "good" genes; if it fails to do so, even its most advantageous genes will not be transmitted into the next generation. Although the focus among most evolutionary biologists has been on selection at the level of the individual, this example raises the possibility that individual genes in genomes are under a kind of group selection. The success of single genes in being transmitted to subsequent generations will depend on their functioning well together, collectively building the best possible organism in a given environment.
When selective change is brought about by human effort, it is known as artificial selection. By allowing only a selected minority of individuals or specimen to reproduce, breeders can produce new generations of organisms (e.g. a particular virus or bacterium) that feature desired traits.
Join eNotes
Over 3,500 study guides, question and answer forums, literature criticism, reference content, and much more!
