From A Storehouse of Knowledge

Natural selection is the change in the frequency of inheritable characteristics in a population as a result of differences in the ability of individuals with those characteristics to survive and produce offspring. This process is colloquially called "survival of the fittest", whereby "fitness" is always a function of the environment. Together with random mutations and inheritance, natural selection is one of the main elements of the mechanism of evolution proposed by Darwin. Creationists agree that natural selection as well as inheritance and random mutations occur, but do not believe that they are in practice capable of producing new characteristics of the sort needed for positive evolution. They therefore find it misleading to refer to observations of natural selection, e.g. in the peppered moths, as "evolution in action".^{[note 1]}

Contents 1 Description 2 Observations of natural selection 2.1 Antibiotic resistance in bacteria 2.2 Industrial melanism in the peppered moth 3 See also 4 Notes 5 References

Description

Natural selection is a process that operates on genetic variation within a population of organisms to eliminate or conserve heritable phenotypic traits due to a selective pressure affecting the ability of individual organisms to reproduce. Natural selection is thus a natural mechanism that determines the frequency of alleles expressing beneficial phenotypes. Natural selection may not always be directional to the extent it does not necessarily produce noticeably radically different phenotypes; it may have the effect of simply eliminating unfit genetic variants in the absence of significant selective pressures.

Selective pressures can be produced by any conditions of an environment that affect the ability of an organism to reproduce, including competition for resources from other organisms, an organism's ability to utilise available resources, an organism's ability to survive pressures like cold or heat, the compatibility of certain combinations of gamates, etc. The theoretical result is that only populations of better-adapted creatures perpetuate genes expressing phenotypes contributing to such adaptations. Traits are said to confer fitness for given selective pressures when they result in a greater ability to reproduce under the circumstances. Once established, a beneficial trait is not fixed for all time: a trait may no longer confer fitness as environmental conditions change.

Possible outcomes of natural selection include adaptation, co-adaptation, speciation, extinction, and, in the evolutionary explanation, co-evolution. Each such outcome is the result of the relative frequency of traits that confer some benefit sufficient to positively affect the survivability of one population of organisms vis-à-vis another. Natural selection does not necessarily operate on the level of species, but also on variations within subspecies varieties as well distinct populations of organisms, such as predator/prey and parasite/host relationships. Natural selection may result in extinction when a population lacks fitness to survive environmental pressures and therefore disappears.

Along with genetic drift and gene flow, natural selection is a major mechanism of the evolutionary explanation. Natural selection is not the only mechanism of or synonymous with evolution.

Natural selection was described by creationist Edward Blyth in 1835 to 1837 as a negative force which eliminated unfit species. In 1859 Charles Darwin, familiar with Blyth's work, incorporated natural selection into his theory of evolution by describing it as a positive force which favoured the fitter variant. Evolutionists often wrongly equate natural selection with evolution.^[1] But as evolutionist John Endler pointed out, "Natural selection must not be equated with evolution, though the two are intimately related."^[2]

While it is often assumed that natural selection requires the action of predators or diseases to actively remove the most 'unfit' members of the population, this is not the case. Differential rates of reproduction alone can result in some phenotypes being simply outbred, even in the absence of a survival difference.

One aspect of natural selection that is sometimes difficult to grasp is that natural selection is not necessarily directional and selection pressures from the environment do not always favor those traits that we might expect, given a naive reading of the term 'fitness'. To be stronger, faster, or longer-lived is not always a sufficient advantage from a reproductive standpoint; the genes of an organism that lives half as long but produces twice as many offspring will become more prevalent in the population. In a species with strong sexual selection, females may favor those males whose plumage, antlers, coloration, or behavior actually increase predation on the male (although Darwin's prediction that sexual selection would favour appearance has been falsified). It is not always possible to predict which traits or combinations of traits will be favored over others. There are examples of carnivores preferring to chase and kill healthy animals rather than weak or unhealthy animals.^[3] Further, in changing environments, traits that were favored in the past may become liabilities.

Observations of natural selection

Antibiotic resistance in bacteria

A current example of natural selection in action is the independent development of antibiotic resistance in bacteria, including the emergence of a methicillin-resistant Staphylococcus aureus (MRSA) bacterium. Due to the widespread use of antibiotics starting in the 20th century, the naturally high variation in the genetic makeup of individual bacteria increases the probability of the existence of a bacterium with a genetic variation that provides a resistance to antibiotics, which then enjoys a reproductive benefit that serves to increase the frequency of that variety in a bacterial population.

A few controlled experiments on acquired antibiotic resistance have been carried out which were able to show that the resistance was the result of a random mutation. Such a direct demonstration is only possible with bacteria because the genome can be continuously monitored, the time per generation is sufficiently short, and the number of individuals in the population can be very large. This experiment demonstrates evolution in the sense of a random mutation in an individual improving the fitness in the given environment, with the mutation subsequently spreading through the population as a result of inheritance and natural selection. Creationists ackowledge this, but argue that the mutation in question does not really entail an increase in genetic information, as would be required to explain the origins of all species as a result of evolution.

Industrial melanism in the peppered moth

The peppered moth, Biston betularia, exists in two forms, a form with light grey speckled wings and body, and a form with black wings and body. The first documented observation of the dark form occurred in 1848. During the following decades, the incidence of the dark form rose rapidly in some areas. By 1900 the black form exceeded 90 percent throughout the industrialized regions of England and in some areas was as high as 98%. Starting about 1960, the fraction of the dark form again declined and is now less than 10 or 20% in most areas.

The frequency of the dark form is highly correlated in both space and time with the level of industrial pollution. For this reason the phenomenon is called industrial melanism. In the regions and in the decades where the dark form was abundant, the surface of trees was significantly darkened by soot and by the death of lichens. This pattern was independently observed in both England and the United States. It is this correlation of the relative frequency of the two forms with the environment, coupled with the demonstration that the form is determined by simple Mendelian inheritance (with the dark form dominant), that makes the genetic history of the peppered moth in the last 160 years perhaps the clearest and best documented example of natural selection known. Consequently, textbooks have often used it as an example of natural selection. To give an impression of the difference in conspicuousness of the forms, photographs of dead specimens of light and dark forms next to each other on light and dark surfaces are usually shown. As with most insect photographs, live specimens are not used because their movement would blur the images and the lighting cannot be controlled. In addition, peppered moths are so rare that two can seldom be found next to each other in the wild.

In 1896 it was hypothesized by Tutt that the mechanism giving the dark form a selective advantage was its relative inconspicuousness when resting on surfaces darkened by pollution, making it less likely to be found and eaten by birds. Starting in the 1950s, Kettlewell and other scientists performed experiments to test and characterize this mechanism. The first study demonstrated a correlation between mortality, form, and environment by releasing and recapturing dark and light moths in regions with more and less pollution. More dark forms survived to be recaptured in polluted regions, and more light forms were recaptured in unpolluted regions. Other experiments showed that moths fixed to surfaces or released into aviaries were more readily eaten by birds if their color contrasted with that of the surface they were resting on.

The experiments can be and have been criticized on a number of grounds. For example to ensure the recapture of enough moths to allow good statistics, Kettlewell minimized the time between release and recapture by releasing the moths during the day. Under those circumstances, however, the moths are likely to alight and remain on the first surface that presents itself, for example tree trunks, rather than seeking out their preferred hiding places, perhaps underneath branches in the forest canopy. (While two thirds of the specimens found were on or near a trunk, only 13% were resting on an exposed portion of a tree trunk.) Despite this and similar limitations, it is hard to see how his results could have been obtained if camouflage and predation were not very important factors.

Population geneticists might call the history of the peppered moth evolution simply because it is a shift in the genetic makeup of a population, but evolution is usually taken to include the origin of different genes, leading to novel features, through mutation as well as changes in their frequencies due to natural selection. Whether the peppered moth illustrates evolution in this sense depends on whether the genes for the dark form were always present or were the result of a specific and recent mutation, and whether that mutation added or lost genetic information. The occurrence of other genes, unrelated to the one responsible for the dark color but near it on the chromosome, have recently been found to be highly correlated with the occurrence of the "dark" gene itself. This is an indication that the dark form is the result of a single, recent mutation, in which case the changes would meet most definitions of evolution, not just of natural selection. The exact nature of the mutation has not yet been determined, so it is not possible to examine the extent to which it may be a model for the type of evolutionary change necessary to produce more complex new features and functions.

See also

• Artificial selection

Notes

References