Natural selection

Natural selection is the differential survival and reproduction of individuals due to differences in the relative fitness endowed on them by their own particular complement of observable characteristics. It is a key law or mechanism of evolution which changes the heritable traits characteristic of a population or species over generations. Charles Darwin popularised the term "natural selection", contrasting it with artificial selection, which is intentional, whereas natural selection is not.

For Darwin, natural selection was a law or principle which resulted from three different kinds of process: inheritance, including the transmission of heritable material from parent to offspring and its development (ontogeny) in the offspring; variation, which partly resulted from an organism's own agency (see phenotype; Baldwin effect); and the struggle for existence, which included both competition between organisms and cooperation or 'mutual aid' (particularly in 'social' plants and social animals).

Variation of traits, both genotypic and phenotypic, exists within all populations of organisms. However, some traits are more likely to facilitate survival and reproductive success. Thus, these traits are more likely to be passed on to the next generation. These traits can also become more common within a population if the environment that favours these traits remains fixed. If new traits become more favoured due to changes in a specific niche, microevolution occurs. If new traits become more favoured due to changes in the broader environment, macroevolution occurs. Sometimes, new species can arise especially if these new traits are radically different from the traits possessed by their predecessors.^{[citation needed]}

Examples

There are now quite a number of examples of natural selection in natural populations.

Antibiotic resistance

Main article: antibiotic resistance

Resistance to antibiotics is increased by the survival of individuals which are immune to the effects of the antibiotic. Their offspring inherit the resistance, creating a new population of resistant bacteria.

A well-known example of natural selection in action is the development of antibiotic resistance in microorganisms. Since the discovery of penicillin in 1928 by Alexander Fleming, antibiotics have been used to fight bacterial diseases. Natural populations of bacteria contain, among their vast numbers of individual members, considerable variation in their genetic material, as the result of mutations. When exposed to antibiotics, most bacteria die quickly, but some have mutations that make them slightly less susceptible. If the exposure to antibiotics is short, these individuals will survive the treatment. The elimination of individuals which have no resistance is an example of natural selection.

Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-resistant bacteria will emerge. This leads to what is known as an evolutionary arms race, or co-evolution, in which bacteria continue to develop strains that are less susceptible to antibiotics, while medical researchers continue to develop new antibiotics that can kill them. Response strategies typically include the use of different, stronger antibiotics; however, new strains of MRSA have recently emerged that are resistant even to these drugs. A similar situation occurs with pesticide resistance in plants and insects, and with malarial resistance to quinine.

Camouflage

Main article: camouflage

One famous case study is the study of peppered moth evolution, and there are many other examples. Most of these day-flying moths were light in colour, but just a few of the moths were dark. At first, the light coloured moths survived better because they were camouflaged against the light colour of the nearby trees. This made it hard for birds to see them.

When factories were built, the pollution made all the trees look black. Now the light coloured moths were obvious against the dark bark. The dark coloured moths had the advantage after the environment changed. The genes controlling dark colour spread through the population of moths. After the second world war, controls against pollution worked to make the environment cleaner. Then the lighter moths once again had the advantage, and are now much more common.

Mimicry is another example: Some harmless insects mimic other insects which are dangerous, or which taste foul. Mimicry evolves because the better mimics survive better. They live to produce more offspring than the less good mimics. The genes of the better mimics become more common in the species. Over time, mimic species get closer to their models.

Sexual selection

Main article: sexual selection

Illustration from The Descent of Man and selection in relation to sex by Charles Darwin showing the Tufted Coquette Lophornis ornatus, female on left, ornamented male on right.

Sexual selection is a special kind of natural selection. It is a theory of Charles Darwin that certain evolutionary traits can be explained by competition within a species. Darwin defined sexual selection as the effects of the "struggle between the individuals of one sex, generally the males, for the possession of the other sex". It is usually males who fight each other. Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc.) and sometimes referred to as 'weapons'. Traits selected by mate choice are called 'ornaments'.

Females often prefer to mate with males with external ornaments—exaggerated features of morphology. Genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism—features that also benefit females. This idea is known as the 'good genes' hypothesis. Sexual selection is still being researched and discussed today.

Ernst Mayr said:

"Since Darwin’s days it has become clear that this kind of selection includes a far wider realm of phenomena, and instead of sexual selection it is better referred to as selection for reproductive success... genuine selection, not elimination, is involved, unlike survival selection. Considering how many new kinds of selection for reproductive success are discovered year after year, I am beginning to wonder whether it is not even more important than survival selection, at least in certain higher organisms".