Natural selection

Natural selection (sometimes Darwinian selection or simply selection) is the mechanism by which heritable traits that are beneficial to an organism (in that they increase chances of survival and reproductive fitness) become more common in a population over successive generations. Along with gene flow and genetic drift, natural selection is considered one of the key mechanisms that drives evolution. The term was introduced by Charles Darwin in his well-known 1859 book, On the Origin of Species.

Fitness, as applied to a biological organism, is defined as how successful it is at reproduction. The more 'fit' an organism is, the more offspring it produces in its lifetime (regardless of its overall lifespan). Natural selection maintains the alleles of fit organisms in this way, while unfit organisms are selected against and their alleles lost from the gene pool.

A typically-given example of natural selection is in the case of peppered moths in the United Kingdom, which may either be light or dark in colour. During the industrial revolution, the trees upon which the moths lived became blackened with soot, making the light-coloured moths more exposed to predators and less likely to survive long enough in order to reproduce. This meant that the black-coloured moths reproduced more frequently and in subsequent generations the allele for black colouration was vastly more common. In this scenario, the black-coloured moths were rendered reproductively fit by the conditions of their environment.

Sexual selection is the term used to describe an organism's specific ability to find a mate (while natural selection is a broader term, describing all the environmental forces that act on the organism's reproductive fitness).

While genetic drift, and to an extent gene flow, are largely undirected processes leading to random changes in allele frequency from generation to generation, natural selection is dictated by the demands of the environment. Although natural selection is directed by the environment, it must have raw genetic material to work on - mutations - and these mutations are statistically random and thus, themselves, undetermined by the environment. This means that a mutation is of equal likelihood to arise whether it poses a selective advantage or a selective disadvantage to the organism.

There are different types of selection, described below:

1. Stabilising selection is the most common form of natural selection. In this form of selection, the population stabilises and tends towards a particular trait, with any extreme deviation against that trait being selected against. A good example is human birth weight: a certain range of weight is stabilised for, since too low a weight leads to poor thermoregulation and too high a weight makes delivery through the pelvis too difficult.

2. Directional selection is when selection favours a particular (extreme) phenotype and the allele frequency of a population constantly moves in the direction of establishing that phenotype. An example is breeding race dogs. The fastest dogs are selected to produce offspring and then, of their offspring, the fastest dogs are selected again for further cross-breeding. Eventually the allele frequency of this dog population tends towards the faster phenotype, with the slower phenotypes being selectively eliminated. Another example is in antibiotic resistance in bacteria. When a new antibiotic is introduced, those bacteria with genetic resistance are more likely to reproduce, and thus resistance alleles are favoured while alleles that do not offer resistance are eliminated. This elimination of undesirable alleles is also called purifying selection.

3. Disruptive selection (also diversifying selection) occurs when either extreme of a phenotypic range is selectively advantageous, while the intermediate phenotypes are selectively disadvantageous. For example, in a population of black rabbits, white rabbits and grey rabbits, where the environment contains black rocks and white rocks, the black & white rabbits will have respective locations where they can hide from predation, while the grey rabbits will be conspicuous in both locations and thus selected against as a result of increased predation. Natural selection in this way is the basis of sympatric speciation, since the black and white rabbits will probably be reproductively isolated by nature of their different habitats and thus be subjected to their own independent selective pressures and genetic drift, which could eventually lead to their divergence as different species.

4. Balancing selection is a mechanism of maintaining genetic polymorphisms by their presence in heterozygotes (where heterozygotes have some advantage over homozygotes); it allows multiple alleles of a gene to be selectively favoured. A classic example is in the case of sickle-cell anaemia, where the heterozygote still has sufficient oxygen-carrying capacity compared to someone homozygous for sickle cell, and also has increased protection from malaria compared to someone homozygous for non-sickle cell. Balancing selection thus ensures that both alleles are maintained in the gene pool within heterozygous genotypes.