Natural selection is the process that removes or adds certain traits in populations of organisms over time. It is driven by the benefit or harm to individuals, with respect to their environment, derived from the genetic changes within the population.
Did You Know?The term 'natural selection' was established by Charles Darwin, the scientist responsible for the discovery of the process of evolution. He intended to contrast it against artificial selection, which is the choosing of traits in other organisms by humans and is now called 'selective breeding'.
Genetic changes (mutations) that allow an organism to exploit its environment and reproduce more are passed on to future generations thanks to the earlier population's success at reproduction. Conversely, organisms that develop mutations that hamper their ability to exploit their environment can't reproduce as successfully as the organisms with beneficial mutations. As a result, the traits die out in that population in that particular environment.
Natural selection and mutation are both equally important drivers of evolution. Mutation is the process of the creation of the new genetic structures. This is not dependent on natural selection. The latter is the process of the changes in a population due to the effects of mutations on the population. It is not driven by mutations, but rather by the environmental response to them. Mutation does not equal natural selection.
It is necessary to gain some basic understanding about genes and their effect on the organism before discussing the types of natural selection.
The genetic structure of an organism defines how the organism appears and behaves. On a larger scale, differences between two distinct genetic structures define two distinct species, genera, families, etc.
Within one single species, though, the genetic makeup functions in two ways. Apart from providing the defining physical structure of the species to the individual, the genetic makeup also varies on a smaller scale between individuals of the same species. There are various characteristics that differ even between members of the same species. In the context of humans, we can consider an example of hair color, eye color, skin color, etc. While the genes in all humans make us all 'human' on a larger scale, varieties of the same type of gene create such superficial variations. The superficial appearance determined by the genetic structure of an organism is called the phenotype, and the genetic structure is called the genotype of the organism.
Such variations in the same gene are called alleles. For example, the same gene decides eye color in all humans. However, a particular allele of the gene produces a particular mixture of pigments, leading to brown eyes. Another allele results in black eyes, while yet another results in blue eyes.
Having understood what alleles are, we can now define natural selection as the process by which the frequency of alleles varies in populations.
In any population, a mean value for an allele gets determined over time. If a bell curve of the distribution of alleles in a stable environment was made, the mean value would fall in the center, with the extremes towards either end. This is demonstrated in the following illustration.
Bell curve showing allele distribution in stable population
For example, if the average height of all Americans were to be determined, we would find that it centers around a mean value, with extreme variations being rare. If, say, the average height was found to be 6 feet, people shorter than 6 feet would be progressively distributed on one slope of the bell according to the number of people standing at a particular height. Similarly, people taller than 6 feet would get progressively distributed on the other slope of the bell curve.
This stable distribution gets disturbed when some selection pressure is applied to the situation.
When environmental pressures act on both extremes (slopes) of the graph, the extreme individuals can't survive. This favors the mean distribution, which experiences unhindered growth. Thus, the mean population is stabilized. This is illustrated in the following bell curve.
Bell curve showing allele distribution due to stabilizing selection
For an example among wildlife, consider the color of oysters. Oysters are found on the seashore, against a backdrop of brown, golden sand. Those with brown shells would have an advantage over those with shells colored too light or too dark. Both the extremes would stand out in the sand, enabling their predators to catch them easily. This results in a decrease in the number of both light- and dark-colored oysters, with the mean color range thriving.
Directional selection occurs when environmental pressures act against or favor one of the two extremes. When one extreme gains an advantage that allows it to reproduce more successfully than both the mean and the other extreme, the latter two die out. This results in a shifting of the mean, because more individuals are born with the allele that was once an extreme. This is illustrated in the following diagram.
Bell curve showing allele distribution due to directional selection
There is an easily understandable example of this type of natural selection in animals. Polar bears are descendants of brown bears living in Arctic regions. When the allele for fur color changed to white in a few individuals, these new individuals gained a significant advantage due to their peculiar environment. Brown bears, though expert hunters, could be easily spotted on the white ice from miles away, whereas the white bears couldn't be detected (by vision, at least) until they got very close. This gave the white bears a huge advantage in feeding, which led to them becoming more successful breeders. The allele for the white fur color was thus passed on to future generations, leading to a radical decrease in the number of brown bears. Thus, white bears became the mean distribution of the allele responsible for fur color. Later, other variations all but completely died out due to stabilizing selection, favoring the new mean.
This example clearly shows the difference between mutation and natural selection. If the original brown bears hadn't been living in ice-covered areas, the white fur wouldn't have given them any advantage. It would have simply become one of the variations rather than the new mean value.
This example also demonstrates the random nature of mutation, but predictable nature of natural selection. For all we know, some brown bears may well have gotten the allele for black fur. Even modern brown bears have considerable differences in the particular shade of brown. However, these changes weren't any more beneficial than the mean value, and thus weren't favored by natural selection.
Disruptive selection occurs when environmental conditions act against the mean allele frequency. When this occurs, the extremities in the population get a boost and prosper. This is depicted in the figure given below.
Consider the hypothetical example of a population of beetles living in an area populated by red flowers. The mean population of the beetles would be red, since it provides a ready camouflage against the red flowers. The two extremes could be a subpopulation of yellow/green beetles and a subpopulation of brownish beetles. As long as the red flowers continue to bloom, the red allele will continue to be the dominant one.
If the red flowers are eliminated by some external agent, the red beetles would be left completely exposed to predators. The red allele would be reduced, with both the extremes experiencing a boost in their population. This is independent of the actual boost in numbers experienced by other variations themselves, which may be negligible. The increase in the predation of red beetles would automatically reduce the predation of other variations.
Disruptive selection is often a precursor to speciation, since the two extremes of the bell curve are already quite different than each other, let alone the mean.