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.
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 genotypeof 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.
This stable distribution gets disturbed when some selection pressure is applied to the situation.
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.
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.