The scientific evolution theory by natural selection was independently conceived by Alfred Russel Wallace and Charles Darwin in the mid – 19th century, and it was set out in detail in Darwin’s book On the Origin of Species. Scientific evolution by natural selection was first demonstrated by the observation that often, more offspring are produced than may possibly survive.
Types of Selection
Stabilizing Selection
Natural selection may be studied by analyzing its effects on the changing gene frequencies, but it may also be explored by examining the effects on observable characteristics – or phenotypes – of the individuals in a population. Phenotypic traits’ distribution scales such as weight, height, number of progeny, or longevity typically exhibit a greater number of individuals with intermediate values and fewer toward the extremes—this is called the normal distribution. When individuals with the intermediate phenotype are favored, and extreme phenotypes are selected against, the selection is called stabilizing.
The distribution and range of phenotypes then remain nearly similar from one generation to the other. Stabilizing selection is common. Individuals with moderate phenotypic values have a better chance of reproducing and surviving. For example, newborn infant mortality is highest when they are either very large or very small; babies of moderate size have a higher chance of survival.
The below figure shows the types of natural selection or the natural selection and its types.
Three types of natural selection representing the effects of each on the distribution of phenotypes within the population. The downward arrows, which point to those phenotypes against that selection act, which is a stabilizing selection example.
Stabilizing selection (left column) acts against the phenotypes at both extremes of the distribution, favoring the intermediate phenotype multiplication. Directional selection (on the center column) works against one extreme of phenotypes by shifting the distribution to the opposite extreme. By dividing the distribution at each extreme, diversifying selection (on the right column) works against intermediate phenotypes.
Often, the stabilizing selection is noticeable after the artificial selection. Chickens that lay larger eggs, cows that produce milk, and corn with a high protein content are chosen by breeders. At the same time, the selection should be reinstated or continued from in time, even after the desired goals have been achieved. If this is fully prevented, natural selection takes over and eventually returns the traits to their original intermediate value.
As a result of stabilising selection, populations often retain a consistent genetic constitution across a range of traits. This attribute of populations is known as genetic homeostasis.
Directional Selection
The phenotypes’ distribution in a population at times changes systematically in a specific direction. The biological and physical aspects of the environment are changing continuously, and over long time periods, the changes can be substantial. The climate and configuration of the waters or land differ incessantly. Also, changes occur in the biotic conditions, which means, in the other organisms present, whether prey, predators, parasites, or competitors. Genetic changes take place as a consequence because the genotypic fitnesses can shift so that various sets of alleles are favored. Also, the opportunity for directional selection arises when the organisms colonize new environments, but the conditions are different from their original habitat.
The below figure shows the light gray peppered moth (Biston betularia)
On the soot-covered oak tree’s trunk, a light grey peppered moth (Biston betularia) and a darkly pigmented form rest near each other. Against this background, the light gray can be noticed more easily than the darker variant.
The directional selection process occurs in spurts. The substitution of one genetic constitution for another alters genotypic fitness at other loci, causing changes in allelic frequencies, which in turn stimulates further changes, and so on in a cascade of events.
Directional selection can be possible only if there is a genetic variation with respect to phenotypic traits under the selection. Natural populations have wide stores of genetic variation, which are constantly replenished by variations that produce new variants. The nearly universal success of the artificial selection and rapid response of the natural populations to new environmental challenges is the evidence, which existing variation provides the required materials for directional selection.
Diversifying Selection
Diversifying selection may favour two or more divergent phenotypes in the same environment at the same time. None of the natural environment is homogeneous; rather, the environment of any animal or plant population is a mosaic consisting of either less or more dissimilar sub environments. There is heterogeneity with respect to the food resources, climate, and living space. And, the heterogeneity can be temporal, with the change taking place over time and spatial as well. Species cope with environmental heterogeneity in diverse ways.
A strategy is a genetic monomorphism, which is a generalist genotype selection that is well-adapted to all of the species’ sub-environments. The other strategy is genetic polymorphism, which is the selection of a diversified gene pool that yields various genotypes, each adapted to a specific sub-environment.