Historical Development and Mechanisms of Evolution and Natural Selection: Natural-Selection Mechanisms

Natural-Selection Mechanisms

Although the genotype of a population is affected by natural selection, it is the environment's effect on the individual phenotype that defines “fittest.”

Polymorphism and Polygenic Traits

Individual genetic variations must also function within the existing genome for polygenic traits and related characteristics to function correctly. Polygenic inheritance provides a continuum of trait expression such as hair color, even to the extent that certain species are polymorphic—having two or more dominant phenotypes that are both commonly expressed in nature. In the case of polymorphic moths, a dark environment favors the black variant over the white, which is easily discovered and consumed by predators. Both polygenic traits and polymorphism create phenotypic variations that are acted upon differently by selection, as described in the next sections.

Directional Selection

Directional selection works by selecting one phenotypic extreme at the cost of excluding the other phenotypic extreme. This allows the offspring of the favored extreme to reproduce more effectively and therefore dominate the population. As a result, the phenotype of successive generations of offspring move in a definite direction because of natural selection. It is most common when a population colonizes a new territory, or when new environmental changes such as an herbicide application are introduced into the system.

Directional selection explains why certain diseases are now resistant to long-standing medicines. Interestingly, it is not because the individual germs grew an immunity or resistance to the medicine (as Lamarck would say), but that as the medicine is applied over the years to a large population, sooner or later the natural variability in the germ population will produce a germ that is unaffected by the medicine. So while all the other germs die and therefore do not reproduce, the naturally resistant germ lives and continues to reproduce until an entire population of resistant germs is formed. The phenotype for that germ has moved in a direction that was defined by natural selection operating on and favoring an extreme phenotype.

Directional selection can be demonstrated by analyzing the following graphs. In the illustration Graph 1, the bell-shaped curve typifies the normal variation found within a species. The shaded area exemplifies those organisms favored for reproduction by natural selection.

In the illustration Graph 2, the population is the same after many generations. The shaded area indicates that the entire population is a descendant of the extreme phenotype found in Graph 1. The phenotype of the entire population has changed because of the directional selection forced by natural selection.

Diversifying Selection

Diversifying selection favors both phenotypic extremes at the expense of intermediate phenotypic ranges. From Genetic Variation and Natural Selection, remember that black-and-white moths are more camouflaged in backgrounds of sharp contrasting colors than the intermediate gray type, thus making the black-and-white type more likely to escape predation and reproduce in greater numbers. Diversifying selection is the opposite of stabilizing selection.

Diversifying selection forms multiple phenotypes that exhibit successful breeding seasons within a population. The result is the prevalence of two (or more) morphs within a population. Examine the illustration Graph 3. Again the bell-shaped curve represents the normal variation found in all populations. The shaded areas represent the two different phenotypic extremes favored by natural selection.

The illustration Graph 4 represents the population after many generations. The intermediate, or gray, phenotype has failed to maintain reproductive vigor, so that species has been eliminated from the population. The remaining are descendants of the phenotypic extremes in Graph 3. Graph 4 demonstrates the phenotype diversification of a population by natural selection.

Stabilizing Selection

The opposite of diversifying selection is stabilizing selection because it favors the intermediate phenotype at the expense of the extremes. Whereas a diversifying mechanism signals a changing environment, stabilizing selection indicates a stable environment that tends to reduce phenotypic variation.

Stabilizing selection favors the intermediate phenotypic range at the exclusion of the extremes. The illustration Graph 5 is similar to Graph 3, except natural selection favors the nonshaded, bulk of the population.

The illustration Graph 6 demonstrates the effect of stabilizing selection. The phenotype extremes have been eliminated. The result is a more homogeneous population.

Sexual Selection

The preference of one mate over another is sexual selection. In some cases, a demonstrative mating dance, elegant plumage, or overall size and physical/mental characteristics may influence the decision to mate or not mate with a given individual. In sharp contrast to evasive and camouflaged behaviors to avoid predation, these acts often appear ostentatious, regardless of the negative possible consequences, such as increased predation. Some will mate and pass on their genes; others will not. The results of this type of selection are most closely aligned with directional or diversifying selection as it affects the overall population structure. Sexual selection is different in that the form of selection is generated by factors within other members of the same species as opposed to an outside event.

The favoring of one phenotype over another by the biotic and abiotic factors in an environment create unequal reproductive rates for individuals in that population. Survival of the fittest essentially means who can reproduce in greater numbers. Reproductive success is the essence of natural selection.