Often, when Hardy-Weinberg equilibrium is violated, natural selection is occurring which will shift allelic frequencies. We can identify four different types of selection which alter these frequencies in different ways.
Directional selection:Sometimes one allele, or a combination of alleles from different genes, provide an adaptive advantage for an organism. Since these organisms are more likely to survive and reproduce, the frequencies of these advantageous alleles will increase. Directional selection is when the allelic frequency shifts in a steady, constant direction.
For example, pesticide resistance in insects is due to alleles which provide protection against these chemicals. Thus, all insects that survive will carry these alleles and will pass them onto their progeny. Soon, all the individuals in a population will have these alleles.
Stabilizing selection: In this type of selection, an intermediate form of an allele or trait is favored over the extremes. A good example of this is with human birth weight. The average weight of a new born is 7 pounds. Much lower than this results in medical problems and infant mortality. Much higher and the mother will have difficulty carrying and delivering the child.
Disruptive Selection: Disruptive selection occurs when environmental conditions favor the two extremes of the trait. In this case, the population will, in essence, be split into two. Given enough time, evolution may result in two different species.
Sexual selection: When competition occurs between individuals of the same sex over mating rights, it is often times the bigger, more aggressive animal, or the animal with the more pleasing appearance or courtship display that is allowed to mate. Therefore, these alleles will be passed on to the next generation. Sexual selection has also been implemented in sexual dimorphism, the dramatic difference seen between members of the opposite sexes in the same species (for example, the beautifully plumed peacock and his drab counterpart, the peahen.)
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