Types of natural selection

Population geneticists have categorized three types of natural selection that can occur in nature. These are as follows:

1. Directional selection

• The variation that can occur when a population shows a particular trend through time is referred to as directional selection. It happens when the environment changes constantly. The directional selection process favors individuals that are better adapted to a new environment or situation or new ecological situation.
• The directional selection also transforms the gene pool of a species toward the highest level of adaptedness that can be reached in the new environment. 
• A very simple example of directional selection is the neck of a giraffe. Each giraffe has a different neck length. This variation in the trait gives each giraffe various advantages like a giraffe with a longer neck is easy to reach the food source, so better able to survive and reproduce. Over time, long-neck giraffes will reproduce more offspring and pass on their gene to the next generation. Thus, the future generation of giraffes has longer necks. If directional selection remains, this new population can eventually become a new species.

2. Stabilizing selection

• In this type of selection, average values for the given trait are favored eliminating extreme value in a population. It is the most common type of selection occurring in the population and is homeostatic that maintains the status quo. This selection reduces the variability in the population. 
• For example: In a plant, plants that are a more height are exposed to more wind and are at risk of being blown over, whereas a short-height plant fails to get an abundant amount of sunlight to prosper. Thus, the plants are average height between the two get both enough sunlight and protection from wind.

3. Disruptive selection

• In this selection, two adaptive traits are selected when the population exists in a heterogeneous environment. It favors the extreme if they have better traits or fitness and intermediates are disadvantageous. This selection results in diversification concerning traits. 
•   For example, an area that has black, grey, and white bunnies contains both white and black rocks. Natural selection will favor both the traits of white and black since they both prove useful for camouflage. The intermediate trait of grey does not prove useful so selective pressure act against the trait.  

The evolutionary biologist also recognized sexual selection and group and kin selection.

1. Sexual selection 

• Most species of animals are dimorphic (male and female). Males and females are different in terms of color, specialized song patterns, behaviors, sex organs, capability, etc. 
• Sexual selection is directly related to differential reproduction, including finding and acquiring a mate, copulation, fertilization, and parental care.
• Those organisms whose more capable of securing mates and is more fitted to the environment. Sexual selection aims to reproduce in which an individual needs to be able to find and protect a mate and produce viable offspring.
• In 1938, Huxley recognized two types of sexual selection; Epigamic selection (based on the choice made between male and female) and Intrasexual selection (this selection is based on the interaction between animals of the same sex, generally between males).

2. Group and kin selection

• Group selection is a selection of a group of individuals, favoring one group over the other, leading to the evolution of a trait that is group advantageous.
• The altruistic behavior of an individual involved in kin selection occurs when natural selection favors a trait that benefits related group members.