There are three main mechanisms of natural selection:
- Directional selection: This occurs when individuals with a particular variation of a trait are better adapted to their environment and are more likely to survive and reproduce. As a result, the frequency of that variation increases over time, and the population may evolve in a particular direction. An example of directional selection is the evolution of longer beaks in Galapagos finches during droughts, as longer beaks allow them to access harder-to-reach food sources.
- Stabilizing selection: This occurs when individuals with intermediate variations of a trait are better adapted to their environment and are more likely to survive and reproduce. As a result, the frequency of extreme variations of the trait decreases over time, and the population becomes more uniform. An example of stabilizing selection is the weight of human newborns, which is often optimized to be within a narrow range that maximizes survival chances.
- Disruptive selection: This occurs when individuals with extreme variations of a trait are better adapted to their environment and have higher fitness than individuals with intermediate variations. As a result, the frequency of intermediate variations decreases over time, and the population may split into distinct subpopulations with different trait values. An example of disruptive selection is the beak size of African seedcrackers, which diverged into two subpopulations with either large or small beaks that specialize in eating different sizes of seeds.
These mechanisms of natural selection can operate in different combinations and can lead to different patterns of evolution depending on the environmental conditions and the genetic variation present in the population.