Directional Selection: This type of natural selection occurs when one extreme phenotype is favored over other phenotypes, leading to a shift in the population's traits in that direction. Over time, this can result in a change in the average trait within the population, as individuals with the favored trait reproduce more successfully. An example of directional selection is the evolution of antibiotic resistance in bacteria, where bacteria with mutations allowing them to survive in the presence of antibiotics are favored.
Disruptive Selection: Disruptive selection occurs when extreme phenotypes at both ends of the spectrum are favored over intermediate phenotypes. This can lead to increased variation within a population and potentially result in speciation, as intermediate forms may be less fit in certain environments. An example of disruptive selection can be seen in certain species of birds where individuals with either very large or very small beaks are better suited to exploit different types of food resources, while those with intermediate beak sizes may struggle.
Stabilizing Selection: Stabilizing selection is the process in which intermediate phenotypes are favored, and extreme phenotypes are selected against. This type of selection tends to reduce variation and maintain the status quo for a particular trait. An example of stabilizing selection is human birth weight, where infants of average weight have higher survival rates compared to those who are significantly smaller or larger, leading to a preference for that intermediate trait over generations.