Natural selection is a biological process driving evolutionary change. It describes how populations adapt to their environments over generations. Individuals with traits better suited to their surroundings are more likely to survive, reproduce, and pass on those characteristics to their offspring. Charles Darwin significantly contributed to its understanding and popularization in the mid-19th century.
Directional Selection
Directional selection occurs when one extreme phenotype is favored over others, causing the average trait value in a population to shift over time. Organisms possessing the favored extreme trait have a higher rate of survival and reproduction, leading to an increase in its prevalence in subsequent generations.
An example of directional selection is the development of antibiotic resistance in bacteria. When a population of bacteria is exposed to an antibiotic, individuals with little or no resistance are eliminated. However, any bacteria with even slight resistance can survive and multiply, passing on their resistance genes. Over time, the bacterial population shifts towards a higher average level of antibiotic resistance, making the antibiotic less effective.
This selection is commonly observed in environments undergoing consistent changes, such as shifts in climate or available food sources. The result is a population where the formerly extreme trait becomes the new norm.
Stabilizing Selection
Stabilizing selection favors intermediate variants within a population, acting against individuals at both extremes of the phenotypic range. This reduces phenotypic variation. Individuals with average traits are more likely to survive and reproduce, while those with extreme characteristics are selected against.
Human birth weight provides a classic illustration of stabilizing selection. Historically, infants with very low birth weights faced higher mortality rates due to being underdeveloped, while those with very high birth weights also had increased mortality, often due to complications during childbirth. Babies born with an intermediate weight have the highest survival rates.
This selection is common in stable environments where conditions remain consistent. It leads to a decrease in variation within the population, as the average phenotype becomes increasingly prevalent. Stabilizing selection preserves advantageous traits suited for a consistent environment.
Disruptive Selection
Disruptive selection, also known as diversifying selection, favors individuals at both extremes of the phenotypic range over intermediate phenotypes. In this scenario, individuals with average traits are disadvantaged, while those with extreme characteristics have a higher chance of survival and reproduction. This can lead to increased genetic variation within a population.
An example of disruptive selection can be seen in certain species of African finches. These birds feed on seeds, and the available seeds might be either very large and hard or very small and soft, with few seeds of intermediate size. Finches with either very large beaks (suited for large seeds) or very small beaks (suited for small seeds) thrive, while those with medium-sized beaks are less efficient at cracking either type of seed.
This selective pressure can eventually lead to the divergence of a population into two distinct groups, each adapted to one of the extreme conditions. Disruptive selection can increase the diversity of traits within a population and, in some cases, may be a precursor to speciation, where new species arise from the original population.