Natural selection is a fundamental process explaining how organisms adapt to their surroundings. Individuals with traits better suited to their environment are more likely to survive and reproduce. These advantageous traits are passed to offspring, leading to a gradual shift in population characteristics over time. This process is a cornerstone of evolutionary biology, accounting for Earth’s vast biodiversity of life.
Directional Selection
Directional selection favors one extreme phenotype, causing the average trait value in a population to shift. This often occurs during environmental change or when a population moves to a new habitat. The favored trait becomes more common across generations, leading to population changes.
Antibiotic resistance in bacteria is a well-documented example. Antibiotics kill most susceptible bacteria, but some have mutations allowing survival. These resistant bacteria reproduce, passing on their genes, leading to populations dominated by resistant strains.
Stabilizing Selection
Stabilizing selection favors intermediate phenotypes over extreme variations, reducing genetic diversity by selecting against individuals at both ends of the trait spectrum. The result is a more uniform population, as the average characteristic is consistently favored.
Human birth weight illustrates stabilizing selection. Babies with very low or high birth weights face challenges. Infants born within an intermediate range, typically around 7 pounds, have the highest survival rates. This selective pressure stabilizes human birth weight around this optimum, as extremes are selected against.
Disruptive Selection
Disruptive selection (diversifying selection) favors individuals at both extreme ends of the phenotypic range, selecting against intermediate forms. This increases diversity, encouraging two or more distinct phenotypes. It often occurs in varied environments where different extremes offer unique advantages.
The African finch Pyrenestes ostrinus shows disruptive selection in beak sizes. They feed on very hard or very soft seeds. Large-beaked birds efficiently crack hard seeds, while small-beaked birds are better suited for soft seeds. Finches with intermediate beak sizes struggle with both, placing them at a disadvantage and leading to selection against them.
Driving Evolutionary Change
These three types of natural selection collectively drive evolution and adaptation. Directional selection allows populations to adapt to changing conditions by shifting traits. Stabilizing selection maintains well-adapted traits in stable environments. Disruptive selection promotes diversification by favoring specialized traits for different ecological niches.
Natural selection, through these mechanisms, shapes populations, contributes to new species, and is a fundamental force behind Earth’s rich biodiversity. The continuous interplay of these selective pressures ensures species respond to their surroundings, fostering life’s diversity and complexity.