Natural selection is the fundamental process driving evolution, describing how organisms better suited to their environment are more likely to survive and produce offspring. This mechanism operates based on four core components. There must be variation in traits among individuals, which must be heritable (passed down genetically). Because populations produce more offspring than the environment can support, a high rate of population growth leads to competition. This results in differential survival and reproduction, where individuals with advantageous, inherited traits contribute more to the next generation. The ways this selective pressure acts upon a population’s traits define the distinct types of natural selection.
Directional Selection
This mode of selection occurs when environmental pressure systematically favors one extreme phenotype over the average or the opposite extreme. This results in a sustained shift in the population’s average trait value over successive generations. If the traits within a population are plotted as a bell-shaped curve, directional selection causes this entire curve to slide toward the favored extreme. A compelling example is the evolution of antibiotic resistance in bacteria. When bacteria are exposed to an antibiotic, individuals with higher resistance levels are most likely to survive and reproduce. Consequently, the average resistance level of the subsequent population increases dramatically. Another case is the increasing body size of horses over geological time, where larger individuals were consistently favored.
Stabilizing Selection
Stabilizing selection preserves the status quo by favoring the intermediate or average phenotype and actively selecting against both extreme variations. This process reduces the overall spread of a trait within a population, making the distribution curve taller and narrower. Individuals at the far ends of the spectrum are less successful at survival or reproduction, resulting in reduced genetic variance for that trait. A well-known example is human birth weight. Infants who were very small or very large historically had lower survival rates than those of average weight. Birth weights around 7 pounds are optimal for survival, maintaining the trait’s average. Similarly, the clutch size (number of eggs laid by birds) is subject to stabilizing selection. Laying too few eggs results in a low number of offspring, but laying too many eggs can strain the parents’ ability to provide enough food, decreasing overall survival.
Disruptive Selection
Disruptive selection favors individuals at both ends of the phenotypic spectrum while selecting against the intermediate average trait. This pressure can cause a population’s trait distribution to become bimodal, appearing as two distinct peaks. The average phenotype is often the least successful because it is poorly suited to either of the available environmental niches. This process is a precursor to the formation of new species as it drives the population to diverge into two distinct groups. A clear illustration is found in African seedcracker finches, where beak size is key. Small-beaked finches crack small, soft seeds efficiently, while large-beaked finches crack large, hard seeds. Medium-beaked birds are poor at both tasks and struggle to compete for food, leading to selection for both small and large beak extremes.
Sexual Selection
Sexual selection is a specialized form of natural selection driven by an organism’s ability to secure a mate, rather than its ability to survive environmental pressures like predation. It often results in the evolution of traits that may appear detrimental to survival but significantly boost reproductive success. This selection is divided into two mechanisms. Intrasexual selection involves direct competition between members of the same sex, typically males, for access to the opposite sex. This drives the evolution of armaments, such as the large antlers on male deer used for fighting or the robust size of elephant seals used to defend breeding territories. Intersexual selection, or mate choice, occurs when one sex (usually the female) chooses a mate based on desirable traits. This preference leads to the evolution of elaborate ornaments, such as the peacock’s ornate tail feathers or the bright plumage of many male birds, which signal genetic quality or fitness to the choosy female.