Natural selection is the fundamental process that drives evolutionary change by influencing which individuals survive and reproduce successfully. This mechanism operates when environmental pressures, known as selection pressures, act upon observable characteristics, leading to changes in trait frequency over many generations. The way these pressures are applied determines the resulting pattern of evolution, categorized into distinct modes, such as directional and disruptive selection. Understanding these patterns reveals how populations adapt and diversify.
The Foundation of Selection: Phenotypic Variation
Evolutionary change requires that individuals within a population possess a range of different characteristics, a concept called phenotypic variation. Many traits, such as height or beak size, are quantitative, meaning they are influenced by multiple genes and exist on a spectrum. When the frequency of these traits is plotted, they typically form a normal distribution, often visualized as a bell curve, with most individuals clustering around an average value, or mean.
Natural selection acts on this bell curve by determining which phenotypes have the highest fitness, a measure of reproductive success. The environment selects for individuals at a certain point on the curve, while selecting against others. The type of selection is defined by which part of the distribution—the average, one extreme, or both extremes—is most successful in passing on its genes.
Directional Selection: Shifting the Mean Trait
Directional selection occurs when individuals at one extreme end of the phenotypic distribution have a clear advantage over the average or the opposite extreme. This pressure causes the entire bell curve to shift consistently in one direction over successive generations. The overall effect is a change in the average trait value (the mean) within the population toward the favored extreme.
A well-documented example is the evolution of antibiotic resistance in bacteria, where the introduction of a drug creates a strong selection pressure. Only those bacteria with the highest resistance survive and reproduce, causing the population’s mean resistance level to increase rapidly. Similarly, the fossil record of horses shows a directional trend toward increased body size, driven by environmental factors favoring larger individuals.
Disruptive Selection: Favoring Extremes
Disruptive selection, also known as diversifying selection, acts in opposition to the average, favoring individuals at both extreme ends of the phenotypic spectrum. The intermediate phenotype is selected against because it is less successful in the given environment. This mode of selection is often seen in diverse environments where different niches require specialized traits.
The result of disruptive selection is the development of a bimodal distribution, where the single original peak splits into two distinct peaks. A classic illustration is the African seedcracker finch, where individuals with either very large or very small beaks thrive because their food source consists primarily of either very hard or very soft seeds. Finches with medium-sized beaks are inefficient at cracking both types, giving them a lower survival rate. This process can be an early stage in speciation, the formation of new species, as the two extreme groups become increasingly distinct.
How the Three Modes of Selection Compare
Directional and disruptive selection are two of the three primary ways selection influences a trait distribution; the third is stabilizing selection. Stabilizing selection is the most common mode, where the average phenotype is favored, and both extremes are selected against. The classic example is human birth weight, where infants closest to the mean have the highest survival rates.
The three modes are distinguished by their effect on the population’s mean and its overall variation. Directional selection causes the mean to shift toward one extreme, but the population retains a single peak. Disruptive selection selects against the mean, causing the distribution to split into two peaks, which increases overall variation. Stabilizing selection maintains the mean at its current position while reducing the width of the bell curve, thereby decreasing phenotypic variation.