Natural selection is the fundamental process driving evolutionary change, acting as a filter that shapes the characteristics of living things. It is defined as the differential survival and reproduction of individuals based on the traits they possess. This process acts on the collective group, affecting entire populations over generations. The core effect is a shift in the frequency of heritable traits, ensuring that characteristics best suited for a given environment become more common over time. Natural selection is the mechanism that results in populations becoming better matched to their surroundings.
The Essential Conditions for Natural Selection
Natural selection requires three simultaneous conditions within any population. The first is variation in traits among individuals. Without differences in characteristics, there would be no raw material for the environment to select from. This variation originates primarily from random genetic mutations, providing a spectrum of phenotypes upon which selection can act.
The second condition is heritability, meaning varied traits must be reliably passed down from parent to offspring. If an advantageous trait is not genetically determined, it cannot increase in frequency across successive generations. For instance, a bird’s increased strength due to better nutrition is not heritable, but a genetically determined difference in wing shape is.
The final condition is differential survival and reproduction, which links heritable variation to the environment. More offspring are produced than the environment can support, leading to competition for resources. Individuals with certain heritable traits will survive and reproduce more successfully than others, contributing a disproportionate number of genes to the next generation. This unequal reproductive success drives the change in the population’s genetic makeup over time.
Modes of Selection: How Trait Distributions Change
Differential reproduction expresses itself through distinct patterns, known as the modes of selection, which describe how a trait’s frequency distribution changes. Directional selection occurs when one extreme phenotype is favored over the average or the opposite extreme. This pushes the population’s trait average toward that favored extreme, such as the increase in beak size in Galápagos finches during drought, where larger beaks were better for cracking hard seeds.
Stabilizing selection favors the intermediate phenotype while selecting against both extremes of the trait distribution. This is the most common type of selection because it maintains the status quo for traits that do not change drastically over time. A classic example is human birth weight, where infants who are neither too small nor too large have the highest survival rates. This selection decreases genetic variation by narrowing the distribution around the mean.
Conversely, disruptive selection favors individuals at both ends of the phenotypic spectrum while selecting against the intermediate average. This results in a population with two distinct groups, potentially leading to a bimodal trait distribution. For example, in African seedcrackers, birds with very small or very large beaks are favored because they specialize in cracking different types of seeds, disadvantaging medium-beaked birds. Disruptive selection increases variation and can be a precursor to the formation of new species if the two extreme groups become reproductively isolated.
The Long-Term Outcome: Adaptation and Increased Fitness
The continuous action of natural selection over many generations leads to the cumulative outcome known as adaptation. An adaptation is a heritable characteristic that has evolved because it improves an organism’s ability to survive and reproduce in a specific environment. This results in populations that are increasingly better suited to their local ecological conditions. For example, the thick, white fur of a polar bear is an adaptation that increases its chances of survival in the Arctic environment through insulation and camouflage.
The ultimate measure of success under natural selection is fitness, which is defined by an individual’s lifetime reproductive success relative to other members of the population. Individuals that produce more viable and fertile offspring than the average individual are considered to have higher fitness. Therefore, a trait is considered an adaptation only if it confers a reproductive advantage, not simply because it promotes long life or strength.
Natural selection does not result in perfectly designed organisms, as it can only work with the variation available at the time. Adaptation is always relative to the current environment and the existing traits in the population. If environmental conditions change, a previously advantageous trait may no longer confer high fitness. The continual filtering of traits ensures the population tracks environmental changes, maintaining its “fit” to the world around it.