Natural selection is the process by which populations of living organisms adapt and change over time, serving as the central mechanism of evolution. This concept explains how life on Earth has diversified into millions of species through entirely natural processes. Natural selection operates as a non-random process where individuals better suited to their environment tend to survive and leave more offspring.
Variation Exists
The process of natural selection begins with the observation that no two individuals within a species are exactly alike; diversity is a fundamental characteristic of all populations. This diversity, or phenotypic variation, includes observable differences such as size, coloration, and behavior. Without this range of individual differences, there would be nothing for the environment to “select” from, and evolutionary change could not occur.
The underlying source of this variation is genetic, primarily driven by random mutations, which introduce new alleles or gene variants into the population’s gene pool. Existing genetic material is further shuffled and recombined through sexual reproduction, creating unique combinations of traits in each generation. These genetic differences manifest as the observable traits upon which selection can act.
Traits are Inherited
For variation to lead to a lasting change in a population, the differences between individuals must be transmissible from parent to offspring. This condition is known as heritability, which links the observable trait, or phenotype, to the underlying genetic code. A heritable trait is one that resembles the parent’s corresponding trait more closely than it resembles the same trait in a random individual.
The mechanism for this transmission is the passing of genes, encoded in DNA, across generations. Acquired characteristics, such as a scar or a trait gained through environmental factors, are not subject to natural selection because they are not encoded in the genetic material passed on to progeny. Only variations with a genetic basis can be reliably passed down and contribute to the evolutionary trajectory of the species.
Differential Survival and Competition
The third condition for natural selection to operate is the differential success of individuals in surviving and reproducing. Organisms typically produce more offspring than the environment can possibly sustain, leading to a “struggle for existence” due to limited resources. This overproduction creates intense competition for necessities like food, shelter, and mates, which acts as a filter on the population.
The environment introduces selective pressure, which determines which variations provide an advantage or disadvantage in this struggle. Individuals possessing beneficial traits are more likely to survive to reproductive age. Biological fitness is measured by the reproductive success of an individual, meaning the number of viable offspring they contribute to the next generation. This non-random survival and reproduction based on heritable traits is the core action of natural selection within a single generation.
Increased Frequency of Adaptive Traits
The cumulative result of the selection process over successive generations is a directional change in the population’s genetic makeup. Individuals who survived and reproduced successfully pass their advantageous, heritable traits to their offspring. As this pattern repeats, the frequency of those beneficial genes, or alleles, increases within the population’s gene pool.
This shift in allele frequency is the definition of evolution. The traits that confer a survival or reproductive advantage become more common, leading to the species becoming better suited, or adapted, to its specific environment. Adaptation is the cumulative outcome of this process, where the population gradually accumulates characteristics that enhance fitness.