Natural selection is a fundamental process that drives evolutionary change in populations of living organisms. It explains how species adapt to their surrounding environment over generations. This mechanism, central to the theory of evolution, accounts for the diversity of life observed on Earth. Organisms that are better suited to their environment tend to survive and reproduce more successfully, passing on their advantageous characteristics. Natural selection continuously shapes populations as environments shift.
Variation Among Individuals
For natural selection to operate, a population must exhibit variation among its individual members. Individuals within a species possess differing traits, such as variations in size, color, strength, or disease resistance. These differences provide the raw material upon which selection can act. Without such inherent variability, there would be no distinct characteristics for environmental pressures to favor or disfavor.
These variations primarily originate from random genetic mutations, which are changes in an organism’s DNA sequence. Mutations can occur spontaneously during DNA replication or be induced by environmental factors like radiation. Genetic recombination during sexual reproduction also significantly contributes to variation by shuffling existing genes into new combinations, ensuring offspring are genetically unique from their parents. This constant input of new genetic arrangements ensures that populations maintain a diverse pool of traits.
Heritability of Traits
The variations present within a population must be heritable, meaning they can be passed down from parents to their offspring. If a trait is not encoded in an organism’s genes and cannot be transmitted, natural selection cannot increase its prevalence. For instance, an acquired skill or a physical change due to environmental factors during an individual’s lifetime would not be subject to natural selection because it is not genetically inherited.
Heritability ensures that traits offering an advantage in a given environment can accumulate over successive generations. The genetic basis of these traits allows for their consistent transmission, enabling populations to evolve and adapt through the selective increase of beneficial traits.
Overproduction and Competition
Most species produce more offspring than the environment can support. For example, fish can lay thousands of eggs, and plants release numerous seeds, far exceeding the number that will survive to adulthood. This overproduction leads to competition for limited resources, such as food, water, shelter, and mates.
This competition creates selective pressure, as not all offspring can survive to reproduce. Individuals must contend with rivals from their own species, as well as with other species and environmental challenges like predators, diseases, or unfavorable climate conditions. Only a fraction of those born will reach reproductive age, setting the stage for differential survival and reproduction.
Differential Survival and Reproduction
In the face of competition and limited resources, individuals with heritable traits better suited to their environment are more likely to survive and reproduce successfully. This concept, known as “fitness” in an evolutionary context, emphasizes reproductive success over longevity. An organism with high fitness produces more viable offspring that survive to reproduce, passing on its advantageous genes.
This process is directly influenced by environmental conditions that favor specific traits. Traits that enhance an individual’s ability to obtain resources, evade predators, or withstand environmental stresses confer a reproductive advantage. Consequently, these favorable traits become more common in the population over successive generations, as individuals carrying them contribute disproportionately to the gene pool.