The terms evolution and natural selection are frequently used interchangeably, often leading to a misunderstanding of these foundational concepts in biology. They are not synonyms, but represent distinct levels of biological change. Evolution describes the overarching historical pattern of life, while natural selection is a powerful process that acts as a primary engine for that change. Understanding the difference requires separating the large-scale outcome (evolution) from the specific mechanism (natural selection).
Defining Evolution: Change Across Generations
Evolution is defined as a cumulative change in the heritable characteristics of a biological population over successive generations. This is most precisely understood as a change in the frequency of alleles within a population’s gene pool over time. An allele is a variant form of a gene, and its frequency is the proportion of that variant relative to others in the population.
When the proportion of a certain allele changes from one generation to the next, the population has evolved. This shift in genetic makeup can manifest as changes in visible traits, physiology, or behavior. Evolution is the observed pattern of descent with modification, representing the historical record of life’s transformation, and it occurs regardless of the specific cause of the change.
Defining Natural Selection: The Adaptation Mechanism
Natural selection is a mechanism that leads to adaptive evolution, explaining how organisms become better suited to their environments. This process requires four necessary components:
- Individuals must exhibit variation in their traits, such as differences in body size or color.
- These traits must be heritable, meaning they can be consistently passed down from parent to offspring.
- There must be a high rate of population growth, producing more offspring than local resources can support, which creates competition for limited resources.
- Differential survival and reproduction must occur, where individuals with favorable traits contribute more offspring to the next generation.
Individuals possessing traits that make them better suited to their specific environment survive and reproduce more successfully. This causes favorable, heritable traits to become increasingly common in the population over time, leading to adaptation. The environment itself is the selective pressure, favoring certain traits over others.
The Relationship Between Mechanism and Outcome
The most straightforward way to differentiate the two concepts is to understand that natural selection is a process, while evolution is the result of that process, among others. Natural selection is the how—the specific driver that causes populations to become better adapted to their environment. Evolution is the what—the cumulative change in allele frequencies observed across generations.
Natural selection acts as a filter that consistently favors certain genetic variants, driving adaptive change. Evolution is a broader term that encompasses all changes in the genetic composition of a population, whether they are adaptive or not. A population can evolve through mechanisms other than natural selection, but natural selection always results in evolution if the four conditions are met. Natural selection is a specific cause of change, whereas evolution is the general phenomenon of change itself.
Other Influences on Evolutionary Trajectories
Evolution is not solely driven by natural selection; other non-selective forces also contribute to the change in allele frequencies over time. These forces include genetic drift, gene flow, and mutation, all of which modify the genetic composition of a population.
Genetic Drift
Genetic drift involves random changes in allele frequencies due to chance events, particularly in small populations. This randomness means that traits can become more or less common irrespective of whether they benefit the organism.
Gene Flow
Gene flow involves the exchange of alleles between different populations through migration. When individuals move, they introduce or remove genetic variants, altering the gene pool of both populations. Gene flow tends to make populations more genetically similar, working against local differentiation.
Mutation
Mutation is the ultimate source of new genetic variation. Mutations are changes to the DNA sequence and are the raw material upon which all other evolutionary forces act. They ensure that new alleles are always being generated, providing the necessary diversity for evolutionary change.