Evolution is the fundamental process by which life changes over time, defined as descent with modification. This biological change occurs across successive generations, leading to the diversity of all living things. Understanding evolution requires identifying the precise biological level where this continuous process of modification can take place. The minimum requirement for evolution is a unit that can possess and pass on heritable variation that changes in frequency over time. While the answer might seem to be the individual organism or the gene itself, the smallest unit that can actually evolve is a collective—the population.
The Smallest Evolving Unit is the Population
A biological population is a localized group of individuals belonging to the same species that can interbreed. Evolution is a change in the frequency of alleles—different versions of a gene—within this group from one generation to the next. This change in allele frequency is termed microevolution. The population is the only biological unit small enough to be affected by the immediate pressures of the environment while still being large enough to contain the necessary genetic variation. The collective gene pool of the population is what allows for the dynamics of selection and change to occur over time.
Why Individuals Do Not Evolve
An individual organism cannot evolve during its own lifetime. The genetic makeup, or genotype, of an individual is fixed at conception. Any changes an organism experiences during its life, such as gaining muscle or developing a scar, are changes to its body (somatic changes) and are not heritable. Evolution requires that changes be passed down to the next generation through reproductive cells. While an individual might be selected for or against by the environment, only their reproductive success or failure contributes to the evolution of the population.
The Raw Material of Evolutionary Change
For a population to evolve, there must be variation in heritable traits among its members. This genetic variation is the raw material upon which all evolutionary forces act. New alleles, the ultimate source of this variation, are introduced into the population primarily through random mutation in the DNA sequence. These mutations can occur spontaneously and introduce novel characteristics that might be beneficial, neutral, or harmful. In sexually reproducing organisms, the process of meiosis further shuffles these existing alleles through recombination, creating unique combinations in every offspring.
Forces That Drive Population Evolution
Four primary mechanisms constantly interact to change the allele frequencies within a population, collectively driving the process of microevolution:
- Natural selection is the non-random process where individuals with traits better suited to the environment survive and reproduce at higher rates than others. Over generations, this differential success causes advantageous alleles to become more common in the population’s gene pool.
- Genetic drift is a random phenomenon that describes how allele frequencies can fluctuate unpredictably from one generation to the next, particularly in small populations. Events like the sudden death of a few individuals can remove certain alleles from the gene pool entirely.
- Gene flow involves the transfer of alleles between populations, typically through the migration of individuals or gametes. This mechanism tends to make different populations of the same species more genetically similar to one another.
- Mutation itself, while the source of new alleles, is a slow but steady force that continuously introduces new genetic information into the population, providing the fuel for the other three forces.