Evolution

Evolution is the change that occurs in the inherited traits of a population of living organisms over successive generations. These traits are carried by genes, which means that change in a population is measured across the group, not in individual organisms alone. Individual organisms grow and change during their lifetime, but evolution refers to cases where the frequency of a particular genetic variant changes within the population.

The idea of evolution is the foundation of modern biology. It explains how different species of organisms descended from common ancestors, and why different species share similar structures and genetic material. In the nineteenth century, Charles Darwin and Alfred Russel Wallace presented explanations based on natural selection; later, the science of genetics added support and reinforcement to their work.

Sources of Variation

Change cannot occur if there is no genetic variation to work on. The first source is mutation, which is an error in DNA replication. Most mutations have no observable effect, some are harmful, and others provide the organism with an advantage. The second source is genetic recombination, which results from sexual reproduction and shuffles the genes inherited from both parents.

Natural Selection

Natural selection is the most important mechanism of evolution. When organisms of the same species differ in inherited traits, those whose traits aid survival and reproduction leave more offspring. As a result, genes conferring advantage increase in frequency within the population over generations. Selection is not a choice; it is a statistical outcome arising from differences in reproduction.

Other Mechanisms

Selection is not the only force. Genetic drift is random change in gene frequency, particularly powerful in small populations. Gene flow is the movement of genes when organisms migrate to another population and interbreed, reducing differences between populations. The interplay of these mechanisms determines the direction of evolution.

Speciation

When two populations of the same species become separated—by geography, distance, or behavioral change—their genes evolve along different paths. After sufficient time, they may no longer be able to interbreed; then the two are said to be different species. This process (speciation) explains the diversity of life on Earth.

Evidence

Evidence for evolution comes from many sources: fossils showing how organisms have changed over time, comparison of body structure across different species, and comparison of DNA sequences between organisms. Observable change in living organisms, such as bacteria developing resistance to antibiotics, provides a testable example.