Evolution is the change in the inherited characteristics of a population over successive generations. Charles Darwin proposed natural selection as the primary mechanism: organisms with traits better suited to their environment tend to survive longer and produce more offspring, passing those advantageous traits to the next generation. Over many generations, this process shifts the genetic composition of a population.
Natural selection operates on four conditions: variation exists within a population, some variations are heritable, organisms produce more offspring than the environment can support, and individuals with favorable traits have higher survival and reproductive success (fitness). For example, beetles with coloring that blends into their environment are less likely to be eaten by birds, so they survive to reproduce more frequently.
Adaptation is the process by which populations become better suited to their habitats over time. Structural adaptations include the thickness of a polar bear's fur or the streamlined shape of a dolphin. Behavioral adaptations include bird migration and nocturnal hunting. Physiological adaptations include the ability of desert animals to conserve water. These traits arise through mutation and are refined by natural selection.
Speciation occurs when populations of the same species become so genetically different that they can no longer interbreed. Allopatric speciation happens when a physical barrier (like a mountain range or ocean) separates populations. Sympatric speciation occurs without physical separation, often through polyploidy in plants or behavioral isolation in animals. The Galapagos finches Darwin observed are a classic example of adaptive radiation — one ancestral species diversifying into many species adapted to different ecological niches.
Evidence for evolution comes from multiple independent sources. The fossil record shows transitional forms (like Tiktaalik, which bridges fish and tetrapods). Comparative anatomy reveals homologous structures — similar bone patterns in different species' limbs (human arm, whale flipper, bat wing) suggesting common ancestry. DNA comparisons show that humans and chimpanzees share approximately 98.7% of their DNA. Biogeography shows that closely related species tend to be found near each other geographically, and island species often resemble mainland relatives.