Natural Selection & Evolution

Evolution is the gradual change in the inherited characteristics of a population of organisms over many generations

Notice three things about that definition:

• It happens to populations, not to individual organisms. A single rabbit cannot evolve during its lifetime. The population it belongs to evolves over generations.
• It is gradual and takes many generations. For most species evolution is slow, taking hundreds or thousands of years to produce visible changes. For organisms with very short generation times (bacteria, viruses, insects) it can happen in months or years.
• The changes are inherited. Differences caused by environment (a sunburned skin, a leg muscle built up at the gym) do not pass to offspring and are not evolution. Only changes coded in the DNA can be inherited and accumulate over generations.

The mechanism by which evolution happens was worked out by Charles Darwin and Alfred Russel Wallace in the 1850s. They called it natural selection.