Evolution: How It Works
Part of Hall of Human Origins.
Part of Hall of Human Origins.
Humans, and all forms of life, have evolved from these primitive ancestors. Over time, countless changes in the shape, size and function of various parts of the body gave rise to new forms of life—from ants to amoebas, hummingbirds to humans. These evolutionary changes have resulted from changes in DNA and the information it contains.
Variation is the source of all evolutionary change. Each human, for example, has a slightly different combination of thousands of different features, such as eye color, blood type and sensitivity to certain tastes. Some variants, or versions of a feature, may help an organism to survive. Others may be harmful. Still other variants are neutral—neither helpful nor harmful.
Mutations are random changes in DNA. Some mutations affect a visible feature of the individual, such as the size of a butterfly's wing spots. Others can create resistance to a virus, such as HIV. The same mutation can be harmful, helpful or neutral, depending on the environment.
DNA changes when chromosomes shuffle, or recombine. This happens normally in the process of making a sperm or an egg. As a result, every new child or offspring inherits a unique assortment of DNA from each parent.
The fate of any trait in evolution depends on survival. In the process of natural selection, organisms with an advantageous trait survive and reproduce, passing that trait on to future generations. Those with a harmful trait may die before they reproduce. Which traits are advantageous? It depends on the environment.
When a trait is completely neutral—neither helpful nor harmful—an unpredictable process called drift causes random changes in the frequency of a trait over many generations. If wing spots were neutral, both butterfly types would be equally likely to get eaten. But eventually, one type would become more common than the other, simply by chance.
Drift explains many subtle differences between people of different ancestry. For thousands of years, the Basques lived in relative isolation along the mountainous border between France and Spain. As a result of drift, Basques today have one of the highest rates of the Rh-negative blood type in the world.