Scientists estimate that there are between three million and thirty million species of living things. Where did this staggering diversity come from?

The answer lies in natural selection, the gradual adaptation of plants and animals to their environments.

Natural selection happens because some individuals within a species have better chances for survival than others. This diversity is the result of mutations, random changes in an organism's DNA that can give one individual advantages over another. These better-equipped organisms produce the most offspring, which inherit the traits that allowed their parents to survive, and the pass on those traits to their own offspring.

Over time, more and more individuals with these favorable traits survive, while the traits that might put them at a disadvantage are slowly weeded out of the gene pool.

At the same time, environmental pressures—from a change in climate to the loss of a food source—can offer more specific challenges. As local populations adapt to the areas in which they live, groups may split. When two groups can no longer reproduce with one another, they become separate species.

This is the process that drives evolution, shaping entire species, rather than individual organisms, over very long periods of time.

Darwin’s Finches
Naturalist Charles Darwin (1809–1882) first noticed the evidence for natural selection while visiting the Galapagos Islands in 1835. On these isolated islands, Darwin found finches that resembled those living on the South American continent, some 1,300 kilometers away. But the Galapagos finches, he realized, showed a range of beak sizes that corresponded to the food sources available where they lived.

Darwin concluded that these birds originated from a single species that migrated from the mainland millions of years ago. Since birds faced distinct challenges depending on where they settled, finches with different traits survived in different locations. Through the process of natural selection, the bird populations eventually split into many species, which still retaining common characteristics.

With little more than a notebook and his own powers of observation, Darwin collected a host of other species, and in 1859 published The Origin of Species , which set out the theory of evolution. Today, biologists still use many of the same simple techniques that led Darwin to his revolutionary conclusions.