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 publishedThe 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.

For biologists, categorizing living things is the key to understanding life on earth. They do this through taxonomy, the classification of living things within hierarchically-organized categories. One of their main goals is to identify new species, the most specific of the categories they use to group organisms.

All organisms can be classified within seven levels of taxonomic categories. The waspEncarsia arabica, for example, is part of the kingdom animalia, which includes all animals. This kingdom is divided into several phyla, such asMandibulata, which is made up of animals that have hard, segmented bodies and characteristic jaw bones. This phylum is further divided into several classes. One of these classes,Insecta, includes organisms with antennae and a pair of compound eyes. The wasp's other traits allow biologists to place it into increasingly refined categories, including the most specific level of classification: species.

To determine whether specimens belong to the same species, biologists look at their body form, their body chemistry, how they develop, and how they behave. In general, only organisms that can produce healthy offspring together are part of the same species. Many types of orchids, though they look similar, are unable to breed with one another, and therefore don't belong to the same species. Dogs, on the other hand, come in a huge variety of shapes and sizes. But since they can all mate with one another, they share the same species name.
If defining a species is difficult, it may be because life is continually—although very slowly—changing. Scientists' current classifications are like snapshots of a moving scene. But by studying the species that exist today, biologists can better understand how the living world came to be the way it is, and how it might be changing.