Species

A species is a group of living things that are closely related and can have babies together. Dogs are one species. House cats are another. Humans are a third. Every living thing on Earth, from the biggest whale to the tiniest bacteria, belongs to some species. Scientists have named about 2 million of them. Many more have not been found yet.

The usual rule for what counts as a species is simple. If two animals can mate and make babies that can also have babies of their own, they belong to the same species. A poodle and a labrador can have puppies, and those puppies grow up and have more puppies. So both dogs belong to one species. A horse and a donkey can have a baby, called a mule. But mules almost never have babies of their own. So horses and donkeys are two different species.

Every species has a two-part scientific name in Latin. Humans are Homo sapiens. House cats are Felis catus. Tigers are Panthera tigris. This naming system was invented in the 1700s by a Swedish scientist named Carl Linnaeus. The same system is still used all over the world today. It lets a scientist in Japan and a scientist in Brazil know they are talking about the exact same animal.

New species form over long periods of time. When one group of animals gets cut off from the rest, maybe by a new river or a new mountain, the two groups change in different ways. After thousands or millions of years, they may become so different that they can no longer mate. Charles Darwin saw this on the Galápagos Islands. Finches on different islands had grown different beaks to eat different foods. They had slowly become separate species.

The simple rule has tricky edges. Some species cannot breed but look almost the same under a microscope. Bacteria do not mate at all; they just split in two. Scientists also argue about groups like wolves, coyotes, and dogs, which can sometimes breed with each other. Some biologists say the word "species" is more of a useful tool than a perfect line in nature.

Species are disappearing faster than new ones are forming. When the last member of a species dies, that species is extinct and gone forever. About 900 species are known to have gone extinct in the last 500 years. Many more are in danger right now.