What are species?

When looking at different animals, it’s pretty easy to tell the difference between a dog and a cow, between a bird and a fish.

That’s because these animals are different species: a set of organisms that can interbreed and produce living and fertile offspring. This is called the biological species concept, separating animals based on whether they can possibly interbreed.

The ability to make fertile offspring is an important quality. For example, donkeys and horses can interbreed to make a baby mule. However, mules are infertile, which means donkeys and horses are separate species.

There’s also another way to define species, called the phylogenetic species concept. This concept separates species that could interbreed, but don’t because they live so far apart. If they live far apart for long enough, then they could eventually change too much to interbreed and become different biological species as well.

The definition of species is a difficult concept to define; these are two commonly agreed upon definitions, but the exact separations are still debated. Species classification isn’t a natural phenomenon, but rather one used by biologists to help them talk about the plants and animals that they study.

Centuries ago, scientists used the physical appearance or anatomy of the organism to figure out its species. Does it have hair, fur, feathers or scales? How many legs does it have? Does it live in oceans, lakes or on land? If it’s a plant, how many leaves or flowers does it have?

Appearances can be deceiving, though. For example, one species can also have multiple breeds, like dogs. All domesticated dogs are considered the same species because they can make crossbreeds, but their widely varied appearance makes it easier to talk about separate breeds.

Today, scientists have more advanced technology. Researchers use the DNA of the organism to determine its species; the more similar DNA is to another animal, the more likely it is that they belong to the same species.

Learn more about how different species evolved

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