Drawing a line that nature blurs
What Makes Two Animals the Same Kind?
A great dane and a chihuahua are one species. Two identical-looking birds can be two. The test is not what they look like.
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Step 1 of 5
Looks are a poor guide.
The amount of difference tells you nothing about the answer Dogs vary more in size and shape than many separate species do, and yet they are one. Meanwhile some near-identical birds never interbreed at all.
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Step 2 of 5
The usual test is breeding.
The common rule is that two animals are one species if they can have young together and those young can have young of their own. That last part matters.
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Step 3 of 5
Some crosses stop one generation short.
The test is not whether two can breed. It is whether their young can. A horse and a donkey have a mule, and almost every mule is infertile. The line comes back at the next generation, which is why they stay two species.
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Step 4 of 5
The rule breaks in plenty of places.
It says nothing about animals that never breed sexually, or about fossils, or about populations that could interbreed but live too far apart to try.
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Step 5 of 5
Nature does not draw sharp lines.
Species blur into one another because they are still splitting. A rule that works most of the time is the honest most we can ask of it.
Nobody knows how many species exist. Around two million have been named and most estimates of the real number are several times that.
The short version
Two animals are the same species if they can breed and their young can breed too, though nature keeps blurring the line.
Try it yourself
Look at pictures of a wolf and a husky. Almost the same animal — and they are, near enough, one species.