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Species and the mule

The biological species concept says that individuals belong to the same species when they can have fertile offspring together. A horse and a donkey can mate, and the offspring is a mule, but the mule is sterile. Horse and donkey are therefore counted as two species. The reason lies in the chromosomes. The horse has 64 and the donkey 62. A mule gets one set from each, and the two sets pair poorly in meiosis. Without an even pairing there are no functional gametes. Species can also arise when populations are separated long enough that they no longer reproduce together.

32 + 31 = 63

chromosomes in a mule

Symbols

$horse$	64 chromosomes
$donkey$	62 chromosomes

Example

The horse contributes $64/2 = 32$ and the donkey $62/2 = 31$ .

A mule: $32 + 31 = 63$ chromosomes, and it is sterile.

That two animals look alike is not enough for them to be the same species. Look at the offspring.

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Part of Biology 1: Evolution.