system
Given the following post, respond as if you're commenting on Reddit:
human
Why do biologists distinguish between odd-toed and even-toed ungulates?

What makes this distinction useful? If it has to do with ancestry, why did these animals keep odd or even numbers of toes? Is there an evolutionary pressure to lose/gain toes in pairs?
gpt
It has to do with ancestry.  The odd-toed ungulates are a clade called Perissodactyla, while the even-toed ungulates are the clade Artiodactyla.  These two groups diverged in the Palaeocene, over 60 million years ago.

The perissodactyls have a stance that centers their weight on their middle (third) toe.  In equids (horse relatives) all four of the other toes were eventually lost, while in most other perissodactyls only digits 1 and 5 were lost.  Very early perissodactyls had 4 or 5 digits.

Artiodactyls have a stance that centers their weight on digits 3 and 4.  Many artiodactyls today still have digits 2 and 5.

So to answer your question, if the digit isn't being used for primary weight-bearing there's a possibility of it becoming vestigial or disappearing.  Since the animals' weight is more or less centered on the limb, this reduction of digits tends to happen from the outside (digit 1 for artiodactyls, or 1 and 5 for perissodactyls) and move inward.