system
Given the following post, respond as if you're commenting on Reddit:
human
With an estimated 1,000,000 nematode species, what distinguishes them all?

I was on a Wikipedia safari, and read that there are 25,000 known nematode species, and 1,000,000 estimated total species.

I'm wondering what could possibly distinguish between 1,000,000 variations of a little worm.  Could someone explain the minimal distinguishing characteristics of a species of nematode?
gpt
Many of the distinctions are on the genetic level.  Two worms may look the same to the unaided eye and even live in the same habitat, but be vastly different genetically and totally unable to crossbreed.