system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: considering that some animals, plants, and fungi have tens of thousands of species in a family who can look very similar, how do you know you've discovered a new species and it isn't just intraspecific variation?

I've been reading about people in the modern day discovering new species, with some like fungi growing in raised beds and parking lots for decades, but no one identifying that they're actually an undiscovered species. 

The same with ants, more than 10,000 have been identified, but there is an estimated 22,000 species. Let's say someone stumbles upon a new species of ant, number 10,001. How would they actually verify that it's a new species and not species #6841 which looks the exact same?
gpt
These days we have relatively fast and cheap genetic analysis tools to compare these species on a genetic level rather than trying to squint real hard at their mandibles to try and note physical differences.

This has had a pretty significant impact on the classification of species and subspecies, and how they’re placed on evolutionary family trees.  Lots of populations have been re-classed as unique species, and some animals that were thought to be close relatives have been moved further apart on the tree.

Actually defining a unique species when you have two adjacent populations that are genetically different but still occasionally interbreed is more complicated.  Nature doesn’t always care about your attempts to classify things neatly.