You know what a species is, right?
Duh, yeah, of course you do!
OK, let's put it into words. A species is: a... uh... group of animals, er, I mean organisms, that can interbreed and make fertile babies.
So, this definition of species is actually one of many.
Yep. One of many! Is your mind blown yet?
It is true that many organisms pick out others "of their own kind" to mate with. We see this all around us; we hear frogs calling, birds chirping, and crickets... uh... cricketing. This "selection" for a mate is done by either the male or the female and they make sure their potential mate has certain attributes or characteristics. Some of these attributes help identify those of the same species; for example, a certain call or smell. However, some species can't select a mate because they do not have males and females!
Ever heard of an asexual? An asexual ('a' = without; 'sexual' = gender) can be thought of an individual without gender, or it can be thought of as one that just does not need a mate to reproduce!
Perhaps you've heard of asexual-ness and "virgin births" from tabloids or other reading material. It's not just a story, it can actually happen. These "virgin births" as a result of parthenogenesis ('parthenos' = virgin; 'genesis' = creation or genesis). Believe it or not, this process occurs in fish, lizards, and of course the creepier crawlies such as daphnia ("water fleas"). The process of parthenogenesis can be divided into further types, but to keep it simple, here is daphnia's parthenogenic process:
With our definition of species above, are parthenogenic organisms actually "species"?
They violate the definition after all!
It turns out that the definition I gave is one under the "biological species concept". This is basically what we, the entirety of the United States (and maybe the world), uses to describe species. There are other species concepts that I can go over later but this gives you a good idea of just how complex describing a species is! We straight up ignore the fact that daphnia, and other pathenogenic organisms, don't follow the rules. Did you even know about this? Pretty crazy!