A group of closely related organisms that are very similar to each other and are usually capable of interbreeding and producing fertile offspring.
Basically if you’re similar enough and can produce fertile offspring you’re considered the same species. This is why one a single mutation is unlikely to produce a new species. If the mutation has such great consequences that it produces new species by definition it cannot have fertile offspring with anyone else. And yes, it says ‘usually’, there are exceptions but I think this is how it works in general.
It’s a important part of the definition:
species (noun):
A group of closely related organisms that are very similar to each other and are usually capable of interbreeding and producing fertile offspring.
Basically if you’re similar enough and can produce fertile offspring you’re considered the same species. This is why one a single mutation is unlikely to produce a new species. If the mutation has such great consequences that it produces new species by definition it cannot have fertile offspring with anyone else. And yes, it says ‘usually’, there are exceptions but I think this is how it works in general.