For sexually reproducing species, progeny result from the fertilization of an egg by a sperm. Organisms that can interbreed and produce fertile offspring are members of the same species. Organisms that can’t interbreed and produce fertile offspring are separate species. Case closed.
Asexual Reproduction Produces Clones
However, there a many organisms that miss out on all the fun and instead reproduce asexually. They accomplish reproduction the same way that the cells of the human body divide to make more cells, as a person grows, develops, or heals from an injury. A cell makes a copy of its genetic material and then splits in two, each new cell with a complete copy of the genome. This method of reproduction results in clones, daughter cells that are exactly the same as the parent cell that generated them.
So if asexually reproducing organisms are clones, how is it that there are different species of them; and how are distinct species defined in organisms that do not reproduce sexually?