What exactly is a species? The question isn’t difficult if you avoid the margins. One can easily discern the difference between a grizzly bear, and us homo sapiens. But where does a species begin and end? That is a trickier question. The best definition by far is in the Oxford Dictionary of Science:
“According to the biological species concept, a species comprises a group of individuals that can usually breed among themselves and produce fertile offspring. Typically, a species consists of numerous local populations distributed over a geographical range. Within a species, groups of individuals become reproductively isolated due to geographical or behavioral factors, and over time may evolve different characteristics and form new and distinct species.”
However, this isn’t always the case. For instance, there is new evidence that indicates neanderthals and homo sapiens interbred, but there are enough genetic variation and anatomical differences for these to be two separate species.
Why should we even bother with the terminology if it is so fuzzy? As Dennis Venema of the Biologos Foundation has stated “All good concepts are fuzzy.” We can discern nature different population groups, and different levels of such, that we can classify. These are our concepts that we are applying to reality, which means that there is bound to be some difficulties in the process of application, but that is ok.
In asexual species there is much more difficulty applying this definition, but it does work (which is the key to any scientific theory or concept) for sexually reproducing organisms. It is important to understand that this is a conceptual classification that we place on these population groups, and not something inherent within the genetic coding. Since there is no fixity to the genes of a species, evolution can and does happen leading to all the diversity of life we see around us today.