The definition, however, doesn't give you a basis for taxonomic ranking. At what level do I chose to rank these populations? I'll give you more examples, because there are problems with this definition.

Even though this thought to be a widely used definition, it is in fact not the most widely used. That is, it may be widely used by the people who promote it, mainly vertebrate zoologists, and in the past, a few, very few, plant systemitists. But most entomologists, most plant people, most of the people who are dealing with all the little creepy crawly things of the world, which is 95 percent of biodiversity don't use the biological species concept.

The biological species concept makes no mention of what we might call the units that are produced by evolutionary processes - what we call evolutionary units. Thus it does not take those isolated populations that are really different from one another and keep them as separate species, but rather it unites them because there is an assumption that they are so similar to one another that they will interbreed if their isolation were to breakdown. So if one takes a group of populations isolated along a mountain range, and they all look similar to one another but yet one can tell from which range they come because they have some characteristics that diagnose them - many times in the past systematists would say "well, I think that if they came back together again, if those populations came back together again, they would interbreed. Therefore, I'm going to call them the same biological species."

Because from one group of organisms to another systematists have different criteria for judging whether populations that are separated from one another might or might not interbreed, you get a broad disparity of how people treat species concepts in these groups. Therefore, species are not equivalent from group to group. As I will show you, you can't reconstruct the historical relationship of groups if you follow this kind of biological species concept and you can't enumerate them without bias. I'm going to give you plenty of real world examples in a minute so some of this will become clearer. The bottom line: you can't recover how species have evolved through space and through time with this concept.

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