[Boogieman]

“All sexually mature humans are quite capable of producing children with any human of the opposite sex, regardless of race.”

That’s not a valid test anymore though, in terms of modern taxonomy. It doesn’t matter if they are capable of producing offspring, if circumstances in nature prevent them from actually ever producing offspring, then scientists classify them as different species. So, for example, if there are 2 groups of chimps who live on opposite sides of a river, and they cannot cross the river, it doesn’t matter if they could produce offspring, they would still be considered separate species.

Humans obviously can traverse any of those kinds of obstacles, so I don’t think scientists would consider us separate species. However, 600 years ago, if they were using the current standards, humans in the Western hemisphere would have had to be considered a separate species from humans in the Eastern hemisphere.

[BroJoeK]

Boogieman: "...for example, if there are 2 groups of chimps who live on opposite sides of a river, and they cannot cross the river, it doesn’t matter if they could produce offspring, they would still be considered separate species."

Obviously some deep confusion on this whole subject...

No, not separate species.
There are only two species in the genus Pan -- Chimps & Bonobos.
Amongst Bonobos there are no sub-species, but Chimps have four sub-species, and here is where you get populations living on different sides of the big river belonging to different sub-species, even though they can and occasionally do interbreed.

Indeed, this whole idea of interbreeding is one, but only one, key factor in naming breeds, sub-species, species, orders, families, etc., etc.

The general rules go like this:

1. Breeds, varieties or races (in humans) of the same species eagerly interbreed and produce viable offspring.
   Examples include domestic dogs and, of course, humans.

2. Among sub-species of the same species we begin to see reluctance, but they still sometimes interbreed and produce viable offspring.
   The most recent example is the discovery of DNA data showing occasional interbreeding between modern humans and Neanderthals, now classified as a human sub-species.

3. Different species of the same genus do not normally interbreed in the wild, but can on rare occasion and also can be forced in captivity.
   Their offspring are normally viable.
   A good example here are Polar and Brown Bears which occasionally mate in the wild, producing fertile offspring dubbed "Grolar Bears".

4. Different genera of the same family do not interbreed in the wild and when forced produce infertile offspring.
   Indian & African Elephants fall into this category.
   Horses & donkeys do also, even though both are in the genus Equs.
   But donkeys belong to a separate sub-genus, Asinus, which recognizes their inability to produce fertile offspring with horses.

5. Different families of the same order cannot be forced to interbreed, at least so far as I've ever heard.

So, if you keep in mind that the degree of difficulty in interbreeding is a major factor in determining which creatures belong in the same breeds, sub-species, species, genera, families, etc., etc., you'll be right about it far more often than wrong.