Multiple human species?

A reader writes:

...stating that there are separate species of humans has implications that I doubt you intend to convey. For example, since I deal with living mammals, I would naturally assume that means that there were significant (biological, not geographical) reproductive barriers between the two groups.

And that would be wrong. Common, but wrong. The biological species concept does not require "significant (biological, not geographical) reproductive barriers." In Mayr's Systematics and the Origin of Species, we read

Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups.

What causes reproductive isolation? All sorts of mechanisms: giving the wrong mating call or doing the wrong mating dance, for example. Two populations might breed at different times or in different places. Individuals from two populations might actually interbreed but never produce offspring, or maybe the offspring is always sterile (like the mule).

But that's all theory. In reality, there are many obviously good species that can not only hybridize but produce fertile offspring. For example, no one would doubt that lions and tigers are separate species, but ligers and tigons are both fertile hybrid offspring. Likewise, zorses (zebra x horse) and zonkeys (zebra x donkey) are also fertile. In fact, a perusal of the hybridization literature gives the impression that hybridization and introgression (the introduction of foreign genes into a population by fertile hybrids) is the rule rather than the exception. Species apparently remain species because they just don't cross with other species. But what about plants? They can't choose which other plants to cross with. True enough, and that's probably why hybridization is rampant in plants, far more so than animals.

I could go on with many other examples of hybrids, but I think those illustrate my point. I should note that the idea of hard, biological barriers between species is an old one. It used to be thought that species were endowed with these barriers to prevent confusion. Some naturalists who were acquainted with hybridization drew the obvious conclusions: species must be much bigger than we think. In a 1787 letter to the Royal Society, John Hunter wrote,

The true distinction between different species of animals must ultimately, as appears to me, be gathered from their incapacity of propagating with each other an offspring capable again of continuing itself by subsequent propagations: thus the Horse and Ass beget a Mule capable of copulation, but incapable of begetting or producing offspring. ... [Discusses dog/wolf and dog/jackal hybrids.] ... Here then is absolute proof of the Jackal being a Dog; and it appears to me, that the Wolf is equally made out to be of the same species.

Needless to say, his claim did not catch on. Today, there are three species of jackal recognized, and none are considered subspecies of dogs.

So here's the question: Should I modify my discussion of the multiple good species of humans to accommodate people's misunderstandings of species? Or should I use this opportunity to teach people what species really are? I think you know which one I'll choose.

Feedback? Email me at toddcharleswood [at] gmail [dot] com.

Hunter. 1787. Observations Tending to Shew That the Wolf, Jackal, and Dog, are All of the Same Species. Philosophical Transactions of the Royal Society 77:253-266.

Liger photo: Wikipedia