Monday, May 24, 2010

Human species

My post on species concepts and humans generated even more feedback. One reader asks:
I assume you are fully aware that experts in a given field often disagree on species designations and that politics can strongly influence whether or not something is classified as a separate species.
Were you going to talk some more about what species really are, or have you already done that in the past? You said that see no evidence of speciation in extant humans ... what is the criteria for speciation that you are looking for?
I will be interested in hearing you elucidate your take on the definition of species and, more specifically, why you believe Neaderthals fit these criteria.
I had not planned to go into greater detail on species, mostly because of the disagreements over species concepts or identifications I view as much less important than baramins. I am well aware of the "lumpers" and "splitters" and the many, many competing species concepts. I just don't think they're all that important, at least not when compared to the question of baramins. So in some sense, I really don't care whether Neandertals are a separate species or just a subspecies of Homo sapiens. On the other hand, I do think there's reasonable evidence to favor "splitting" Neandertals into a separate species rather than "lumping" them with sapiens. I warn you in advance, though, that I haven't given this a lot of systematic thought. I'm mostly shooting from the hip here.

If we go with the biological species concept, as articulated by many biologists long before Ernst Mayr's version, we have to look at evidence of interbreeding between populations. I take the reality of interbreeding as much more important than the potential for interbreeding. The fact that grizzlies and polar bears can interbreed I consider far less important than the fact that they usually don't interbreed. Add to that the observation of diagnosable differences between grizzlies and polar bears (like the anatomy and coloration), which implies they have not interbred (at least extensively) in the past. Therefore, I see them as distinct species.

In the case of the Neandertals, we have morphological differences that suggest separation into a different species. Evidence also supports a different developmental pathway from modern humans (for example, see Thompson and Nelson 2000; Ponce de León and Zollikofer 2001). With modern humans, development is all pretty much the same, and the morphological differences are quite superficial. I don't think that's quite enough to settle the question of species or subspecies, though.

The real clincher for me is the genetics. Here's a quick neighbor-joining tree of the complete mitochondrial genomes from 106 diverse Homo sapiens (in blue), Neandertals (in red), and the newly-described Denisova sequence:


In this tree, the branch length is proportional to the number of differences. You'll notice that most of the modern human branch of the tree is really crowded. It's hard to distinguish any sort of topology or branch order. The individual Neanderthals are all quite similar as well, but they are very different from modern humans. In that set of sequences, I counted 99 fixed differences where the Neandertals all had one nucleotide but modern humans all had the same different nucleotide at the same position (for example, Neandertals all had a G while modern humans all had an A). The neat separation between Homo sapiens and Neandertals on this tree tells me that they did not interbreed very much. That is corroborated by the Neandertal genome, which suggests that interbreeding led to at most 4% of modern nonAfrican human DNA being of Neandertal origin.

So we have a morphologically different form of Homo (Neandertal) with a different development and evidence of low interbreeding. That's a good species. That's not the case with modern humans, which are genetically very similar, morphologically uniform, and can and do freely interbreed. Modern humans are all one species.

Ultimately, though, I don't think it matters that much. If you want to keep calling Neandertals a subspecies, knock yourself out. That's what NCBI does. I'm certain that that won't work for erectus, habilis, or sediba, though. In those cases, the morphological differences are far more pronounced, and I suspect that the genetic evidence (if there ever is any) will support their classification as separate, good species even more than the Neandertals.

So again, what's the big deal? Who cares? One reader suggests that the problem is lingering belief in species fixity:
I think this is part of what brings up the association between racism and species of humans - if God created all the species separately, then obviously if there are different species of humans then they must be different from the Adam and Eve species of humans.
I've argued before that belief in species fixity persists in creationism, partly because of inconsistent presentation to the public. Perhaps there is an equation of species with special creation in the minds of many creationists, thus leading to a negative backlash against the very idea of separate species of humans.

All the more reason to consistently teach them that species generally aren't special creations; baramins are.

Thompson and Nelson. 2000. The place of Neandertals in the evolution of hominid patterns of growth and development. J. Human Evol. 38:475-495. [PDF]

Ponce de León and Zollikofer. 2001. Neanderthal cranial ontogeny and its implications for late hominid diversity. Nature 412:534-538.


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