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June 23, 2005

Breakin' free of biology?

On page 73 of Speciation the authors offer:

...Wilson et al. are probably correct in their main conclusion: although some distantly related species of birds can produce viable hybrids despite more than 15 millions of divergence (Price and Bouvier 2002), it is absurd to suppose that equally old mammalian species (e.g., humans vs. gibbons), could yield the same result....

This passage jumped out at me for the following reason, they reiterate many times that many of the contentions or truisms relating to speciation are grounded more in intuition than data. To correct this their book is chock full of references, some of them quite old (Orr and Coyne seem positively Gouldian in their quest for ancient references, I am somewhat shocked to see dates like 1912 in a work that isn't primarily historical more than once or twice). In the section above the authors refer to studies which do suggest that mammals might develop hybrid inviability faster than other taxa, though with caution. But to me their dismissal of the human-gibbon hybrid is too flippant in light of the rather qualified and provisional tone that suffuses their text in general.

Do I think ape-human hybrids would be viable? I wouldn't bet on it. But I don't have anything more to go on than intuition. Coyne and Orr, who are giants in their field, have more to draw on to base intuitions than I, but they are generally very careful not to pull rank in this way from what I can tell, and in contrast to other assertions in the book I do not see a citation to support their use of the term "absurd."

I am dwelling on this because Speciation hits another topic that I find interesting, the reality of the Biological Species Concept and the concordance between "professionals" and indigenous peoples in species categorizations, on the other of 70-80%. Orr and Coyne use this finding to suggest that species are objective across human cultures (though higher order taxa are not). This is not surprising, I tend to lean toward the idea that humans have some sort of innate biased folk biology that transcends culture, and likely has biological antecedants. Another contention of this paradigm is that humans also tend to also fixate on our own species, our own group/tribe, as a special kind in and of itself (even though we are formed of the same molecules as other species, to ourselves we have an ontological significance).

Biologists, being human, are subject to the same bias. I will go out on a limb and suggest that Coyne and Orr might find the idea of a viable human-ape hybrid absurd because it is instinctively abominable. In a similar vein my personal impression is that the idea of human-animal chimeras are very disturbing to people, imagine if you will if someone engineered a dog with the face of a man? To some extent our attitudes toward the great apes prefigure these modern discomforts, we see in them a "warped" reflection of ourselves. From a cognitive perspective the chimpanzee might be triggering mental faculties that respond to animal and human input cues, resulting in a mixed cascade of inferences.

But this "mental block" may have more than just ethical and aesthetic consequences, it might have skewed the progress of science in paleoanthropology. Over the past few months Greg Cochran has brought up the issue of "2s" a few times, and I have begun to entertain it seriously. Reading Coyne and Orr's review of the literature on debates about the various species concepts, with extreme partisans of the species-are-social-constructs coming out of botanical fields where hybridization is ubiquitous (or so they claim), I am in hindsight shocked at the peculiar duality of Out-of-Africa-Alone and Multi-regionalism. In light of the literature reviewed in Speciation both models seem somewhat extreme, but they were offered as the only viable and fleshed out alternatives for two generations. Granted, there were regular admissions by many of possible hybridization events, in The Third Chimpanzee Jared Diamond offered in an aside that it was possible that hybridization occurred in Eastern Asia (an easy speculation since the fossil record for "archaic" H. sapiens is so scanty). But these musings never really made it very far and were stray thoughts that seemed to never congeal into a model. Anagenetic Multi-regionalism and Out-of-Africa-Alone might simply have been geared toward our cognitive "sweet spot," we are after all a very special species, a One-of-a-Kind. Either we came out of Africa in one fell sweep, all descendents of a mitochondrial Eve, or we were always one worldwide species slowly hurtling simultaneously toward our inevitable sapiency.

Addendum: Orr and Coyne express repeated caution about inferring species phylogenies from only a few loci, and make the important point that hybridization events can result in homogenization on some loci while selection maintains differentiation on others. For example in many plant species there seems to be a number if signatures of hybridization events in the mtDNA, but the autosomal genome remains differentiated.

Update: John's post reviewing baboon hybridization is relevant....

Posted by razib at 02:07 AM