Last week I expressed my qualms about allopatric speciation the biological species concept. I didn’t quite say it that way, but that’s really what I was getting at. When talking about the phylogenetic relationship of populations the level of species is just another systematic layer. Arguably its the only one that’s “natural” or “fundamental.” And that is because of the role of the biological species concept. Basically, that refers to the fact that two species are labeled as such when they can’t exchange genes through hybridization. As I have noted before this is very much an instrumental standard, and not an iron law. For whatever reason the new vogue among paleoanthropologists seems to be to label Neandertals a separate species (e.g., H. neanderthalensis). That doesn’t bother me, but, please do remember that there’s now a rather overwhelming amount of circumstantial evidence that ~2% of the ancestry of most modern humans, H. sapiens, derives from this population. Because of the deficiency of Neandertal ancestry in the X chromosome, a common indicator of hybrid breakdown, we can infer that the ~500,000 year old separation between the African root stock of sapiens and neanderthalensis, was not without consequences. The biological species concept is useful, but not as an iron law. Rather, it is a reference point at one end of the distribution of the viscosity of gene flow between two populations.
And this is one area where whole genome sequences seem to be resulting in changes in perceptions. A paper in PNAS emphasizes this for equids, primarily donkeys and zebras, Speciation with gene flow in equids despite extensive chromosomal plasticity.
Horses, asses, and zebras belong to a single genus, Equus, which emerged 4.0-4.5 Mya. Although the equine fossil record represents a textbook example of evolution, the succession of events that gave rise to the diversity of species existing today remains unclear. Here we present six genomes from each living species of asses and zebras. This completes the set of genomes available for all extant species in the genus, which was hitherto represented only by the horse and the domestic donkey. In addition, we used a museum specimen to characterize the genome of the quagga zebra, which was driven to extinction in the early 1900s. We scan the genomes for lineage-specific adaptations and identify 48 genes that have evolved under positive selection and are involved in olfaction, immune response, development, locomotion, and behavior. Our extensive genome dataset reveals a highly dynamic demographic history with synchronous expansions and collapses on different continents during the last 400 ky after major climatic events. We show that the earliest speciation occurred with gene flow in Northern America, and that the ancestor of present-day asses and zebras dispersed into the Old World 2.1-3.4 Mya. Strikingly, we also find evidence for gene flow involving three contemporary equine species despite chromosomal numbers varying from 16 pairs to 31 pairs. These findings challenge the claim that the accumulation of chromosomal rearrangements drive complete reproductive isolation, and promote equids as a fundamental model for understanding the interplay between chromosomal structure, gene flow, and, ultimately, speciation.
There’s a lot in this paper which I will elide. For example, like most large species during the Pleistocene it seems that we need to start rethinking the importance of meta-population dynamics. But interestingly the evidence of gene flow across lineages reinforces the result that chromosome number differences are no barrier to hybridization. This should be obvious in some ways with equids, Przewalski’s horse has a different number than the domestic horse! I wouldn’t be surprised if this pattern of gene flow, especially near the root of the emergence of a lineage which we define as a “species”, is not uncommon with mobile genuses which span the world island.
And going back to hominins and hominoids, the title is a reference to a 2006 paper, Genetic evidence for complex speciation of humans and chimpanzees. The argument in that paper was that there was evidence of hybridization events after the initial divergence. This is highly disputed, and I haven’t followed the literature closely. But after all that we’ve found out about hybridization between distant lineages within Homo since 2006 should we surprised that this would occur within Africa in the incipient Pan and Homo lineages?
Last month Aeon Magazine asked: Genes that leap from one species to another are more common than we thought. Does this shake up the tree of life? Perhaps. But it shouldn’t. The universe is far more wondrous than can be imagined in our small philosophy.
Comments are closed.