As man is, the dog shall be

Screenshot from 2015-08-26 20:34:38
51LNowRMg9L._SY344_BO1,204,203,200_When is a jackal a wolf? All the time apparently. At least according to a persuasive new paper, Genome-wide Evidence Reveals that African and Eurasian Golden Jackals Are Distinct Species.

First, let’s put this in context. Canids area big deal. They’re big social mammals whose distribution and speciose character have undergone big changes across the Pleistocene. Sound familiar? Is it any surprise that one of their kind is our “best friend.” And, according to the anthropologist Pat Shipman the symbiotic relationship between dog and man is responsible for the victory of our lineage of hominins in the evolutionary war of all against all. About six months ago that thesis would seem a stretch, as the origin of dogs does not date until almost the Holocene according most genetic scholarship (the paleontologists have found rather old suggestive skulls thought). So tens of thousands of years after modern humans replaced other lineages. But ancient DNA suggests problems with the calibration of earlier work, which may have dated their divergence from wolves too recently. That and the fact that the emergence of dogs as a distinct group of canids might be concurrent with the arrival of modern humans to Eurasia make Shipman’s thesis at least feasible, if not probable. And note that I stated divergence from wolves, not derivation. It turns out that dogs are a sister lineage to Palearctic wolves, not derived from them. As observed in this paper extant lineages of wolves are genetically rather homogeneous, and seem to have diversified relatively recently, within the last 20,000 years, on the order of 10 to 20 thousand years after the last common ancestor of extant wolves and dogs.

Screenshot from 2015-08-26 20:44:18Where do jackals play into this? The golden jackal has a distribution which covers both Eurasia and Africa. The species’ was determined morphologically. In other words, they look similar across their range. But sometimes you can’t judge a book by its cover. As an obvious example, most people would think that a hyrax on superficial inspection was a rodent. But a close examination of anatomical details indicated a relationship to elephants to classical taxonomists, which has been validated by DNA. But, as the paper above states plainly in the title the DNA here contradicts inferences made from morphology. Wolves and dogs, and African golden jackals, form a monophyletic lineage, to which Eurasian golden jackals are an outgroup! This determination was achieved through mtDNA analyses, as well as phylogenetic reconstruction from specific genetic regions, and, genome-wide comparisons on millions of polymorphisms.

But wait there’s more! One major difference between the example above of the hyrax vs. elephant and jackal vs. wolf is that the phylogenetic distance in the latter case is far smaller across the tips of the branch. That probably explains why morphological characters were not sufficient to discern the shared ancestry and derived characteristics of the wolf and the African jackal, as opposed to the Eurasian jackal. And, a corollary to this is that hybridization between these lineages is possible. In other words, this isn’t a phylogenetic tree, it’s a phylogenetic graph! Using D-statistics the authors show that there has been a fair amount of gene flow between Eurasian wolves and Eurasian jackals. And, in particular a lot of admixture from the Eurasian jackal to the dingo and basenji breeds.

Is this starting to sound a bit familiar? As population genomics has increased coverage of human populations, modern and ancient, as well as increasing marker density and accuracy, first approximation coarse phylogenetic trees have given way to threads of gene flow edges tracing their away across the thick branches. The trees have given ways to myriad graphs which force us to make more subtle our understanding of the genetic background of our own lineage. I see no reason why the same will not be true for large mammals, or, frankly, an innumerable number of clades.

In the near feature sequencing will be ubiquitous in ecological and systematic studies. At the coarsest big picture scale we’ll still see a confirmation of the tree of life as it’s classically envisioned, exploding outward from node to node, in subdivisions of clean monophyletic lineages, pruned by extinction diversified by drift and selection. But as you focus in closely the bifurcations will turn in on themselves or thread together in tangle, as the branches begin to be stitched together by gene flow. Look even closer and you’ll see that even within a young species, like humans, our local geographic pedigrees also collapse in on themselves, and tangle and coalesce down to a set finite number of individuals, rather than the infinite space of genealogical possibilities.

Posted in Uncategorized

Comments are closed.