Genetics has numerous uses. There are some biologists for whom genetics implies very specific chemical and physical properties of a particular flavor of DNA molecule. Consider a scientist focused on the biophysical properties of zinc finger proteins and the ZYF gene. Then there are biologists for whom genetics is a more abstract and evolutionary enterprise. David Haig and the late W. D. Hamilton fall into this class of thinkers. This is a way of looking at genetics as the scaffold or currency of evolutionary process. Finally, there are those for whom genes are simply discrete convenient markers to trace out historical and spatial patterns. The field of molecular ecology describes this attitude, though the application of phylogenetic techniques from the life sciences in linguistics illustrates the generality of these methodologies.
Last month I noted that a paper on speculative inferences as to the phylogenetic origins of Australian Aborigines was hampered in its force of conclusions by the fact that the authors didn’t release the data to the public (more accurately, peers). There are likely political reasons for this in regards to Australian Aborigine data sets, so I don’t begrudge them this (Well, at least too much. I’d probably accept the result more myself if I could test drive the data set, but I doubt they could control the fact that the data had to be private). This is why when a new paper on a novel phylogenetic inference comes out I immediately control-f to see if they released their data. In regards to genome-wide association studies on medical population panels I can somewhat understand the need for closed data (even though anonymization obviates much of this), but I don’t see this rationale as relevant at all for phylogenetic data (if concerned one can remove particular functional SNPs).
To understand nature in all its complexity we have to cut down the riotous variety down to size. For ease of comprehension we formalize with math, verbalize with analogies, and visualize with representations. These approximations of reality are not reality, but when we look through the glass darkly they give us filaments of essential insight. Dalton’s model of the atom is false in important details (e.g., fundamental particles turn out to be divisible into quarks), but it still has conceptual utility.
Likewise, the phylogenetic trees popularized by L. L. Cavalli-Sforza in The History and Geography of Human Genes are still useful in understanding the shape of the human demographic past. But it seems that the bifurcating model of the tree must now be strongly tinted by the shades of reticulation. In a stylized sense inter-specific phylogenies, which assume the approximate truth of the biological species concept (i.e., little gene flow across lineages), mislead us when we think of the phylogeny of species on the microevolutionary scale of population genetics. On an intra-specific scale gene flow is not just a nuisance parameter in the model, it is an essential phenomenon which must be accommodated into the framework.
A new short communication in Scientific Reports suggests that most demographic expansion as ascertained using mtDNA occurred before the Neolithic. MtDNA analysis of global populations support that major population expansions began before Neolithic Time:
Agriculture resulted in extensive population growths and human activities. However, whether major human expansions started after Neolithic Time still remained controversial. With the benefit of 1000 Genome Project, we were able to analyze a total of 910 samples from 11 populations in Africa, Europe and Americas. From these random samples, we identified the expansion lineages and reconstructed the historical demographic variations. In all the three continents, we found that most major lineage expansions (11 out of 15 star lineages in Africa, all autochthonous lineages in Europe and America) coalesced before the first appearance of agriculture. Furthermore, major population expansions were estimated after Last Glacial Maximum but before Neolithic Time, also corresponding to the result of major lineage expansions. Considering results in current and previous study, global mtDNA evidence showed that rising temperature after Last Glacial Maximum offered amiable environments and might be the most important factor for prehistorical human expansions.
I put up kind of a ridiculous title. But I do hope that at some point in the near future we’ll have some of the same flavor of debates on the macroevolutionary time scale that we have on the human microevolutionary time scale. There’ll be a surfeit of sequence at nearly every node of interest on the tree of life, and computational power galore devoted to analyzing variation and reconstructing any phylogeny we can conceive of. To be fair, one could argue we aren’t there even with human phylogenetics either. But it is rather strange we’re debating the origin of mammals and the nature of the lineage’s phylogenetic tree at this time. This is the kind of thing that I hope a more robust and assertive molecular phylogenetics can resolve (and paleontology as well, but I’m not up on the latest in computational analysis of morphological characters).
ADMIXTURE and STRUCTURE tests aren’t formal mixture tests. Yes! In fact, in the “open science” community this issue is repeated over and over and over, because people routinely get confused (our audience does not consist of population geneticists and phylogeneticists by and large). So sometimes it is necessary to lay it out in detail as in the post above. The key point to always remember is that population genetic & phylogenetic statistics and visualizations are a reduction and summary of reality in human palatable form. They tell us something, but they do not tell us everything. A common issue is that for purposes of mental digestion it is useful to label ancestral elements “European,” or on PCA refer to a “European-Asian” cline, as if the population genetic abstractions themselves are the measure of what European or Asian is. But European and Asian are themselves human constructions, and subject to debate (e.g., do Turks count as Europeans? Indians as Asians?) The population genetic statistics are not themselves subjective, but the meanings we give them are.
The NJ tree is from Genome-Wide Analysis in Brazilian Xavante Indians Reveals Low Degree of Admixture. It’s a visualization of a genetic distance matrix. Am I strange, or do these sorts of trees really leave a lot to be desired in terms of actually getting across any extra information beyond a table?
A few years ago a paper came out which suggested that the brown bears of the ABC Islands of southeastern Alaska were more closely related to polar bears than they were to other brown bears. More precisely, polar bears and ABC brown bears formed a distinct clade set apart from other brown bears, so that the class “brown bear” was not monophyletic. This meant that all the descendants of the hypothetical ancestral lineage of brown bears are not brown bears. Like reptiles, brown bears may then be paraphyletic. If this is correct polar bears can be thought of as a derived and specialized lineage of brown bears, despite all their morphological differences.
This is not just systematic arcana. The phylogenetic relationships of species has important implications for their conservation status, something all the more salient due to changes in the arctic habitat of the polar bear.
But there is a catch with the science: it focuses on mitochondrial lineages. In other words, the matriline, the female line of descent. There are technical reasons for this, primarily having to do with the tractability of generating phylogenetic trees from nonrecombining data sets of mtDNA as well as the ease of extractions of this genetic material (it’s abundant). And, in the case of ancient DNA abundance is still critical.
Last week a new paper in Current Biology reexamined the phylogenetic relationships of polar bears and brown bears using ancient DNA samples. Unfortunately it resulted in some weird titles: ‘Polar bear’s ancestor is Irish brown bear, study finds’. We’re revisiting the problem of ‘mitochondrial Eve’ all over, conflating mtDNA lineages with the total history of the species (granted, the fine print of the journalism usually alludes to this detail, but the headlines do not).
Let’s look at the paper itself. Ancient Hybridization and an Irish Origin for the Modern Polar Bear Matriline:
Last summer I made a thoughtless and silly error in relation to a model of human population history when asked by a reader the question: “which population is most distantly related to Africans?” I contended that all non-African populations are equally distant. This is obviously wrong on the face of it if you look at any genetic distance measures. West Eurasians, even those without recent Sub-Saharan African admixture (e.g., North Europeans) are closer than East Eurasians, who are often closer than Oceanians and Amerindians. One explanation I offered is that these latter groups were subject to greater genetic drift through a series of population bottlenecks. In this framework the number of generations until the last common ancestor with Sub-Saharan Africans for all groups outside of Africa should be about the same, but due to evolutionary factors such as more extreme genetic drift or different selective pressures some non-African groups had diverged more from Africans than others in terms of their genetic state. In other words, the most genetically divergent groups in relation to Africans did not diverge any earlier, but simply diverged more rapidly.
Dienekes Pontikos disagreed with such a simple explanation. He argued that admixture or gene flow between Africans and non-African groups since the last common ancestor could explain the differences. I am now of the opinion that Dienekes may have been right. My own confidence in the “serial bottleneck” hypothesis as the primary explanation for the nature of relationships of the phylogenetic tree of human populations is shaky at best. Why my errors of inference?
There were two major issues at work in my misjudgments of the arc of the past and the topology of the present. In the latter instance I saw plenty of phylogenetic trees which illustrated clearly the variation in genetic distance from Africans for various non-African groups. Why didn’t I internalize those visual representations? It was I think the power of the “Out of Africa” (OoA) with replacement paradigm. Even by the summer of 2010 I had come to reject it in its strong form, due to the evidence of admixture with Neanderthals, and rumors of other events which were born out to be true with the publishing of the Denisovan results. But to a first approximation the clean and simple OoA was still looming so large in my mind that I made the incorrect inference, whereby all non-Africans are viewed simply as a branch of Africans without any particular differentiation in relation to their ancestral population. Secondarily, I also was still impacted by the idea that most of the genetic variation you see in the world around us has its roots tens of thousands of years ago. By this, I mean that the phylogeographic patterns of 25,000 years in the past would map on well to the phylogeographic patterns of the present. This assumption is what drove a lot of phylogeography in the early aughts, because the chain of causation could be reversed, and inferences about the past were made from patterns of the present. My own confidence in this model had already been perturbed when I made my errors, but it still held some sort of sway in my head implicitly I believe. It is one thing to move on from old models explicitly, but another thing to remove the furniture from your cognitive basement and attic.
I have moved further from my preconceptions between then and now. It took a while to sink in, but I’m getting there. A cognitive “paradigm shift” if you will. In particular I am more open to the idea of substantive back migration to Africa, as well as secondary migrations out of Africa. A new paper in Genome Research is out which adds some interesting details to this bigger discussion, and seems to weigh in further against my tentative hypothesis that serial bottlenecks and genetic drift can explain variation in distance to Africans of various non-African groups. Human population dispersal “Out of Africa” estimated from linkage disequilibrium and allele frequencies of SNPs:
Sometimes in the comments of this weblog people get into heated disagreements about one figure and its proper interpretation. I don’t get much involved most of the time because different visualization techniques often differ on the margin, so getting obsessed with minor details is a fool’s errand. For example, in the paper I reviewed below there was a neighbor-joining phylogenetic tree. Take a look. The length of the branches are proportional to genetic distance.
Mitochondrial DNA from 147 people, drawn from five geographic populations have been analysed by restriction mapping. All these mitochondrial DMAs stem from one woman who is postulated to have lived ab7out 200,000 years ago, probably in Africa. All the populations examined except the African population have multiple origins, implying that each area was colonised repeatedly
And so was published in the year 1987 the paper which established in the public’s mind the idea of mitochondrial Eve, which gave rise to a famous cover photo in Newsweek. This also led to the Children of Eve episode on the PBS documentary NOVA. Here is the summary:
NOVA examines a controversial theory that traces our ancestry to a small group of women living in Africa 300,000 years ago.
As Milford Wolpoff has complained it is probably accurate to characterize the documentary as not particularly “fair & balanced.” Mitochondrial Eve may have been controversial, and subsequently plagued by issues of molecular clock calibration as well as spurious interpretations of the cladograms, but the tide of history was on its side, and PBS was telling that story. And the story was not just the primary science, rather, one had to understand the controversy in light of the debates among paleontologists and between paleontologists and molecular biologists. A group of researchers, spearheaded by Chris Stringer argued for the recent origin of modern humans from Africa on the basis of fossils alone. They were challenged by an established school of multiregionalists who argued for deeper roots of modern human populations, which derived from local hominins which diversified after the the migration of H. erectus out of Africa. The argument of the multiregionalists was that selective sweeps across the full range of the human populations gave rise gradually to modern humanity as we know it, a compound of specific ancient local features and trans-population characters which unified us into a broader whole. Stringer and company presented a simpler model where anatomically modern human being arose ~200,000 years ago in Africa, and subsequently expanded to other parts of the world, by and large replacing the local hominin populations. In the multiregionalist telling Neandertals became human beings, while Out of Africa would imply that Neandertals were replaced by human beings.
One of the most persistent debates about the process of evolution is whether it exhibits directionality or inevitability. This is not limited to a biological context; Marxist thinkers long promoted a model of long-term social determinism whereby human groups progressed through a sequence of modes of production. Such an assumption is not limited to Marxists. William H. McNeill observes the trend toward greater complexity and robusticity of civilization in The Human Web, while Ray Huang documents the same on a smaller scale in China: A Macrohistory. A superficial familiarity with the dynastic cycles which recurred over the history of Imperial China immediately yields the observation that the interregnums between distinct Mandates of Heaven became progressively less chaotic and lengthy. But set against this larger trend are the small cycles of rise and fall and rise. Consider the complexity and economies of scale of the late Roman Empire, whose crash in material terms is copiously documented in The Fall of Rome: And the End of Civilization. It is arguable that it took nearly eight centuries for European civilization to match the vigor and sophistication of the Roman Empire after its collapse as a unitary entity in the 5th century (though some claim that Europeans did not match Roman civilization until the early modern period, after the Renaissance).
It is natural and unsurprising that the same sort of disputes which have plagued the scholarship of human history are also endemic to a historical science like evolutionary biology. Stephen Jay Gould famously asserted that evolutionary outcomes are highly contingent. Richard Dawkins disagrees. Here is a passage from The Ancestor’s Tale:
…I have long wondered whether the hectoring orthodoxy of contingency might have gone too far. My review of Gould’s Full House (reprinted in A Devil’s Chaplain) defended the popular notion of progress in evolution: not progress towards humanity – Darwin forend! – but progress in directions that are at least predictable enough to justify the word. As I shall argue in a moment, the cumulative build-up of compelx adaptations like eyes strongly suggest a version of progress – especially when coupled in imagination with of the wonderful products of convergent evolution.
100 years ago a science based physical anthropology offered up very little as to a systematics of mankind beyond what you could intuit from visual assessments of phenotypic similarity alone. Instead, there were fantastical taxonomies which had little basis in the true pattern of variation and more in the nationalistic debates of that period. The Nordic, Mediterranean, and Alpine trichotomy of the European peoples had only marginally more concrete reality than the division between the Vanyar, Noldor, and Teleri.
We don’t live in such a fantastic age. Much of the mystery, and so potential for mischief, is gone. The “post-genomic” era means that old questions only vaguely perceived in the past are now well resolved. Quite often readers will ask a question as to the phylogenetic relationship between population A & B. If I don’t know off the top of my head, which is the norm, I’ll go to the search engine and look up what I’ve written on the topic. This has started to become tedious, in part because WordPress’ search engine leaves something to desired. So I have some papers bookmarked for immediate reference. They’re of wide scope (i.e., they don’t focus on just one population such as the Jews) and draw from a large number of markers to get a good picture of total genome relatedness. The focus within these papers tends to be genetic distances and relationships, not other topics of great interest such as natural selection. Also, I’ve tried to find links accessible to people without institutional access (for the Science link free registration will do it). If you can think of other papers, please leave the link in the comments.
Years ago an evolutionary biologist mentioned to me almost offhand that with the emergence of genomics and the necessity to master computational techniques a lot of the labor hours which may have gone into a more thorough understanding of specific organisms had gone by the wayside. He believed that his Ph.D. advisor was going to take a lot of knowledge with him when he retired because there was just no time to devote to discussing details of specific organismic life history, anatomy, and behavior. I obviously think that the sacrifice has been worth it, the new methods are powerful and answer long standing questions (or hold promise to do so), but something has no doubt been lost. Biological variation is such that a gestalt “big picture” sense of the lay of the land is useful. Much of biology is a historical science, and like history the details are of the essence. But unlike history biology is a natural science, and amenable to experimentation and observation, as well as laced with a more thorough formalization (yes, I am aware of cliometrics). The mileage one gets out of theory in biology is far greater than in history, as evidenced by the high prestige of an evolutionary framework, and the obscurity of cliodynamics (and the relative marginal reputation of Arnold Toynbee).
But evolution purely as logic often fails. The old debate between the balance & classical schools in evolutionary genetics was upended by empirical findings in molecular evolution in the 1960s, which subsequently stimulated neutral theory. Natural science has to extend itself through a long-term dance between system building and empirical verification or falsification. The seeds of new systems don’t come from a vacuum, rather, the prior set of observations and experiments lay the groundwork and serve as points of embarkation.
The combination of biology’s variation and its reliance on theories, heuristics, and rules-of-thumb (e.g., 19th century biology’s love affair with “laws”), often leads to perplexing surprises when a more systematic or deeper read of the data flies in the face of expectations. So it is with a new paper in PNAS which upends some specific relationships between mammalian characteristics and encephalization, as well as some more general prejudices. Brain size, life history, and metabolism at the marsupial/placental dichotomy:
In that their demographic history is complicated. The Origin and Genetic Variation of Domestic Chickens with Special Reference to Junglefowls Gallus g. gallus and G. varius:
… domestic chickens diverged from red junglefowl 58,000±16,000 years ago, well before the archeological dating of domestication, and that their common ancestor in turn diverged from green junglefowl 3.6 million years ago. Several shared haplotypes nonetheless found between green junglefowl and chickens are attributed to recent unidirectional introgression of chickens into green junglefowl. Shared haplotypes are more frequently found between red junglefowl and chickens, which are attributed to both introgression and ancestral polymorphisms. Within each chicken breed, there is an excess of homozygosity, but there is no significant reduction in the nucleotide diversity. Phenotypic modifications of chicken breeds as a result of artificial selection appear to stem from ancestral polymorphisms at a limited number of genetic loci.
I wonder if domesticates in particular exhibit these more complex reticulated patterns in their phylogenies because they spread along human trade routes.
Fascinating post by Bayes, Phylogenetics, cultural evolution and horizontal transmission:
For some time now, evolutionary biologists have used phylogenetics. It is a well-established, powerful set of tools that allow us to test evolutionary hypotheses. More recently, however, these methods are being imported to analyse linguistic and cultural phenomena. For instance, the use of phylogenetics has led to observations that languages evolve in punctuational bursts, explored the role of population movements, and investigated the descent of Acheulean handaxes. I’ve followed the developments in linguistics with particular interest; after all, tracing the ephemeral nature of language is a daunting task. The first obvious road block is that prior to the invention of writing, the uptake of which is limited in geography and history, language leaves no archaeological record for linguists to examine. One particular note I’d like to make is that when Charles Darwin first formulated his theory of natural selection, he took inspiration from linguistic family trees as the basis for his sketch on the evolutionary tree of life. So it seems rather appropriate that phylogenetic approaches are now being used to inform our knowledge regarding linguistic evolution.
Like many other attempts applying evolutionary thinking in culture, phylogenetic approaches are, at times, met with contempt. This stems from assertions that cultural evolution and biological evolution differ greatly in regards to the relative importance of horizontal transmission….
I guess the general points to take away from this post are: 1) Do not necessarily assume horizontal transmission is dominant in shaping culture; and, 2) Even with certain levels of reticulation, it does not necessarily invalidate a phylogenetic approach in investigating cultural and linguistic evolution.
Last week Nature published a paper which may have found a new ‘branch’ of the hominin evolutionary bush which may have been coexistent which modern humans and Neandertals. I recommend The Atavism, Carl and John Hawks on this story. Interesting times.