I’ve reposted this once already, but it is so important and cool …. we’re going to do it again. (And by “it” I mean the research, not this post!)
The perfect bird family tree …
… is certainly still in the future. But we have seen a step in that direction in a new paper, coming out this week in Science. This research applies intensive and extensive genomic analysis to the avian phylogenetic tree. The results are interesting.
This paper is summarized in a number of locations, most notably here on Living the Scientific Life. Here, I will summarize it only very briefly. However, there are two observations I would like to make about this paper and its apparent meaning. One has to do with the nature of science, and the other has to do with the nature of evolution in particular. I’ll argue that we can quantify (almost non-trivially) the number of times science is wrong. I’ll also argue that Stephen Jay Gould was wrong (not totally, but not trivially) about one of his most important assertions (other than his musings about the myth of vaginal orgasms … we’ll talk about that another time).
The paper in question comes out of a project called the Early Bird Assembling the Tree-of_Life Research Project, based at the Field Museum in Chicago. The project is huge, with people assembled from all across the world, collecting piles of data and coming up with a new bird phylogeny.
Check out this description:
… we made more than 26,000 DNA-DNA comparisons among ca. 1700 bird species, representing all of the orders and 168 of the 171 families in Wetmore’s … classification. A new classification was proposed [based on this research] …
Those words are NOT about the present paper, but rather, were written by Charles G. Sibley describing the research he did with Alquist prior to 1986. Sibley and Alquist’s DNA bird phylogeny did everything the current paper proports to do, but earlier, and in the context of much less certainty about the relationships among the groups of birds. Sibly and Alquist’s studies were also among the first uses of DNA to build phylogenies of ANY large groups of species and served as the template for much of the work done on other groups through the 1980s and 1990s.
So what about the current paper?
For more than five years, the Early Bird Assembling the Tree-of-Life Research Project, centered at The Field Museum, has been examining DNA from all major living groups of birds. Thus far, scientists have built and analyzed a dataset of more than 32 kilobases of nuclear DNA sequences from 19 different locations on the DNA of each of 169 bird species. The results of this massive research, which is equivalent to a small genome project …
Comparatively, the earlier research (which was more than one paper, of course) seems more intensive and extensive. The work by Sibley and Alquist used ALL of the DNA (sort of … I’m glossing details here) as it was a hybridization rather than sequence comparison technique, and Sibley and Alquist used WAY more species.
So given the intensity of the earlier work, how can this later work actually contribute so much, and virtually re-write bird phylogeny?
Because hybridization techniques produce results that can be garbled … and you can’t really tell when they are garbled as well as you might want to …. for early branches of an ancient phylogeny. So, if you think about bird phylogeny as a book with lots of chapters, and the chapters have paragraphs and such, Sibley and Alquist got the data sorted out within each chapter very nicely, and put all the chapters together in a structure that was reasonable but subject to revision. The present study is the revision. The present study uses techniques that purport to allow resolution of these problem areas of the phylogeny. (Again, I oversimplify, but only a little.)
Certain aspects of the avian phylogeny were confirmed by the present study, but the reason everyone is going gaga over this work is the set of unexpected findings. Some of these have to do with wacky divergences. Hummingbirds
evolved from a group characterized by the nightjars
(nighthawks, whip-poor-will, etc.)
That shows that evolution is amazing in how it can produce great diversity. As we knew. But what is more interesting is the new cases of convergence. For instance, Falcons and hawks are in separate groups and do not necessarily share a common ancestor that is hawk-like. Falcons are with the song birds and parrots while hawks are with the some of the owls and the hornbills.
What is the meaning of all of this? In some sense, it is too early to say. This is all very complicated. Think of a phylogeny as a set of hypotheses. A taxon with species defined as its members is a proposal that can be tested with new data or analysis. Well, many of the hypotheses regarding major taxa of birds are damaged by this analysis. Beautiful hypotheses are getting wiped out by complex genomic facts. And each of these hypotheses has an associated group of researchers working with it. Some will be delighted to find out this new information … it will confirm suspicions or clear up questions. Others will not like it. Some will play, some will fight. Either way, there will be dust, and the dust will take time to settle.
But for now, I’d like to propose two items for your consideration, as promised.
First, a way to think about science. Of N attempts to understand or describe a given system (in this case, the system is bird phylogeny) the number of incorrect attempts is either N or N-1, such that it is N until the description is stable. That should be obvious. But the second part of my assertion is never obvious: It is very difficult to tell the difference between the N state and the N-1 state.
I was at a major scientific meeting 20 some years ago when Sibley and Alquist’s research was really dawning on people. They gave a talk that was shortly thereafter embodied in a chapter of a publication on birds, the culmination of the research I cited above. It changed the way we think in a big way (about phylogenies) and in many many small ways (one per species, in a sense). The present paper does not overthrow Sibley and Alquist, but it does provide major and important revisions. The point is, Sibley and Alquist was the big kahuna of bird phylogeny. Sibley and Alquist figured it out. It was obvious that only minor fixing up would be needed.
Apparently not.
Are we at N-1? (Where N is the sum of all those major bird taxonomy/diversity/phylogeny moments from just before Darwin to more recent.) I’m betting not.
The second point I want to make is much more important and utterly contentious. Toss me to the dogs, if you want, but I’m going to say it:
Stephen Jay Gould stated, again and again (I’m sure I’ve seen him mutterting this on the Redline Subway on the way back from Red Sox games….) that if you re-ran the history of evolution, like re-playing a movie, the story would never be the same twice.
This non-sameness seems to be (but is not) critical to the non-teleology of evolution. But I think it is wrong.
I think that if you replayed the evolution movie again and again, starting with the Cambrian Explosion, you would get woodpeckers, humingbirds, and raptors again and again and again and again and again. You might not get birds every time, but you’d get these guilds. They may be in very different places on the phylogenetic tree, but these adaptive syndromes would be represented every time (and others, too, of course).
So it would not be like playing the same movie over and over again, but it would be like staging the same play again and again with different actors and different interpretations. But it would NOT be as different as different movies of the same genre again and again.
No, the history of life, played again and again, would be the same each time at a level somewhere between, say, Lethal Weapon (again and again) or Jaws (again and again) on one hand and multiple iterations of A Midsummer Night’s Dream on the other
Hackett, S.J., Kimball, R.T., Reddy, S., Bowie, R.C., Braun, E.L., Braun, M.J., Chojnowski, J.L., Cox, W.A., Han, K., Harshman, J., Huddleston, C.J., Marks, B.D., Miglia, K.J., Moore, W.S., Sheldon, F.H., Steadman, D.W., Witt, C.C., Yuri, T. (2008). A Phylogenomic Study of Birds Reveals Their Evolutionary History. Science, 320(5884), 1763-1768. DOI: 10.1126/science.1157704
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That is so cool. I think birds are just about the most interesting animal group on earth, especially since scientists tied them to the dinosaurs…which makes so much sense! Chickens=tasty, tasty dinosaurs!!
“If you replayed the evolution movie again… you would get woodpeckers, hummingbirds and raptors again…”
That presumption is not very controversial when joined with your proviso that the particular actors need not be birds: bats, bees, moths, and hummingbirds are phylogenetically diverse known pollinators, for example. If an exploitable ecological niche exists, multiple species are likely to arise to exploit it.
The more compelling proposition is whether a substantially identical species would be likely to arise if you roll the evolutionary dice again: is a taxonomic type more or less likely, even inevitable? In general, I suspect, probably not.
For example, the early Homo sapiens population in east Africa appears (based on genetic diversity arguments) to have undergone a population bottleneck perhaps 70,000 years ago, not too long before some of their descendants migrated out of Africa to populate the rest of the planet. If this inference is correct, our species dwindled to within a few thousand ancestors from total extinction. One can easily envision a plausible sequence of events – a prolonged drought, a new disease epidemic – which might have exterminated such a small and localized population. The impetus for either science or blogs would then have passed to the next of kin, namely Homo neanderthalensis. They were big-brained and hardy survivors, but seem to have lacked the same creative impulse. Whether they would have evolved a comparable level of intellectual sophistication is an intriguing speculation, and the ongoing genome comparisons may even offer some insight as to how close they might have been to developing an innovative culture.
But consider how critically dependent on small historical incidents is the fate of evolution in our own time. For example, China under Mao had a billion people living under communism in extreme poverty. In the late 1970s, Mao died and was succeeded by Chao En-Lai, who instituted modest capitalist reforms. Over the next thirty years, the Chinese economy grew at double-digit annual rates. By 2010, 300 million Chinese had attained a middle-class or higher standard of living. Whereas their parent’s generation was lucky to have a full rice bowl, they could afford an occasional luxury like shark-fin soup. In response to this demand, the harvest of sharks for their fins has led to a world-wide decline in many shark species.
Had Chao died before Mao, would China have embraced capitalist reforms? Had they not, the Chinese would have remained dirt-poor, and the demand for shark fins would never have developed. Odd to think that the evolutionary survival of hammerheads in the Gulf of Mexico might be affected by which Chinese octogenarian autocrat outlived the other; yet it seems to be the case.
Re above comment: I should have said Deng Xiaopeng rather than Chao En-Lai; apparently I can’t keep my Chinese autocrats straight without a scorecard.
Hello.
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Thanks, good luck!