We left Darwin in a troubled frame of mind. The fossil record seemed to offer little support to his theory (then again, it offered little support to any other theory). By the time we reach the end of chapter 10, 'On the geological succession of organic beings', he's feeling far more chipper:
"[A]ll the chief laws of palaeontology plainly proclaim, as it seems to me, that species have been produced by ordinary generation: old forms having been supplanted by new and improved forms of life, produced by the laws of variation still acting round us, and preserved by Natural Selection."
What happened? I'm not entirely sure. Chapters 9 and 10 are sisters -- they are summarized together -- but they tell different stories. Chapter 9 was big on geology, and low on biological specifics. Reading chapter 10 in isolation, you'd never guess that the fossil record was such a tattered and faded document as its predecessor made out. We've seen, in chapters 6 and 7, for example, how good Darwin is at arguing with himself, at pre-empting our objections and addressing them, but these two geological chapters don't talk to one another as much as you might expect.
There is one message, however, that we can carry over from the previous chapter -- the world is very old. When you combine that with one of this chapter's messages -- that, if you look at the rocks, you see fossils appearing, sticking around for a bit, and disappearing, that starts to look like something not predicted, or explained, by natural theology. Once again, Darwin shows that the idea that species are specially and individually created is a poor fit to the data. Of course, species might appear fully formed and vanish like pieces removed from a game board. But Darwin has a better idea.
Descent with modification, says Darwin, can explain why once a group such as trilobites or ammonites goes extinct it never comes back. It can explain why Australia and South America's fossil animals belong to the same groups as those continents' living animals, and why history is a much better guide than environment to the species you find in a place ("how slight is the relation between the physical conditions of various countries, and the nature of their inhabitants"). And it can explain why, as you look back through the fossil record, the gap between modern groups narrows, and missing links appear. To illustrate this, he turns to the work of the man who would become his, and evolution's, enemy, Richard Owen:
"[W]hole pages could be filled with striking illustrations from our great palaeontologist, Owen, showing how extinct animals fall in between existing groups. Cuvier ranked the Ruminants and Pachyderms, as the two most distinct orders of mammals; but Owen has discovered so many fossil links, that he has had to alter the whole classification of these two orders; and has placed certain pachyderms in the same sub-order with ruminants: for example, he dissolves by fine gradations the apparently wide difference between the pig and the camel."
To explain the patterns of extinction and origination, Darwin turns, not surprisingly, to natural selection. Things disappear because something better comes along. As an analogy, he uses invasive species. Invaders must be fitter -- many British species have naturalized successfully in New Zealand, so they "may said to be higher than those of New Zealand", even though "the most skilful naturalist from an examination of the species of the two countries could not have foreseen this result".
He also turns to domestic species. Pigeons et al. aren't just something Darwin wheels out early on and then dispenses with. He uses them in just about every chapter.
"[W]hen a new and slightly improved variety has been raised, it at first supplants the less improved varieties in the same neighbourhood; when much improved it is transported far and near, like our short-horn cattle, and takes the place of other breeds in other countries. Thus the appearance of new forms and the disappearance of old forms, both natural and artificial, are bound together."
Although that doesn't mean that we should expect evolution to create regularity. His theory explains more than it predicts:
"I believe in no fixed law of development, causing all the inhabitants of a country to change abruptly, or simultaneously, or to an equal degree. The process of modification must be extremely slow. The variability of each species is quite independent of that of all others. Whether such variability be taken advantage of by natural selection, and whether the variations be accumulated to a greater or lesser amount, thus causing a greater or lesser amount of modification in the varying species, depends on many complex contingencies, on the variability being of a beneficial nature, on the power of intercrossing, on the rate of breeding, on the slowly changing physical conditions of the country, and more especially on the nature of the other inhabitants with which the varying species comes into competition. Hence it is by no means surprising that one species should retain the same identical form much longer than others; or, if changing, that it should change less."
Once again, as when he discussed the blurry boundary between species and varieties, Darwin is using irregularity and uncertainty as evidence.
Is life, every day, in every way, getting better and better? Whether palaeontology gives Darwin quite as much support as he thought is an open question. The fact that many organisms remain recognizable across massive stretches of geological time suggests that stabilizing, or purifying selection is also an important force, selecting against the extremes. Stasis is data, to quote the punctuated equilibrium guys (seeing as 'data' is plural, shouldn't that be 'stases are data'?). And there's no guarantee that a selection pressure will be in place long enough to create changes that show up in the fossil record.
On the other hand, adaptation is all around us -- in the evolution of antibiotic resistance, say. And we are getting better (although still not perfect) at reading the imprint of different forms of selection, both positive and stabilizing, in genetic data.
Understanding the links, or differences, between microevolution and the patterns seen in the fossil record is a tricky business. Take two recent papers in PNAS.
In 2007, Gene Hunt published a study arguing that directional change in the fossil record was rare: of the 250 lineages he looked at, 95% showed either stasis or a random walk -- only 5% showed any evidence for directional selection.
"The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria and suggests that truly directional evolution is infrequent or, perhaps more importantly, of short enough duration so as to rarely register in paleontological sampling."
"These results provide a rare demonstration of a large-scale and probably driven trend occurring across multiple independent lineages and influencing both the form and number of species through deep time and in the present day."
Whether macroevolution is microevolution writ large is one of the big questions of evolutionary biology. It's also one of the things that some evo-devo researchers, such as Sean Carroll, hope they can help to answer by revealing the genetic (i.e. microevolutionary) bases of change to form (i.e. what shows up in the fossils).
I seem to be ending all these posts by going, well, there's this one bunch of guys says one thing, and then there's this other bunch of guys says something else. But that's the way it is, and like most of the questions that Darwin set loose, it'll be a long time before this one is settled.
On Wednesday: Geographical Distribution.
Data is plural in Latin, and among a minority of English speakers (confined almost exclusively to the sciences).
Among the vast majority of those in the information technology industry, and among the great majority of standard English speakers at large, data is a mass noun.
So the slogan, whatever its factual value to the non-theory of punctuated equilibrium (long periods of stasis fit just fine into standard evolution - no special label required), it's perfectly correct grammatically.
Even after 20 years in science, I find it unnatural to use "data" in the plural, as in "these data". It's good to hear that for most people, it is unnatural :-)
And speaking of punctuated equilibrium, I saw some foreshadowing of this idea when Darwin theorized that species that disappear in the fossil record and reappear may have gone locally extinct in the region being looked at but that they survived elsewhere in other regions and later repopulated that area.
Data/datum/datas/dati/whatever - I know it's nerdy to say 'these data'. It's one of the scars of having worked as a Nature copy editor. But these quaintnesses ought to hang on somewhere, dammit.
Diego - I thought Darwin's section On the sudden appearance of whole groups of allied species also foreshadowed the punk eek debate.
In the section entitled On the state of Development of Ancient Forms, Darwin makes a couple of references to embryonic development reproducing earlier forms of life:
Thus the embryo comes to be left as a sort of picture, preserved by nature, of the ancient and less modified condition of each animal. This view may be true, and yet it may never be capable of full proof.
This seems to indicate a nascent form of Haeckel's "phylogeny recapitulates ontogeny" theory, which I have read has both been debunked or otherwise discredited in some quarters, and embraced in a lesser or modified form in others.
Can you give a quick rundown of the current thinking on this matter?
Thank you, and thanks for blogging your reviews. I'm really enjoying reading along.
Hi James. Thanks for pointing that out - I caught a whiff of recapitulation theory there too (I didn't realize at the time that the phrase 'ontogeny recapitulates phylogeny' hadn't been coined when Darwin wrote this). Darwin returns to embryology and evolution at greater length in chapter 13, and I've written a bit about it there.
I think I'm right in saying that people no longer think that ontogeny recapitulates phylogeny - we don't think that organisms are linked in a great chain of being from higher to lower, and so groups don't evolve by reassembling every previous link of the chain in their development and then adding a new and 'higher' link. Instead, diversity has arisen because evolution tinkers with developmental processes throughout the embryo's life.
Ah, thanks for that. I should have read ahead :-) (and gotten my phrasing correct). It did seem dubious, can't recall where I sourced the information showing that it was still a viable theory.
I look forward to the review in Chapter 13.
I'm surprised no one has mentioned Steve Gould's "Full House" concept that complexity increased because that was the only general direction that life could go in. (IIRC, "Full House" might have published in Europe under a different title. Characteristically compelling ideas and great writing by Gould in this book, highly recommended, and especially for baseball fans.)
I wonder if the apparent trend of increasing complexity is simply life's way of solving the problems of existence. My personal experience in science and engineering is that practical solutions almost require always adding complexity to systems, because it is hard obviously to do more with less (as much as my boss would like that!).
Haeckel was an excellent comparative anatomist, perhaps not as capable as Huxley, but so overwhelmed by hubris and racist notions, even extreme in his own time, that it is difficult to look at him with objectivity, never mind compassion. That he "blurred" his embryological work has been demonstrated.
The passage of time has not been kind to Steve Gould: the ghost of Maynard Smith and the poundings of Daniel Dennett and Richard Dawkins and others, as well as the very careful and real biostratigraphy of Phil Gingerich and his students at the Tertiary end, Conway Morris at the Paleozoic end have worked through most perspectives. Gould's ponderous and quirky "The Structure of Evolutionary Theory," muddles matters still more. In the final analysis the rocks and fossils win out for the geological record, just as the cladistic knife has been settling most evolutionary matters of the dead and living. Through it all, the amazing detail of Darwin's work in the Origin, as well as everything from barnacles to coral atolls to earthworms and plants, still stands up better than well.
"Data" is plural like "stamina" and "agenda" are plural: if you're speaking Latin.
I really liked your article! I just have a few questions regarding the difference between species and varieties. To clarify, is a "variety" just a random mutation in an individual within a species that gets passed down to some of that individual's offspring, therefore slightly changing/"varying" from the original population? I read chapter two, and I just wanted to check my definition of variety. You mentioned that Darwin had a hard time defining the difference between a species and varieties. Do you (in Darwin's definition) know at what point the physical or behavior differences between two populations descended from the same ancestor(s) makes the two populations different species? I know that if two species can interbreed, they're generally considered the same species, but what about hybrids? Thank you for your time!