William H. Calvin, "Evolution of the human brain." handout for Swedish CME, See also http://WilliamCalvin.com/2002/Swedish.htm
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University of Washington
Seattle WA 98195-1800 USA
To: Human Evolution E-Seminar
From: William H. Calvin
51.47794°N 0.29089°W 11m
What, someone asked by e-mail, did Darwin really discover? It probably isn’t what you always thought.
It wasn’t evolution per se. There had been an active public discussion of evolution since before Darwin was born (his grandfather Erasmus even wrote poems on the subject).
It wasn’t adaptations to fit the environment, as the religious philosophers had already seized on that idea as suggesting design from on high.
Nor was it “survival of the fittest.” That idea had been floated by Empedocles 2,500 years ago in ancient Greece, long before Herbert Spencer, in the wake of Darwin, invented the phrase we now use.
It certainly wasn’t the basic biological and geological facts that Darwin discovered, although during his voyage around the world, and after discovering natural selection, Darwin did add quite a bit in the factual line.
What Darwin contributed was an idea, a way of making various disconnected pieces of the overall puzzle fit together, something like trying to solve a jigsaw puzzle without a picture for a model. He imagined the picture.
It wasn’t, however, the idea of descent from a common ancestor. Diderot, Lamarck, and Erasmus Darwin had all speculated on that subject two generations earlier. And there were trees of descent around to serve as examples, given how by 1816 the linguists were claiming that most European languages had descended from the same Indo-European root language.
By 1837 Darwin had concluded that nature was always in the process of becoming something else, though again there had been other attempts like Lamarck’s along this line. Darwin just looked at the biological facts in a different way than his predecessors and contemporaries, not forcing them to fit the usual stories about how things had come about. Fitting facts to an idea is a primary way in which progress is made in science, but a fit in one aspect has often blinded scientists to more overarching explanations.
But even that wasn’t his main contribution. Charles Darwin had an idea that supplied a mechanism, something to turn the crank that transformed one thing into another.
Basically, Charles Darwin (in 1838 and, independently, Alfred Russel Wallace in 1858) had a good idea about the process of evolution, how one thing could turn into another without an intelligent designer supervising. Out of all the variation thrown up with each generation (even children of the same two parents can be quite unlike one another), some variants fit the present environment better. And so, in conditions where only a few offspring manage to reach adulthood (both Wallace and Darwin got that insight from Malthus and his emphasis on biological overproduction), there is a tendency for the environment to affect which variants get their genes into the next generation.
Many are called, few are chosen by the hidden hand of what Darwin labeled “natural selection.” The name comes from the contrast to animal breeding, so-called “artificial selection.” It is, as Ernst Mayr noted, an unfortunate term, as it suggests an agent doing the natural selecting.
As Thomas Huxley said, when reading Darwin’s book manuscript before its publication in 1859, “How stupid not to have thought of it before.” Two and a half millennia of very smart philosophers trying to solve the problem, and then the answer turns out to be so simple. Like the Necker cube and similar perceptual phenomena, there are often several ways to look at the same facts, just as there are two equally valid roots of a quadratic equation, both of which give satisfaction. And “seeing” the alternative form can be difficult when your culture guides you to see the usual explanation. But the alternate form may lead you to a more coherent solution, one that also explains a much bigger jigsaw puzzle.
A few years later, Darwin realized that he needed to add an “inheritance principle,” to emphasize that the variations of the next generation were preferentially done from the more successful of the current generation (the individuals better suited to surviving the environment or finding mates). This means, of course, that the new variations were not just at random, but were centered around the currently-successful model. In other words, they were little jumps from a mobile starting place, variations on a theme, not big jumps where the starting place becomes irrelevant because the jump carries so far. (Warning: Except for the pros, half of the people who write about evolution, whether pro or con, may be confused about this important short-distance randomness aspect.)
Many variations, of course, are not as good as the parents – nature appears not to worry about this waste, to our distress – but a few variants are even better than their parents. And so, with passing generations, there is a chance for drift to occur towards the better solutions to environmental and mate-finding challenges. Perfection you don’t get, but occasionally you do get something that, locally, could be called “progress” – that ill-defined something that makes us so impressed by the Darwinian process. Nature can be seen to pull itself up by its own bootstraps, amidst a huge waste in variations that go nowhere.
You can summarize Darwin’s bootstrapping process in various ways, from our modern perspective. A century ago, Alfred Russel Wallace emphasized variation, selection, and inheritance. It reminds me of a three-legged stool: evolution takes all of them to stand up.
But there are some hidden biological assumptions in that three-part summary and, when trying to make the list a little more abstract to encompass non-biological possibilities for a Darwinian process, I wound up listing six ingredients that are essential (in the sense that if you’re missing any one of them, you’re not likely to see much progress):
1. There’s a pattern of some sort (a string of DNA bases called a gene is the most familiar such pattern, though a cultural meme – ideas, tunes – may also do nicely).
2. Copies can be made of this pattern (indeed the minimal pattern that can be semi-faithfully copied tends to define the pattern of interest).
3. Variations occur, typically from copying errors or superpositions, more rarely from a point mutation in an original pattern.
4. A population of one variant competes with a population of another variant for occupation of a space (bluegrass competing against crabgrass for space in my backyard is an example of a copying competition).
5. There is a multifaceted environment that makes one pattern’s population able to occupy a higher fraction of the space than the other (for grass, it’s how often you water it, trim it, fertilize it, freeze it, and walk on it). This is the “natural selection” aspect for which Darwin named his theory, but it’s only one of six essential ingredients.
6. And finally, the next round of variations is centered on the patterns that proved somewhat more successful in the prior copying competition.
Try leaving one of these out, and your quality improvement lasts only for the current generation – or it wanders aimlessly, only weakly directed by natural selection.
Many processes loosely called “Darwinian” have only a few of these essentials, as in the selective survival of some neural connections in the brain during development (a third of cortical connections are edited out during childhood). Yes, there is natural selection producing a useful pattern – but there are no copies, no populations competing, and there is no inheritance principle to promote “progress” over the generations. Half a loaf is better than none, but this is one of these committees that doesn’t “get up and fly” unless all the members are present.
And it flies even faster with a few optional members. There are some things that, while they aren’t essential in the same way, affect the rate at which evolutionary change can occur. There are at least five things that speed up evolution.
First is speciation, where a population becomes resistant to successful breeding with its parent population and thus preserves its new adaptations from being diluted by unimproved immigrants. The crank now has a ratchet.
Then there is sex (systematic means of creating variety by shuffling and recombination – don’t leave variations to chance!).
Splitting a population up into islands (that temporarily promote inbreeding and limit competition from outsiders) can do wonders.
Another prominent speedup is when you have empty niches to refill (where competition is temporarily suspended and the resources so rich that even oddities get a chance to grow up and reproduce).
Climate fluctuations, whatever they may do via culling, also promote island formation and empty niches quite vigorously on occasion, and so may temporarily speed up the pace of evolution.
Some optional elements slow down evolution: “grooves” develop, ruts from which variations cannot effectively escape without causing fatal errors in development. And the milder variations simply backslide, so the species average doesn’t drift much. Similar stabilization is perhaps what has happened with “living fossil” species that remain largely unchanged for extremely long periods.
You’ll notice that I didn’t even mention changes in the rate of mutations. Since sex and gene shuffling were invented, mutation rate may have fallen pretty far down the list of important factors controlling the pace of evolution, even though mutations are the usual beginner’s example. Species shifts more often involve changes in the relative proportion of existing gene versions (gene frequencies). It’s the committee’s composition that counts; sometimes all it takes is removing one member to break a deadlock or open up new paths.
[later, at p.178]
Darwin saw that climate had repeatedly changed but, unlike others before him, he successfully figured out a mechanism whereby animal species could change with it, to adapt body and behavior to the new climate regime. Just spawn a lot of variations in each generation and, given the high mortality among the young, only those variants better adapted to the current environment will survive long enough to reach reproductive age. And those lucky variants will spawn additional variations around their body-and-behavior traits, to further explore “fits” to the environment’s opportunities and perils. Those variants better suited to some other climate simply tend not to grow up and reproduce.
But note that this need not be sustainable change. When the climate changes back to the original, the adaptations can track it back again. (Remind me later to explain how speciation can ratchet the adaptation, so it doesn’t drift back so easily). Furthermore, adaptations may mostly happen when there is no other choice. At least on some time scales, climate’s influence needs to be viewed with some skepticism, as most species react to cooling and drying episodes by moving elsewhere, places where their suite of adaptations still works.
Well, moving is something of an euphuism; if there are regional subpopulations, some of them may die out while others continue. With serious climate change, this may leave only a few subpopulations in refugia, places where the species still has all of the essentials for making a living and reproducing. Let climate improve, and they will “expand their range” to live in more places, with refugia pioneers rediscovering those old places where the species once thrived.
Population size is always fluctuating like this. A shrink-and-expand cycle produces more evolutionary change than adaptations-in-place, as I mentioned earlier, but other factors that truly fragment the central population may prove even more important in transforming the species, particularly (as I’ll mention in a minute) because of the chance aggregations that occur when things fragment.
Population fragmentations are what happens when a lake almost dries up. As the water level drops, you get a series of small ponds and puddles, in which life continues – but there’s now a lot of inbreeding because they are trapped and cannot circulate. There may be some selection for living in the increasingly salty ponds. Most little ponds dry up completely, and the life in them doesn’t contribute to what happens later (there are some exceptions, animals whose lay-them-and-leave-them young can survive desiccation). If only the population in one pond survives and then re-expands, we see a classical “population bottleneck” where the re-enlarged population is comprised of only closely-related individuals. Note that much of the pre-existing variety may vanish, even though little natural selection affected the survivors directly (it just eliminated much of their competition by chance). Refugia are common on land, too, and land animals can be similarly restricted to inbreeding for awhile, with a great reduction in genetic variety because of sheer chance. Cheetahs, all very similar genetically, likely re-expanded from one such small surviving population. This means that natural selection no longer has much variation to operate on, preventing evolution until mutations and cross-over breaks eventually generate some new variation on which recombination can act.
But more often, multiple ponds survive the downsizing and fragmentation. When the old lake refills with the rains, multiple small groups form the basis of the re-enlarged animal population. Each group may have survived for a different reason, some developing adaptations but most not. It is presumably only when the land refugia are also under stress, when they too become excessively cool or dry or dusty, that selection can efficiently operate to improve thermal regulation or kidneys or noses. Or to select for rare abilities that, for once, make a difference. This seems fundamentally different from “who survives” during ordinary population contractions into unstressed refugia.
And there is really nothing to suggest that it was all that cold in tropical refugia for our ancestors. Drought, however, is another matter, as is an ecosystem that fire has severely disrupted. Cooling is just the easiest thing to measure in reconstructing paleoclimate, and not necessarily the most relevant thing to survive and thrive.
Lake Victoria, right on the equator over in East Africa, dried up during the last ice age and abruptly refilled about 15,000 years ago. The cichlid fish in the East African lakes split into many new species about then.
Selection during downsizing isn’t the only way that evolution operates. There are also opportunities to be exploited when conditions improve. And for an ape-like creature already adapted to making a living on the savanna, the opportunities were of boom-time proportions.
READINGS: For the paleoanthropological and ape aspects, try reading.
For the brain side of things, you will find many of the references in my earlier books, The Cerebral Code, How Brains Think, Lingua ex Machina (with the linguist Derek Bickerton), and Conversations with Neil’s Brain (with the neurosurgeon George Ojemann), all at http://faculty.washington.edu/wcalvin. For the connection with behavior, see:
Especially for evolutionary biology, some fine writers have also been at work, adding to the books written by the biologists.