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Biological organisms in general, and human brains particularly, contain complex adaptations; adaptations which involve many genes working in concert. Complex adaptations must evolve incrementally, gene by gene. If gene B depends on gene A to produce its effect, then gene A has to become nearly universal in the gene pool before there's a substantial selection pressure in favor of gene B.
A fur coat isn't an evolutionary advantage unless the environment reliably throws cold weather at you. And other genes are also part of the environment; they are the genetic environment. If gene B depends on gene A, then gene B isn't a significant advantage unless gene A is reliably part of the genetic environment.
Let's say that you have a complex adaptation with six interdependent parts, and that each of the six genes is independently at ten percent frequency in the population. The chance of assembling a whole working adaptation is literally a million to one; and the average fitness of the genes is tiny, and they will not increase in frequency.
In a sexually reproducing species, complex adaptations are necessarily universal.
"The laws of physics and the rules of math don't cease to apply. That leads me to believe that evolution doesn't stop. That further leads me to believe that nature —bloody in tooth and claw, as some have termed it —will simply be taken to the next level...
"[Getting rid of Darwinian evolution is] like trying to get rid of gravitation. So long as there are limited resources and multiple competing actors capable of passing on characteristics, you have selection pressure."
—Perry Metzger, predicting that the reign of natural selection would continue into the indefinite future.
In evolutionary biology, as in many other fields, it is important to think quantitatively rather than qualitatively. Does a beneficial mutation "sometimes spread, but not always"? Well, a psychic power would be a beneficial mutation, so you'd expect it to spread, right? Yet this is qualitative reasoning, not quantitative—if X is true, then Y is true; if psychic powers are beneficial, they may spread. In Evolutions Are Stupid, I described the equations for a beneficial mutation's probability of fixation, roughly twice the fitness advantage (6% for a 3% advantage). Only this kind of numerical thinking is likely to make us realize that mutations which are only rarely useful are extremely unlikely to spread, and that it is practically impossible for complex adaptations to arise without constant use. If psychic powers really existed, we should expect to see everyone using them all the time—not just because they would be so amazingly useful, but because otherwise they couldn't have evolved in the first place.
"So long as there are limited resources and multiple competing actors capable of passing on characteristics, you have selection pressure." This is qualitative reasoning. How much selection pressure?
Followup to: Evolutions Are Stupid
It is a very common misconception that an evolution works for the good of its species. Can you remember hearing someone talk about two rabbits breeding eight rabbits and thereby "contributing to the survival of their species"? A modern evolutionary biologist would never say such a thing; they'd sooner breed with a rabbit.
It's yet another case where you've got to simultaneously consider multiple abstract concepts and keep them distinct. Evolution doesn't operate on particular individuals; individuals keep whatever genes they're born with. Evolution operates on a reproducing population, a species, over time. There's a natural tendency to think that if an Evolution Fairy is operating on the species, she must be optimizing for the species. But what really changes are the gene frequencies, and frequencies don't increase or decrease according to how much the gene helps the species as a whole. As we shall later see, it's quite possible for a species to evolve to extinction.
"Individual organisms are best thought of as adaptation-executers rather than as fitness-maximizers."
—John Tooby and Leda Cosmides, The Psychological Foundations of Culture.
Fifty thousand years ago, the taste buds of Homo sapiens directed their bearers to the scarcest, most critical food resources—sugar and fat. Calories, in a word. Today, the context of a taste bud's function has changed, but the taste buds themselves have not. Calories, far from being scarce (in First World countries), are actively harmful. Micronutrients that were reliably abundant in leaves and nuts are absent from bread, but our taste buds don't complain. A scoop of ice cream is a superstimulus, containing more sugar, fat, and salt than anything in the ancestral environment.
No human being with the deliberate goal of maximizing their alleles' inclusive genetic fitness, would ever eat a cookie unless they were starving. But individual organisms are best thought of as adaptation-executers, not fitness-maximizers.
I've previously dwelt in considerable length upon forms of rationalization whereby our beliefs appear to match the evidence much more strongly than they actually do. And I'm not overemphasizing the point, either. If we could beat this fundamental metabias and see what every hypothesis really predicted, we would be able to recover from almost any other error of fact.
The mirror challenge for decision theory is seeing which option a choice criterion really endorses. If your stated moral principles call for you to provide laptops to everyone, does that really endorse buying a $1 million gem-studded laptop for yourself, or spending the same money on shipping 5000 OLPCs?
We seem to have evolved a knack for arguing that practically any goal implies practically any action. A phlogiston theorist explaining why magnesium gains weight when burned has nothing on an Inquisitor explaining why God's infinite love for all His children requires burning some of them at the stake.
There's no mystery about this. Politics was a feature of the ancestral environment. We are descended from those who argued most persuasively that the good of the tribe meant executing their hated rival Uglak. (We sure ain't descended from Uglak.)
Before 1966, it was not unusual to see serious biologists advocating evolutionary hypotheses that we would now regard as magical thinking. These muddled notions played an important historical role in the development of later evolutionary theory, error calling forth correction; like the folly of English kings provoking into existence the Magna Carta and constitutional democracy.
As an example of romance, Vero Wynne-Edwards, Warder Allee, and J. L. Brereton, among others, believed that predators would voluntarily restrain their breeding to avoid overpopulating their habitat and exhausting the prey population.
But evolution does not open the floodgates to arbitrary purposes. You cannot explain a rattlesnake's rattle by saying that it exists to benefit other animals who would otherwise be bitten. No outside Evolution Fairy decides when a gene ought to be promoted; the gene's effect must somehow directly cause the gene to be more prevalent in the next generation. It's clear why our human sense of aesthetics, witnessing a population crash of foxes who've eaten all the rabbits, cries "Something should've been done!" But how would a gene complex for restraining reproduction—of all things!—cause itself to become more frequent in the next generation?
Yesterday, I wrote:
Science has a very exact idea of the capabilities of evolution. If you praise evolution one millimeter higher than this, you're not "fighting on evolution's side" against creationism. You're being scientifically inaccurate, full stop.
In this post I describe some well-known inefficiencies and limitations of evolutions. I say "evolutions", plural, because fox evolution works at cross-purposes to rabbit evolution, and neither can talk to snake evolution to learn how to build venomous fangs.
So I am talking about limitations of evolution here, but this does not mean I am trying to sneak in creationism. This is standard Evolutionary Biology 201. (583 if you must derive the equations.) Evolutions, thus limited, can still explain observed biology; in fact the limitations are necessary to make sense of it. Remember that the wonder of evolutions is not how well they work, but that they work at all.
Human intelligence is so complicated that no one has any good way to calculate how efficient it is. Natural selection, though not simple, is simpler than a human brain; and correspondingly slower and less efficient, as befits the first optimization process ever to exist. In fact, evolutions are simple enough that we can calculate exactly how stupid they are.
Followup to: An Alien God
The wonder of evolution is that it works at all.
I mean that literally: If you want to marvel at evolution, that's what's marvel-worthy.
How does optimization first arise in the universe? If an intelligent agent designed Nature, who designed the intelligent agent? Where is the first design that has no designer? The puzzle is not how the first stage of the bootstrap can be super-clever and super-efficient; the puzzle is how it can happen at all.
"A curious aspect of the theory of evolution," said Jacques Monod, "is that everybody thinks he understands it."
A human being, looking at the natural world, sees a thousand times purpose. A rabbit's legs, built and articulated for running; a fox's jaws, built and articulated for tearing. But what you see is not exactly what is there...
In the days before Darwin, the cause of all this apparent purposefulness was a very great puzzle unto science. The Goddists said "God did it", because you get 50 bonus points each time you use the word "God" in a sentence. Yet perhaps I'm being unfair. In the days before Darwin, it seemed like a much more reasonable hypothesis. Find a watch in the desert, said William Paley, and you can infer the existence of a watchmaker.
But when you look at all the apparent purposefulness in Nature, rather than picking and choosing your examples, you start to notice things that don't fit the Judeo-Christian concept of one benevolent God. Foxes seem well-designed to catch rabbits. Rabbits seem well-designed to evade foxes. Was the Creator having trouble making up Its mind?