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April 03, 2005

Why evolution doesn't make sense

When I was in high school I did a project with a girl who was the younger sister of a girl I knew. Our topic was dinosaur biomechanics, and I remember spending some time trying to find information on the Diplodocus stride in those pre-web days. One afternoon my partner said me, "Evolution makes no sense, we don't see anything evolving...." I ignored her and continued, for as you might know where I went to school such was the orthodoxy. My partner was not from a fundamentalist household and was no Creationist footsoldier, evolution just "didn't make sense" to her.

When I was in 7th grade I had to take a test to place in the "gifted" track. The was an in depth interview with a psychologist for the school district. There were the standard looking at shapes, solving geometrical problems, but there was also a battery of questions related to general knowledge topics. At one point the psychologist asked me what "Charles Darwin's theory" was about, and I gave a cookie cutter answer about common descent and selection of the "fittest" and what not. The interviewer told me that he had never received such an accurate and precise response on that particular question. I was shocked and frankly disturbed, as I had been exploring evolutionary concepts since I was about 8 years old, and never considered them anything but banal background facts that framed my universe.

Such is not the case for all, and, it might be possible that it will never be the case for most, and I think the answer itself is ironically our evolutionary history and the imprint it has had on our minds.

To elaborate, consider the idea that evolution is not happening because we don't observe it in our own lifetimes. One thing that I realized is that there is a hint of Lamarckism in that viewpoint, that is, that there is continuous relationship between ontogeny and evolution, that we are evolving in our own lifetimes. This is not the standard Darwinian paradigm at all, though Darwin left room for acquired characteristics in his model the Modern Synthesis of evolutionary theory is clearly one which emphasizes that it is selection from a genetically and phenotypically varied population that is the prime force behind morphological change. The fact that this insight is not clear to those who are not ideologically biased against evolution a priori because of religious or political commitments suggests to me that there is something profoundly "unnatural" about evolutionary theory as it is today when viewed in the context of our minds.

Consider the time scales that evolution operates over. I have just noted above that evolution operates on the scale of generations, individuals within a population have differential fitness correlated with different phenotypes. We will not therefore see evolution occurring before our eyes among creatures with long generations, including humans.

Assume that a human generation is 20 years. That is ten generations over 200 years, the normal limits of verbal/oral memory held through a family. There are 100 generations over 2,000 years, a span of time that usually exceeds the lifespans of national, political and religious entities! There are 1,000 generations in 20,000 years. There are 2,000 generations in 40,000 years, roughly the period of time when H. sapiens sapiens has been the sole hominid on this planet. The common ancestry of H. sapiens neandertalis and H. sapiens sapiens probably extends back about 200,000 years toward various populations of H. heidelbergensis in Western Eurasia. That's 10,000 generations back to our common ancestry with our sister subspecies!

Humans have an innate numeracy, but its discrete and precise range is not much farther than 6 or 8. Beyond that point we live in an analog world or must dig into the realm of abstract mathematics. The last common ancestor between our own species and another homonid lineage might be pushed back on the order of millions of years, that is, tens and perhaps hundreds of thousands of generations back into the past. Unless you are a Methuselah or a god such timescales simply exist outside of the realm of "common" sense and plain conception.

Of course "microevolution" continues, selection on traits within a species, which has resulted in the stocky build of the Inuit peoples in the last 15,000 years, less than a 1,000 generations, or the rise of tall and lanky Nilotic herders with animal domestication on the bright plains of East Africa, perhaps as few as serveral hundred generations. But this is not the "evolution" that many in the public imagine, and Creationists admit to microevolution as within the realm of possibility. The public aims for speciation, and in particular a morphological shift, a reorganization, to grab one's attention and attest to the sweeping salience of mutational induced change in one's lifetime.

This is simply problematic as it is predicated on a "hopeful monster" thesis promoted by the likes of Richard Goldschmidt but repudiated for over two generations. Rather, the promoters of the Modern Synthesis emphasize more gradualistic change, and assert that macroevolution and microevolution are ontologically ultimately the same. Even those who dissent from the orthodoxy, like the late Stephen Jay Gould and Niles Eldridge, rejected the saltationalist idea of hopeful monsters rooted in the possibility of widescale one-generational genetic reorganization. Part of the issue is the public perception that mutation is more important to evolutionary change than selection, when mutation is simply a background given that is shaped by the algorithmic logic of natural selection.

Certainly change can happen in a few generations. Consider the empirical equation for selection response on a trait (used for artificial breeding):

R = h2*S

h2 = realized heritability & S = selection differential of breeding and parental population (breeding as in the subset of the population chosen to sire the next generation).

If you had a population of humans whose mean height was 60 inches, 5 feet, how long would it take their mean heights to reach 72 inches, 6 feet (ignoring consideration of sexual dimoprhism, etc.)? Assume that the selection differential is 1 inch and heritability is .5. Iterate.

It will take about 20 generations (20 successive selection events) to reach a mean of 6 feet in mean height, about 400 years. Of course the model is grossly simplified (humans do not have discrete generations obviously), and I am assuming that genetic variation is not exhausted. Nevertheless, there, you see, evolution happens! In fact, if I selected a group of breeders whose mean height was 7 feet I could hit 6 feet as the mean in one generation (this would require a large population since if the mean is 5 feet and the distribution is normal there wouldn't be that mean people north of 6 feet in the first place).1

This wouldn't impress anyone, because people don't think that change in one trait matches their idea of evolution. T-shirts that depict human evolution show a rise in height (that's wrong, it is possible that H. erectus, or at least many populations of this species, were rather taller than the typical post-neolithic human being) as we evolve upward and onward, but, there are also a host of other morphological changes. What we are looking for is phenotypic change over generations on a host of traits. It happens, but never with the rapidity that will satisify commen sense.

Additionally, beyond the problem of the extent of change as a function of time, there is the essentialist paradigm which demands a particular conception of species difference contemporaneously. Unlike "genus" and other taxonomical categories species is more than a human semantic conjuring, they do seem to exist. But, contra the wisdom of the Creationists, and human common sense, it is not clear that species are always cut & dried in terms of how one delineates one "kind" from another. The various ring species are a clear illustration of morphological variance within interbreeding species. Dogs also illustrate part of the problem within themselves they vary a great deal morphologically, but there is debate whether they should separated from wolves, and the reality that some dogs seem more "wolfish" than domesticated, especially when placed next to the more bizarre lap dog morphs. I have already addressed the ecological species concept vs. the biological species concept, but it seems likely that the public simply dovetails all these nuances together and so expects similar theoretical elegance from evolution. I have not even touched on the topic of defining "species" among asexually reproducing prokaryotes, but since they are outside of the realm of the basic senses they are also generally not admissible to the court of common sense.

Evolution as a model is a scientific paradigm scaffolded by all sorts of peculiar realities, contingencies and theoretical constructs, as a fact it works over inhuman timescales and the underlying processes are not always intuitive. Human beings evolved to process an analog world of information, but we are a shaped on the most fundamental level by a discrete information storage medium and replicative process. I believe this is why random genetic drift often surprises people, it is a process that occurs due to imperfect discrete sampling of the previous generation of alleles, while human beings conceive of our replication as an analog process where we recreate the whole. Whether you believe in the evolutionary psychological massively modular mind with its specialized knowledge domains, or you hew to adaptive thinking or even rely on the importance of general cognition primed toward inputs that shape templates and that over time can trigger almost reflexive associations, you acknowledge that our minds are riddled with biases of thought, process and intuition. If you asked a young adult if two metallic balls of very different sizes would hit the ground at the same time if dropped from the same height they would say "yes," but why, common sense? No, I suspect they've been taught this reality as part of their science curriculum from an early age. In the everyday world we don't encounter many dense objects with low surface areas dropped from on high on a straight line so that we can neglect the reality of drag and other vicissitudes. Human life for the past 100,000 years has not been a controlled experiment rigged to elucidate general scientific principles and the abstract laws of the universe, rather, it has been a sloppy mess and a struggle to reproduce. The biases we have been given by evolution or our proximate day to day experience as human beings are useful. If we spent time considering that Newtonian Mechanics is fundamentally a faulty aproximation, that the world around is 99.999999% vacuum, that does nothing for our life and nothing for our reproductive possibilities. We know on a populational level that clines repudiate essentialist ideas of race, that the term has as much ontological substance as "genus" or "phylum," but, on a day to day level we label people as "black" or "white," not 76.45% African on the neutral genome markers, or 94.5% European on the neutral genome markers. But, if we are to talk about human populations in a scientific fashion, to build models, throw up hypotheses to test, then we need to delve into the deep truth of populational thinking and abandon the superficially useful labels in the "common" world.

Over the past 5,000 years a special social system has emerged which has resulted in the generation of a niche for certain individuals to insulate themselves from the caprice of the world, to live a relatively controlled experiment in their lives. Consider the case of Charles Darwin, an independently wealthy gentleman who devoted his life to the study of Natural History. Consider Aristotle, who made his living as a tutor, exploring his own ideas with youths while being paid for it. We are not Bayesian conditional probability calculators because we haven't had to be, the set of possibilites that we will encounter is a tiny scattering in the vast sample space. A few humans though have found a niche where they could explore the nuances of the order of the universe and let their mental capacities be stretched into bizarre counter-intuitive contortions. Today we call these individuals scientists.

Science is hard, science is abnormal, and beware the bewitchment of "common sense." Qasars and quarks, random genetic drift and DNA, such things are difficult to grasp with common sense precisely because their sensory reality is excluded from our universe, not only do we not have direct experience as individuals our species' minds have never been shaped by the patterns and rules which emerge out of their interlocking dance with the rest of the reality. Evolution is the father of this situation, it has equipped us to recognize faces, to keep track of social relationships and fall forward in a controlled fashion without thought. But, evolution could not shape us to understand itself, because it works over millennia, and there is no fitness advantage is conceiving of possibilities deep into the future when the concerns of the present loom large. Perhaps this is another argument against "species" selection, as a species that could "understand" evolution on a communal level might have been able to game the system, enter into new niches, escape overspecialization that might make them vulnerable to reccurent catastrophic events.

Ultimately the issue is not evolution, it is the profound complexity and technicality of the scientific disciplines in the modern world, and the lack of fluency with them that is the norm among lay persons. Evolutionary theory is also problematic because it intersects with the deeply emotional issue of human origins, and when emotion confronts rationality the wisdom of the crowds is not pretty. For the past few hundred years technological change has given science breathing room, the ability to take the population foward over new horizons and into strange lands, but who knows what the future holds?

1 - As you can see, there is regression to the mean since 50% of the variation on the trait isn't genotypically controlled.

Posted by razib at 10:01 PM