Is life sloppy or not? Our last two posts discussed reasons to think it is, that it's orderly in many ways, but not so precise as to be characterized by laws like those of physical sciences. Yet, if we're just molecules interacting, how can life not be just as law-like? And isn't its law, or at least one of its laws, that evolution works by making every trait tightly adapted to its circumstances, just as Darwin argued and as many if not most life scientists (including those with scant real evolutionary training who plow the seas of biomedical genetics) routinely say it is?
Many years ago, I was doing some epidemiological research in Panama. During a break in data collection, I took a stroll along a path that wound up into the rain-forested hills out of the small town where we were working. I was watching parrots flying across the searing tropical sun overhead, and the lush tropical vegetation, which I'd never seen before, when I stopped to look at a butterfly sitting on a nearby leaf. Admiring its black eye and curvy antenna, I moved to get a closer look, which made the butterfly dart off....backwards!
Well, it didn't really fly backwards, and in fact it was I who was thinking backwards. I'd been tricked by a 'false' eye spot on its hind wing, thinking that was the head end. It was a remarkable example of the effect of protective coloration, and I felt like the predatory bird that such eye spots had evolved to trick, so that the butterfly could live to mate another time. At the time, I did not question the explanation, so impressed was I by my gullibility. Evolution in action!
Then, a few years ago, I had a somewhat kindred experience. Walking a long a path on the south rim of the Grand Canyon, I noticed a butterfly that landed on the sandy path. Its coloration was remarkably like that of the stones and sand. I could hardly see it, til it then flew off. Again, I mused, this is just what the textbooks say about one of the most clear-cut examples of adaptation.
But I've thought about these experiences since, and wondered how accurately they reflect the dogma of evolution as it is so often held, explicitly or implicitly. For example, while I did see the effectiveness of protective coloration in these two instances, I also did, after all, see the butterflies. I wasn't completely fooled. For the moment let's assume that the standard argument for protective adaptation is correct, even though it's based on our human abilities to recognize patterns, not those of the actual predators the butterflies have to deal with.
In the first case, had I been a predator, I'd have missed that butterfly. But I'd not have been tricked a second time! Likewise, while I marveled at the mottled butterfly largely hidden on the sunlit sandy path, I did after all, see it. Had I been hungry, I'd have had my meal.
Sill, one can see the potential that protective coloration gives the prey so that at least sometimes it will escape being eaten and live to reproduce. Even if it's not perfect protection, if it raises fitness just a tad, over the long haul it could represent a force-like, systematic genetic advantage that would work as Darwin said and as any biologist with physics-envy could want.
But then I wondered why most species in my surrounds did not have protective coloration. If it's so force-like and obvious, why are most birds and beasts easily seen? Why hasn't protective coloration worked so perfectly, as in a sense a Darwinian point of view would hold, that the ecosystem breaks down because predators can't find anything to eat? Or, put another way, why is each species' adaptations, whatever they be, different? Why isn't an ecosystem just one huge mottled pattern of camouflage?
Well, the Darwinian fundamentalist argument would say that each species has its own adaptations or it wouldn't be here. That is not much different from saying "God made it that way", except that an adaptationist (that is, one who assumes that every trait must be the result of systematic natural selection) says that there is a physical reason for the traits that relates to its having out-competed other variants in its past. Our job in science is just to find out what the reason was.
So some species, the story goes, fly irregularly in a way that distracts the pursuing bat or bird, or makes it hard to catch. Some have protective coloration. Some only come out at night. Some herd together so predators have a hard time finding which one to pursue and end up missing all of them, except maybe the elderly who are past reproduction anyway (and don't cost the prey species any fitness). Some species taste bad so birds avoid them while others taste good but mimic their bitter contemporaries. Some run fast or fly or climb to escape predators, while others freeze and appear dead to avoid detection. And so on.
Or, because species move around and environments change, some non-camouflaged butterflies might do well to become so, but are recent immigrants from a zone where they didn't need camouflage or perhaps had it in that environment. Of course, they'll now have to undergo adaptation in their new environment if they're to survive and persist there.
Why are there so many kinds of sparrows, or grasshoppers, or grass just in my back yard? The idea that each has its own specific 'niche' sounds good, but is often hard to accept, even just on the face of it.
Now, within each of these stories would be sub-stories of how the individual aspects of the traits mentioned evolved, piece-meal, over long time periods. Think how many changes it makes to make a bird out of a non-flying reptile. Or just to mold and color a butterfly's wing to give it the right sort of eyespot and antenna-like tail shape. And similar ad hoc, or perhaps better described as post hoc, stories need to be told about how the predator hones its detection and pursuit skills so it doesn't just starve into extinction first.
The gist of the point here is that each story is so different that it becomes in a way self-fulfilling in terms of overall adaptation arguments. One can, literally, always concoct some adaptationistic story about every trait in every species in every environment. That is the sense in which evolution is either essentially tautological or so ad hoc as not to reflect the kind of law-like rigor we see in physics.
The strange fact is that at some level these stories seem to be true!
Even if each story is in essential ways completely true--and many of them seem so plausible that one needn't doubt their truth--living nature is much more pixillated and less unitary in its evolution: each story is of the here and now, the factors and forces changing over time and place. Each story is different and indeed is changing all the time. That is an essence of an evolutionary principle. It is, in a sense, the common basis of evolutionary theory. It isn't controversial at all. But it is very far from the kinds of forces we see in physics and chemistry, or the precision of mathematics.
The problem is that most of these stories cannot directly be confirmed. Among other reasons, whether you're eaten, or catch prey, is highly probabilistic--has a high level, even a fundamental, component of chance. Since the difference at any given time between most genetic variants related to some aspect of the evolutionary race are usually very slight, having their purported effect over thousands of generations, the difference at the gene level can be essentially undocumentable: we could not, perhaps even in principle, detect the differences with samples that we could actually obtain.
For the same reason, the outcomes themselves are also unpredictable even in principle. This is so even if one assumes, as many biologists explicitly or implicitly do, that nothing in our makeup or genomes is without function, and natural selection detects all functional variation (a key point in Darwin's own thinking). But that is an assumption and it makes adaptation explanations often unfalsifiable and to a great extent just stories empty of scientific content.
In fact, referring back to our series on whether probability exists, it is impossible in such a probabilistic world to show that something in life has exactly zero effect, or indeed to show that small effects are different from zero. That means that much of our supposedly force-like story can't be confirmed except by using subjective judgments about statistical results in our studies--and that directly implies that Nature isn't doing what we say it's doing either.
Even if every aspect of the stories were true, and even if demonstrably so, it means that much in life is unique and that any law that applies is itself going to have to be ad hoc, different for each pixel at every time. Indeed, and perhaps strangely, this is the essence of life, and is squarely within evolutionary theory if we but think carefully about it. But that's very different from laws like gravity or the ideal gas law. The foundations of modern science include repeatability and the similarity of observations underlying a law of nature. But life is all about local, contingent, accumulating dissimilarity.
Causation in life is a kind of spectrum, with some causes so systematic and obvious that classical theory applies very well to anybody's satisfaction, and that fact is indeed the addictive hook that makes us think it will apply everywhere. But they are the relatively rare exceptions to the general, more pixillated story, where the story seems to fit but really isn't nearly so clearly observed.
In that latter sense, the laws of life are about difference and things not being exactly repeatable. That's why I can spot a camouflaged butterfly....but only sometimes.