Adapted for what?

kw: observations, evolutionary theory

I am part way through a difficult book, and reading it spins off ideas every which way. The current chapter makes much of the concept of "selection-for", in the sense that physical traits manifest a direction imposed on natural selection so as to make a creature optimally adapted to its environmental niche. The notion of selection-for is advanced as a weakness of Darwinian (or Neo-Darwinist) evolutionary theory.

This illustrates a profound misunderstanding of natural selection. Darwin invoked artificial selection, which has produced hundreds of breeds from a few species of wild stock (such as Dobermans, Mastiffs and Pekingese from wolves), and used it as the basis for his introduction of natural selection. The problem is, one is purposeful, the other purposeless. So Darwin, if he leaned too hard on this analogy, as many say he did, left himself open to criticism. The core concept is this: artificial selection is purposeful, driven by conscious entities, and is indeed a series of selections for variations in traits that are desired by those entities; natural selection is selection against variations in traits that are less advantageous to the reproduction or existence of creatures as they cope with their environment.

The heart is used as an example. Two characteristics we might point out are that it pumps blood, and that it makes "lub-dub" sounds. Extreme adaptationism is held up as a straw man who says that both characteristics must have been selected for, because they both exist. I don't know anybody who believes the heart sounds were selected for explicitly. Natural selection is not influenced by possible futures; it cannot have "known", half a billion years ago, that one day doctors would listen to heart sounds to discern the health of their patients! The sounds are a side effect of the heart's action, just as the slapping sounds of your feet on the pavement are side effects of running or walking.

As a matter of fact, I suspect that loud heart sounds were selected against, because they'd make an animal more easily detected by a predator with good hearing. The present level of heart sound is a compromise between the energy needed to circulate blood and the risk of a predator hearing it happening. Perhaps this is why, though fight-or-flight terror causes the heart to race and get louder, total shock makes its beat weak, fluttery and almost silent.

A key point I have very rarely seen addressed, which is so far not mentioned in the book either (I'm halfway through), is that every creature is a work in progress. Evolution isn't finished with us yet. A few items in human evolution that I can think of off the cuff:

• Wisdom teeth. Many people have a mouth too small for them to fit, and have them removed (at least in the developed world; in poor places they just eat poorly and cry a lot). And some people are born with three or fewer, or even none. So if there were no dentists, never fear. In 50,000 years few people will have wisdom teeth anyway.
• Lower back pain. The back has been changing for the 2-3 million years that hominids and humans have walked upright. It is still changing. People 50,000-100,000 years from now will have a very different posture than we do, and there won't be any more chiropractors.
• The appendix. It serves an immune function in most mammals, but primates have a more efficient immune system and it is not needed. Its tendency to become infected, if there were no antibiotics or surgeons, would lead to its gradual elimination. As it is, however, future monkeys and apes are likely to have no appendices, but humans still will.

Selection happens all the time. When an environment has been stable for a long time, the creatures are well adapted to it and to each other, but still some more, some less. Creatures in a stable environment display a narrow range of variation. Let instability come in, and the resulting stresses will increase the variation in all the species, because of variable responses to stress that were masked when stress was low. The death rate will increase as larger numbers of creatures succumb; those that die were well enough adapted before, but closer to the edge than most. Now they are "unfit" for the changed environment and get culled. Meanwhile, generation after generation, mutations at every level continue to introduce new variations, some of which survive and reproduce better than others, leading to genetic drift "toward" adaptation to the new environment.

Introduction of a species into a relatively empty environment leads to adaptive radiation, like that seen for Darwin's finches. At one time, there were no birds on the Galapagos Islands. At some point a breeding colony of finches was established. There was variation in this population, and the plethora of kinds of food available led to variation in the finches. But it worked like this: pre-existing variations in the strength of the beak made some finches prefer grass seeds, while others could eat tougher seeds. Perhaps at first there was so much "easy food" that they all ate that. But of course the population outgrew the easy food source. The birds that could survive on the different foods gradually became several species, each adapted to a diet different from the others.

There were side effects to this. The finches that needed the strongest beaks, which were of course the thickest beaks, became physically larger. It takes a bigger body to support the heavier, larger skull needed to energize a real nutcracker of a beak. Larger body size wasn't "selected for" all by itself. It was part of the package needed to exploit the largest and hardest seeds.

It can be said that life is the ultimate energy filter. Energy from the Sun mostly just radiates to the edge of the Universe. Some strikes the Earth, energizing its biosphere. Left to itself, the Earth would absorb the radiation and re-radiate it at a longer wavelength, mostly to the edge of the Universe. But Earth is not a passive, vacuum-immersed globe. It has an atmosphere full of greenhouse gases such as water (by far the most efficient one). Just the presence of this wet atmosphere changes the energy flow, shifting the temperature of Earth's surface to a new equilibrium, some 30°C warmer on average, but with a much smaller total variation, compared to the vacuum-wrapt moon.

Then life gets involved. Chlorophyll converts some of the light energy into electric charges, enough to promote a chemical reaction that converts oxygen and carbon dioxide into carbohydrate. It incidentally reduces the planet's temperature a couple of degrees in the process. Chlorophyll-bearing plants, algae and cyanobacteria are eaten by animals (and protists and certain bacteria). And, of course, animals eat all of the above, up to top predators that are typically eaten only by scavengers and "decomposers" when they die of elderly infirmities.

Consider an early Earth, with lots of energy flow but little life, yet. The environment seems poor, but is actually rich for the creatures that exist. There is more energy available than they can make use of. Easy sources are used first. The population grows until all of the easiest source is taken. Those creatures that, because of chance variation, can exploit less easy sources, gradually become new species. Over time, at any trophic level, this leads to broad, rich niches becoming narrower and narrower, and species multiplying. This picture is complicated by all the side products of these burgeoning species, which provide energy to creatures that could not have existed before (lots of these become parasites).

No matter where you start looking at the situation, whether today, or during the age of dinosaurs, or a billion or two billion years ago, the central fact is that this is a situation of profound disequilibrium. If all humans were to leave the Galapagos Islands, and keep it quarantined for a million years (we should exist so long), there would then be more finch species than there are today, each zealously exploiting a narrow niche defined by food source. There would also be more species of most kinds of the creatures there, perhaps even of the giant tortoises.

Every one of those species would be a work in progress: well adapted to its niche, seemingly optimally so. But look beneath the surface. Finch species A's population is expanding, putting pressure on Finch species B, which doesn't quite eat the same sort of seed, but there is some overlap of food source. Over time, Finch species B could change, maybe just a little, so it can better withstand the competition from A…or it faces extinction. More likely, a sub-population from either A or B may begin to specialize on the "overlap" food source, and eventually lead to a new species, muscling out both A and B from it, until both of them quit using it (At that point, A and B may have become new species, replacing their former selves; there's more going on than just food preferences here).

Well, I've come far afield with this selection-for rant. This is the take-away. Every species is a work in progress, no environment is totally stable, and tomorrow will be different from today. Natural selection is our name for the differential death rate between variations that can either more easily or less easily take advantage of the resources provided by the present environment. Its partner, mutation, requires a rant of its own (Oh, goody! Now I have a subject for tomorrow or the next day).