v1.2.2 / chapter 17 of 28 / 01 aug 08 / greg goebel / public domain
* Evolution by natural selection does not simply happen in an arbitrary fashion: it is driven and focused by selection pressures. One of the major components of evolutionary science is determining the circumstances, the "game strategies", by which this occurs.
* Critics of Darwinism often point to certain organisms or features of organisms and insist that there is no way that natural selection could explain them. However, Darwin's advocates have been very energetic in considering evolutionary "game strategies", and in fact it's less often true that there is no explanation than that a number of explanations are available.
In many cases, the game strategy is obvious and unarguable. Consider gazelles and the cheetahs that prey on them. A faster gazelle means that slow cheetahs are going to have a hard time making ends meet as predators. That means faster cheetahs survive. Faster cheetahs mean that slower gazelles are likely to end up as lunch -- which means faster gazelles survive. Gazelles and cheetahs are locked in an "co-evolutionary arms race".
The race can drive Darwinian evolution at a very rapid rate. It provides an explanation for the 19th-century illusion of "orthogenesis", the idea that organisms will evolve in a particular "direction" on sheer "momentum" -- when in fact they simply being driven in a particular direction by strong external selection pressures. The race only slows down when it starts running into diminishing returns -- for example, if the adaptations of a female cheetah for running begin to pressure her ability to bear a litter of kittens, or if longer legs tend to break more easily.
As the two players in the arms race begin to reach their limits, entering a "Red Queen's race", their paths of evolutionary change are likely to go off in unexpected new directions. In particular, once gazelles and cheetahs can't get any faster, they obtain survival advantages other than speed, such as better camouflage and better weaponry. Of course they were likely also acquiring such adaptations to an extent while they were engaged in the race for more speed.
One very profitable evolutionary avenue open to them is to become smarter. Once begun down the path of increased intelligence, the evolutionary arms race between smarter predators and smarter prey would continue to drive the evolution of further increased intelligence. Careful analyses of the volumes of brain cases of fossil skulls, compared to estimates of body weight, show that modern mammalian carnivores and herbivores have bigger brains than their ancestors and are presumably smarter.
* The notion of evolutionary arms races suggests the old Social Darwinist concept of a ruthless nature, and that vision clearly has plenty of basis in reality. However, arms races can also drive strategies for cooperation. The most obvious scheme along this lines is the herd (or flock or troop or whatever), in which individuals operate as a group to obtain more eyes, ears, and noses to watch out for predators.
The herd also provides a defense in simple terms of numbers, in that a predator necessarily will only attack one side or the other of the herd -- an animal on the perimeter of the herd is partly protected on the interior side of the herd by the potential sacrifice of a herdmate on the other side, while an animal inside the herd has some protection all around. In some cases, the herd allows ganging up on enemies: a leopard might be willing to take on a single full-grown male baboon, but taking on four or five of them is a different matter. Of course, predators can also adopt the same cooperative tactics, the classic example being the wolf pack.
The harem is a common variation on the theme of the herd or pack, with a single male controlling a small herd of females and juveniles. From the point of view of natural selection and the "selfish gene" the harem concept makes perfect sense: a dominant male obtains reproductive control over a group of females and ensures the propagation of his genes. Such cooperative associations also lead to altruistic behavior, for example mutual grooming in troops of primates.
* Where the theme of cooperation gets more interesting is in interspecies cooperation. The most famous scenario is that of cleaner fish and shrimp -- small striped sea creatures that clean parasites not only from the bodies of larger, often predatory fish, but will even clean the mouths and gills of their cooperative hosts. The scheme obviously got started from small fish daring to dash in and sneak a meal of parasites infesting the bigger fish. The bigger fish that were more tolerant of this trickery would have enhanced health and would reproduce more successfully. At the same time, the cleaners developed distinctive color schemes that allowed them to be recognized as helpers and not as snacks.
There is a wide range of symbiotic relationships between species, for example plants that provide nodules, sweet sap, or even convenient living spaces as an encouragement for ants to take up residence, where they drive off potential plant-eaters. Where these symbiotic relationships become really spectacular is in the relationship between pollinating plants and their pollinators. The relationship between plant and pollinator can become so tight that a particular plant will focus on a limited range of pollinators, and those pollinators will only tend a limited range of plants. For example, red flowers with a long throat are optimized for pollination by hummingbirds -- the red color specifically attracts hummingbirds, and the long throat means that long-billed pollinators are best suited to access the nectar.
This makes perfect sense under Darwinism as well. Plants that developed mechanisms to attract pollinators would improve the odds of propagation; as more specialized mechanisms to support the relationship between plants and pollinators arose, they would tend to branch off in a number of ways, with one strategy being for a plant to channel access to a limited range of pollinators. This ensures that its pollen won't be wasted on unrelated plant species, and the pollinators obtain a "reserved" source of food not available to competitors.
Critics have suggested that such tight symbiotic relationships are inexplicable under Darwinism, along the lines of the challenge of the eye and sea dragons but worse, that there was no way such arrangements could have been created by accident and incrementally. Richard Dawkins once got a letter from a reader who said he had become convinced Darwinism was wrong after watching a nature show on TV that showed plants with "dummy" female wasps to attract pollinating male wasps. The reader insisted this couldn't happen under Darwinism, because the scheme wouldn't work unless the "dummy" female wasp was perfectly convincing.
The letter was almost too easy for Dawkins, since his formal background was as an animal behaviorist. He knew perfectly well from a wide range of experiments by his colleagues that insects are almost completely "hardwired" in their behavior, and trick themselves very easily. As he pointed out, a wasp is about as intelligent as a washing machine -- okay, maybe a high-end washing machine, maybe a bit smarter, but they're still not in different leagues. Could male wasps have been tricked on occasion by plants whose flowers had some vague cues that by accident matched the cues of female wasps? Sure, why not? And once that happened, the natural selection treadmill got going, with plants in the population featuring flowers with more resemblance to a female wasp than others gaining the edge in spreading pollen. The plants gradually evolved from fooling some of the wasps some of the time to fooling most of the wasps most of the time.
The idea that the symbiotic relationships between flowers and pollinators are a challenge to Darwinism doesn't stand up to inspection. Not only is it possible to envision game strategies by which such partnerships could have arisen, there is also the fact that there are so many different types of pollinators: bees, wasps, moths, butterflies, beetles, hummingbirds, lorikeets, bats, and even the Australian honey possum -- the only nonflying mammal to take up the trade. This suggests opportunism and improvisation, with entirely different lineages of animals acquiring adaptations and getting their own piece of the action. The geographic distributions of pollinators suggests an evolutionary origin as well: why are hummingbirds only found in the New World, why are lorikeets only found the Asia-Pacific region? Why Design two different solutions to the same problem? And in the case of the lorikeets, the idea that they are simple adaptive derivatives of the parrot family is hard to avoid.
It is actually hard to see that there is anything fundamentally more inexplicable about the co-evolution of cooperative species than there is about the co-evolution of antagonistic species. Predators can acquire highly specialized adaptations to hunt down prey, while prey species can acquire highly specialized adaptations to defend themselves from predators. For example, our immune system has impressive elaborations to defend us from pathogens, while pathogens have developed some very sneaky ways to evade our immune system. From the point of view of Darwinian evolution, both cooperation and antagonism are useful strategies -- Darwinism is opportunistic, whatever works. In fact, the dividing line between the strategies of cooperation and antagonism is not rigid: there are small fish that impersonate cleaner fish, allowing them to sneak little bites out of unsuspecting host fish, exploiting the strategy of cooperation to antagonistic ends.
* It is also puzzling that the critics make so much of interspecies cooperation because it's so common, coming across on examination as perfectly ordinary, hardly freakish. There are particularly significant and widespread examples of symbiosis at the level of microorganisms. The examples of the eukaryotic cell, with its obvious origins as a symbiotic assembly of microorganisms, and the lichens, cooperatives of fungi and algae, have already been mentioned. There is also the case of the "nitrogen-fixing bacteria", Rhizobium, which lives in nodules on the roots of plants such as peas, being given a home while it converts atmospheric nitrogen into compounds required by its plant host.
A particularly intriguing example of symbiosis at the level of the microorganism is our intestinal flora, the "endosymbionts" that live inside our bodies, some of which we absolutely need to stay healthy -- if we were to kill off all the bacteria that inhabit our guts, we'd be dead in a month from vitamin deficiencies. Herbivores are even more dependent on their intestinal flora, with the endosymbionts digesting the cellulose in grass or leaves to produce by-products that the host can then digest itself. Termites similarly carry gut microorganisms to help digest the cellulose in wood. Indeed, micro-organisms seem to have a near-monopoly on cellulose digestion, with herbivores being as a strong rule dependent on them to make a living.
Some critics have suggested that this is puzzling, that under the endlessly repeated improvisations of Darwinian evolution not all herbivores would have such a dependency. Actually, not all do: most, possibly all, termites can produce their own "cellulases" -- cellulose-digesting enzymes -- though they also rely on endosymbiotic microorganisms to help get the job done. Wood is likely the hardest thing in world that's conceivably edible to actually eat, and termites need all the help they can get to eat it.
Even if termites didn't produce cellulases, it is obvious that microorganisms, with their brief life-cycles, evolve much more rapidly than large herbivorous organisms, and so by the simple odds the microorganisms are likely to beat their hosts to cellulose digestion by a comfortable margin even under the least favorable circumstances. In fact, Darwinian evolution is actually braced up by the fact that there are a number of different endosymbiotic arrangements in the digestion of herbivores. Even just among termites, there are some species that depend on an assortment of bacteria to do the job, while others rely on protozoans. If the system were Designed, why have multiple Designs?
Incidentally, in the case of the protozoan named Mixotricha paradoxa in the gut of the Australian Darwin's termite, the protozoan itself is a marvel of Darwinian improvisation. It has a symbiotic relationship with spiral-shaped "spirochete" bacteria that dock in conveniently accommodating recesses on its surface and, waving about, provide propulsion, and it also has endosymbiotic bacteria that seem to be helping it with the job of digesting wood. In fact, this "patchwork protozoan" has a total of five different species of bacterial symbiotes, suggesting the biological equivalent of a developing-world jitney bus with riders hanging to the top and sides.
Another interesting question raised by critics of Darwinism is why humans haven't obtained endosymbiotic bacteria to help them digest grass. Surely this would provide a significant selective advantage, so why hasn't it happened? The first half of the answer to this question is that, as far as Darwinian evolution is concerned, nothing is specifically fated to happen, and we just didn't draw that ticket in the evolutionary lottery. The second and more interesting half of the answer is that drawing that ticket, becoming an herbivore, is not as simple as just obtaining intestinal bacteria that can digest cellulose. An herbivore needs a mouthful of flat, grinding teeth to pulp leaves and grass, and an expanded digestive tract to cram full of plant matter for the bacteria to digest. Even then, grass and leaves are not a very efficient food source; an herbivore spends a large proportion of its time eating and digesting. Without the specialized teeth and digestive tract, the herbivore might well end up spending more energy trying to stay fed than could be obtained from the food.
In other words, being an herbivore is a way of life and not something that could be picked up from a mutation or two in intestinal flora. Herbivores are adapted and committed to that approach to staying fed, to the general exclusion of alternatives. Claiming to find a fault with Darwinism simply because humans can't eat grass is on the order of finding fault because they can't fly.
There are in fact primates, such as colobus monkeys, that are herbivores, subsisting on leaves, and they have the adaptations to support that lifestyle. They spend roughly three hours a day feeding and seven hours a day just resting, digesting the leaves. Could ancestral leaf-eating monkeys along the lines of the colobus monkeys have ever evolved to human levels of intelligence? There's no way to prove they couldn't, but there are good reasons to see it as a bad bet. An existence of stuffing oneself with leaves and then sitting around waiting to digest them seems unlikely to be one that drives a high degree of intelligence. Furthermore, a big brain requires a lot of energy to support, and a diet of leaves is a poor way to support it.
* We tend to think of most microorganisms as parasites, and indeed they very often are, so it's a bit surprising to find they can be helpful. Actually, it's a perfectly straightforward ESS. A parasite that injures or kills its host is engaged in a "cut and run" strategy -- which is perfectly workable, but it has a disadvantage in eliminating the parasite's gravy train and requiring the parasite to find another. One of Gary Larson's FAR SIDE cartoons of the 1990s illustrated this neatly, featuring a flea carrying a placard to warn his brethren: THE END OF THE DOG IS NEAR.
Killing off a host isn't such a problem if the parasite can easily find a new one, but if it can't, the parasite has a Darwinian incentive to come to an accommodation with its host, or even improve the host's chances of survival. The host similarly has a Darwinian incentive to make things easier for a symbiotic microorganism, calling off the dogs of the immune system and even providing accommodations to make the symbiote's life easier. Unfortunately, while there is a tendency for parasites to become kindlier and gentler to their hosts, the "cut and run" strategy is still a practical option in the Darwinian lottery as well, as demonstrated by intestinal flora that occasionally turn lethal.
The two approaches remain in tension with each other. Symbiosis becomes less neatly Designed and more obviously Darwinian in appearance when it is realized that symbiotic "happy marriages" can go disastrously bad on a mutational whim. It seems even more Darwinian when it is realized there are intermediate cases. For example, there are tapeworms that infect mice that produce analogues of mouse growth hormones to make their mouse hosts grow. Is this a benefit for the mice? Maybe, but it seems more reminiscent of a technique for fattening up livestock. As far as both symbiotes and parasites are concerned, what they do is strictly to promote their own propagation, there's no payoff for them in doing anything else. If it helps the host, fine, if not -- that's just too bad for the host.
* Sometimes evolution seems to go off in bizarre directions. The most famous example is the peacock, with its elaborate set of tailfeathers. Everybody knows that it's to attract peahens, but from a strictly functional point of view it's absurd -- a bit reminiscent of a woman who has been made up with an elaborate coiffure, which can look very nice but is a real pain to keep that way. Incidentally, the peacock's fan technically isn't really a tail, the long feathers being produced from the bird's back, and nitpicky sources prefer to call it a "train", like the train of a bride's gown.
The peacock's tail gave Darwin the shudders, trying to wonder how natural selection might account for it. In his plodding and thorough way, he came up with the notion of sexual selection, that it occurred because peahens had a preference for peacocks with larger tails. The idea was supported by the fact that only one gender had the ornamentation, peahens being generally unadorned creatures; sex clearly had something to do with it. He elaborated on sexual selection in great detail, but his scientific successors all but abandoned the idea because it seemed to arbitrarily depend on the whim of peahens. Why did peahens like the elaborate tails? Darwin could only say: "Just because."
The only problem was that nobody else could come up with explanations that were even as good. Alfred Russel Wallace claimed that all ornamentations such as the peacock's tail were actually a sort of camouflage -- a notion which certainly did not catch on, and later he decided, much more sensibly, that they demonstrated the health of the animal to prospective mates. Julian Huxley thought they were to intimidate other males. Some thought they were flags to tell females they were breeding with the right species -- though why females of other species didn't need such spectacular recognition flags was unclear. Some thought they were warnings to predators, like the colors of nasty-tasting butterflies -- though peacocks were hardly inedible.
Ronald Fisher finally revisited sexual selection and put it on a stronger basis. In a simplified fashion, Fisher's thinking not always being easy to follow, he observed that, with the exception of sex chromosomes, males and females of a species shared the same genes. That would mean that if females were attracted to males with the longest tails, their progeny would include genes for both longer tails and attraction to longer tails, with the first being expressed in males and the second being expressed in females.
Fisher pointed out, in mathematical terms, that this synergy between male ornamentation and female desire created a positive feedback loop, or "cascade" process. Peahens were attracted to proto-peacocks that had long tails; these cocks bred with hens that liked long tails. Cycle through this process thousands of times and it might simply go exponentially out of control, the tail of the peacock growing very rapidly over generations until it became enough of a liability that it balanced out his enhanced ability to attract hens and sire chicks. Fisher wasn't so clear on how the cycle got started, speculating along the line suggested by Wallace that peahens obtained a selective advantage by being attracted to healthy males with long tails.
Incidentally, in some species where the male is gaudily decorated, such as mandarin ducks, the coloration only lasts as long as the breeding season. Once sex isn't the issue of the day, the drake loses his bright plumage and ends up being hard to distinguish from the drab hen. This is another thing that's easy to explain in under Darwinism, since once the drake is no longer in pursuit of hens, the bright plumage is a liability, making him more vulnerable to predators.
A few of the ideas for the rapid and extreme expansion of human intelligence were discussed in a previous chapter. Some researchers have added to that list, not implausibly, the notion that maybe the big human brain is a product of sexual selection. One reason for this belief is that human brain size evolved at an extraordinary rate that is characteristic of the tight feedback of sexual selection. By this notion, it appears that female prehumans didn't generally "like them big and stupid."
On a related note, a human male's penis is on the average four times longer than that of a full-grown gorilla's. Is there any particular advantage from a simple efficiency point of view in the much bigger size -- does size really matter? Or is it just an arbitrary trait of our species, driven possibly by sexual selection and positive feedback? It certainly is a bit comforting for us human males to know that in comparison to our body weight, our penises outmatch those of males of the species near to us. But this line of discussion need not be pursued further here.
* An Israeli evolutionist, Amotz Zahavi (born 1928), later came up with an interesting observation on the peacock's tail and other elaborate sexual ornamentations that became known as the "handicap principle", something of a follow-up to the notions of Wallace, in which Zahavi suggested that there wasn't any element of whim in it at all. The idea is that the peacock's excess of ornamentation is not mere useless flash, it's a billboard that tells the peahen the cock is so healthy and promising as a prospective father of the hen's chicks that he can afford to carry around such excess baggage, and has survived in spite of it.
It was long recognized that the peacock's tail was a form of sexual advertising, but the handicap principle gave the advertising a specific message. The idea is along the lines of a wealthy man about town who lights cigarettes with $100 dollar bills to impress the girls: "I'm so wealthy I can afford to simply burn up money." The important feature of the peacock's tail is that it's an honest message, since it would be very difficult for an unhealthy peacock to support an impressive tail. If there were some way to cheat on the scheme, with unhealthy males performing false advertising, it would not provide as great a selective advantage and would be winnowed out by natural selection; fly-by-night operations are not in business over the long term.
The handicap principle is interesting but has been somewhat controversial, its critics suggesting that it sounds a bit -- or maybe more than a bit -- like yet another "just so" story. Dawkins said that the logical conclusion of the handicap principle would be the evolution of males with one leg and one eye. Zahavi promptly shot back: "Some of our best generals [in Israel] have only one eye!" (For what it's worth, it might be noted that one-eyed Israeli general Moshe Dayan was a notorious and highly successful skirt-chaser.)
However, as Dawkins conceded, the more the idea is considered, the more interesting it becomes. Population genetics models do provide some support for it and it has caught on, at least to an extent, being reinforced by examples from nature such as "stotting". This is a behavior among certain gazelles in which they perform energetic bounds up into the air when they spot a predator, such as a lion. The behavior seemed bizarre, almost as if daring the predator to attack, far more activity than would be needed to provide a warning to the herd and a waste of effort that might be better spent in escaping from the predator. The reality is that predators sensibly prefer to take on older or weaker prey. The stotting may be an honest advertisement to the predator: "I'm so healthy that I can afford to bound up in the air -- you won't catch me, don't bother, try somebody else."
The concept of "honest advertisement" also helps solve another dilemma of sexual selection. If peahens only mate with the showiest males, the population of peafowl gradually converges to a genetic uniformity that will be a liability over the long run. However, the "parasitic Red Queen's race" now enters the picture: without genetic diversity, pathogens soon increase their stranglehold on the species, and peacocks with the more "traditional" genomes increasingly find themselves sickly and no longer able to provide resources to maintain their flashy tails. That gives the edge to peacocks with less "traditional" genomes: the last shall be first, the first shall be last.
* The tail of the peacock ends up making more difficulties for Design than it does for Darwinism. One critic protested: "Why would natural selection ... produce a species whose females lust for males with life-threatening decorations?" The obvious answer is: why would a Design process produce a such a species? Is it the "dimwit Designer" at work, whimsically adding bizarre features to organisms, like the infamous tailfins of a 1959 Cadillac -- raised to a meter in height? Was He on recreational drugs?
As the discussion above shows, it's perfectly explicable under Darwinism. The lengths to which organisms will go to just reproduce may seem surprising, but the answer to that is: just reproduce? In Darwinian evolution, reproduction is the goal of the game, everything else is just in support of that goal. The competition to reproduce can be very intense and can drive significant adaptations. Male field crickets will sing away loudly at night, and the loudest among them will get mates. However, they will also attract parasitic flies that lay eggs in them, the eggs hatching into larva that eat the crickets alive. Where parasitic flies are common, cricket singing fades out; when the flies are suppressed, the crickets will soon evolve back to singing as loud as they can. It might be nice to have some form of sexual display that doesn't attract unwanted attention, but that's not how Darwinian evolution works: the crickets are trapped, and end up being forced to accept the bad with the good.
* Critics of Darwinism tend to find the discussion of evolutionary strategies particularly irritating, since it can come across as an attempt to cover all the bases. Convenient rationales are invoked as needed, with a wild range of strategies invoked to show how different evolutionary processes might have taken place. There is a saying in the sciences that a theory that explains everything explains nothing, and the critics accuse evolutionary game strategies as being exactly such tapdancing.
In reality, nobody ever claimed that Darwinism explains everything. Its only goal is to explain "the origin of species", and that's all it does -- as noted, it doesn't provide the faintest explanation of the origins of life itself. It is not a "Master Theory Of The Universe", as might be found on some crackpot's website, any more than is the law of gravity. If Darwinism is in fact an accurate theory, then it's not surprising that it covers the bases as far as the origins of species are concerned -- just as the law of gravity covers the bases for the attractions of masses. If a theory actually works, then it would be expected to be able to explain things in its domain, and a sign of its sophistication that it can. Science writer Carl Zimmer parodied the critics by imagining their complaints against Darwinism translated to the theory of plate tectonics:
BEGIN QUOTE:
Isn't it amazing how everything seems to provide evidence for plate tectonics? Continents split apart. That's plate tectonics! Continents crash into each other. That's plate tectonics?! Plates sink under other plates. That's plate tectonics. Although some plates actually slide past each other. That's plate tectonics. Not only is everything evidence for plate tectonics, but plate tectonics explains everything! No it's not circular reasoning, it's plate tectonics!
END QUOTE
Is Darwinian evolution circular reasoning? If climate grows colder, a mammal will get a selective advantage from thicker fur; if climate grows warmer, a thinner coat will provide a selective advantage. A coyote from the frigid mountains of Idaho has a thick, luxurious coat, while a coyote from the warm swamps of Florida is a shorthair. In a forest, a predator will get dark coloration, in the arctic, it will have white coloration. Maybe creatures in similar ecological niches will look alike, sharks versus dolphins; maybe they will look different, kangaroos versus gazelles. Where's the problem? If organisms aren't Designed, then we would expect a wild variety of "solutions" -- whatever works -- and even if they are Designed, organisms will have the "solutions" that fit them to the environment and lifestyle.
It is precisely the broad applicability of Darwinian theory that makes scientific confidence in it so high. Serious investigation of the species of organisms on the Earth, at every level of detail, gradually fills in a picture that is entirely consistent with evolution by natural selection. The machine works, we can see its wheels moving and meshing, and with every new wheel we discover, the machine becomes more complete and convincing in its operation.
The critics are simply trying to play "heads I win tails you lose" -- proclaiming Darwinism false if it can't provide explanations, while proclaiming it suspicious if it can. In the meantime, they are carefully avoiding the fact that Darwinism can provide far more detailed explanations than Design, which given its reliance on "just happened" is much more vulnerable to the charge of explaining everything and explaining nothing.
* However, the critics do have a legitimate point. Evolutionary game strategies give scenarios for how organisms might have evolved, but it may be hard to give much solid proof that a given game scenario actually drove a process. In some cases, for example co-evolutionary arms races, the game strategies are so obvious that it takes a minimal amount of imagination to accept them and skepticism to a fault to reject them, but the game scenarios can become increasingly baroque until they seem like nothing more than idle speculation, neither provable nor disprovable. We can look for examples from nature to find corroboration or put together computer models to see how well they might work, but that's about it.
Why, for example, are humans mostly hairless when mammals are typically covered by fur? It hardly seems like a neutral feature, but if it's an adaptation, then for what? The most popular explanation for the time being is that it made it easier to deal with parasites; unlike most other mammals, humans could make clothes and could afford to lose their fur. There are also suggestions that sexual selection played a part, that females preferred hairless males because they were less likely to have parasites. Actually proving any of this would be difficult, and such thinking has to be regarded as not much more than plausible educated speculation. As far as some of the scenarios put forth by evolutionary psychologists go, even many evolutionists find them hard to swallow.
This is not to say that considering the possible paths of evolution by natural selection is pointless. If evolution by natural selection is accepted, then it is an appropriate, in fact unavoidable question to ask: Can it really work, and if so, how? The critics like to point to particular biosystems and claim there is no way to imagine their origin through natural selection; if evolutionists then respond to the challenge and imagine one or more ways of how they could have happened, then the critics are in a poor position to complain. In practice, they inevitably call the scenarios "just so" stories and demand proof, but since the only issue they originally raised was one of imagination, that's "moving the goalposts".
It is admittedly hard to see that evolutionary game strategy provides much persuasive proof of Darwinism in itself, but there's plenty of proof elsewhere -- and it's just as hard to see how anyone could use it to disprove Darwinism. The critics keep trying to play the game of "stump the evolutionists", throwing out an exotic example of a biosystem and demanding an explanation, but it's strictly a propaganda exercise. The evolutionists will always go through their kit of basic principles and enormous stockpile of evidence from nature to piece together a scenario or two. It's just too easy to do.
* The evolutionists may also, if they're feeling snarky, pointedly ask the critics why they find it so difficult to provide any detailed scenarios for their own concepts. How do the critics explain why humans are mostly hairless? "Just happened? Just because?" Or do they just proclaim the issue uninteresting or unanswerable and sweep it under the rug? As far as evolutionists are concerned, as John Maynard Smith put it: "Confronted with feathers, or eyes, or ribosomes, we cannot not ask what they are for."
It is precisely this obsessive curiosity that every good scientist knows is at the core of science. As much as it annoys the critics, when scientists run into an unexplained phenomenon, the first thing the scientists do is start speculating about possible explanations. What else would they do? Stand around and wring their hands? Proclaim there's no chance of figuring it out? Scientists speculate a great deal, throwing out "what if" ideas to see if they stand up to evidence and logic. If they don't, the ideas are modified or discarded; if they do, they provide a basis for more substantial investigation. Some speculations are too flimsy to be taken seriously, but some seem too insightful to be easily dismissed.
Consider the issue of "morning sickness", the inclination of otherwise perfectly fit human females in the first trimester of pregnancy to suffer from nausea and vomiting. It is a puzzling phenomenon -- if it were unusual, it could be dismissed as a health difficulty, but the problem is that it is normal, it is unusual for a woman not to have morning sickness. From an evolutionary perspective, it would seem there was a reason for morning sickness, since if it was merely a useless inconvenience, selection would have got rid of it. That would imply that it was either a side-effect of an adaptation that served a useful function or actually served a useful function in itself.
In the 1990s, a theory was put forward that morning sickness actually serves a useful function. The first trimester of pregnancy is the time when the human fetus is developing rapidly, assembling its basis elements, and during this time it is highly vulnerable to developmental interference from plant toxins and the like. It would then provide a selective advantage if the mother was very touchy about what she eats, avoiding toxins and allowing the fetus to develop normally. It might be argued that this is an example of Design at work, but it would seem inconsiderate of the Designer to inflict such a punishment on females for bearing children -- He might have come up with a less unpleasant fix. Its callous lack of consideration is perfectly compatible with Darwinism, and in case the Darwinian analysis turns something that seems to make no sense into something that makes perfect sense.
More recently, a study performed of pygmy tribes produced a Darwinian evolutionary model that was very difficult to credibly reject. While the small-statured pygmies of the African jungles are best-known, there are also unrelated jungle pygmy tribes in the Philippines and South America. Why should tribes of people who live in tropical jungle environments tend to be so small?
Careful analysis of statistics on these tribes produced two clear results: first, living in an environment where disease and parasites are very common, few pygmies lived into their late twenties; second, pygmy women had their first children at an unusually young age. Further examination of the data showed that pygmies grow at the same rate as other humans, they just sexually mature at a younger age. The conclusion was almost unavoidable: members of pygmy tribes who sexually matured at a more normal age didn't live long enough to produce children in the same numbers as those members who sexually matured earlier, and so selection gradually eliminated the members who matured at normal ages from the tribal populations.
* In any case, evolutionary scenarios and models, for all their clear limitations, do not reveal the weakness of modern evolutionary science so much as they demonstrate its sophistication and logical integrity. Once again, if Darwinism is really correct, then finding evolutionary scenarios would be expected. As Darwin himself put it, at the end of his discussion of the evolution of the eye: "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down. But I can find no such case."
