Natural and sexual selection produce adaptations. It seems so obvious to us now that Darwin has explained it to us.
But it turns out to not be so simple. Natural and sexual selection may act directly upon certain traits, and indirectly upon others; the trait may have been a side-effect of the real story of natural selection. Stephen Jay Gould and Richard Lewontin realized this as they looked at the spaces between the arches of a cathedral, and the artwork contained therein. The artwork was constrained by the spaces. The design of the cathedral was focused on the arches; the spaces between them, or spandrels, were a side effect. Later, Gould and Elizabeth Vrba expanded this concept. Some evolutionary changes occurred, like artwork in spandrels, within necessary constraints. Other evolutionary changes in traits resulted from natural selection acting on other traits; they called these changes exaptations rather than adaptations.
Some characteristics of organisms are, like spandrels, structurally inevitable. Consider the patterns of allometry. Large trees must have relatively thick trunks, and large animals must have relatively thick legs. An animal that is twice as large as another in all linear dimensions would weigh eight times as much but have legs only four times as strong; to make up for this, the legs must be more than twice as thick. They have to be the square root of eight times as thick. There is no choice in the matter. Natural selection may have made the animal larger, an adaptation; but the thicker legs are an exaptation.
One humorous example of an exaptation (three exaptations walked into a bar…) is the human chin. Why do we have chins, and chimps do not? Some scientists have really used their imagination on this one. They imagined that the chin served as a way of expressing threat gestures. No kidding; I read that. (Post a comment to tell us your favorite story about why humans have chins.) But it turns out that the human chin is not an adaptation at all; it is an exaptation. One of the major characteristics of human evolution has been neoteny, the retention of juvenile characteristics into adulthood. Neoteny affected the growth patterns of the bones of the skull; in addition, natural selection favored jaws and teeth suitable for an omnivorous diet in human ancestors. The chin was a side-effect of these different facial bone and jaw growth patterns. The chin was probably not, itself, selected for anything; natural selection acted upon neoteny, not directly upon the chin.
Sometimes a feature can evolve for one reason and then turn out to be useful for another. Here are a couple of examples. Today, birds use feathers primarily for flight. But the earliest birds (according to fossils discovered in 2009) did not use feathers for flight; they had down feathers, which held in body heat. Once they had feathers, natural selection could act upon the feathers to make them into flight feathers. Another example is one of the most famous evolution just-so stories, the neck of the giraffe. What a weird adaptation. Giraffes have to spread their legs awkwardly just to drink from a pond. (Three giraffes walk into a bar…) To most observers, it would appear obvious that long necks are an adaptation that allows giraffes to feed at the tops of trees. But if this is so, why do female giraffes have shorter necks than male giraffes, and why are giraffes so frequently observed actually bending down to eat leaves? Careful observations have shown that males with longer necks prevail in battles with one another, and that females choose the males with longer necks. Long necks apparently evolved by competition for mates. Now that giraffes have long necks, they can use them for eating leaves from treetops, but that is not the original reason that long necks evolved in these animals.
Creationists could come up with their own version of exaptation. They could say that God created sex so that humans could make babies. But once sex existed, it could be used just for fun. Sexual fun is an exaptation, or sexaptation, or ecstatic-taptation. But I am not aware that any creationists have made this argument.
Adapted from the entry “Adaptation” in Encyclopedia of Evolution (Facts on File, 2006).
Home » Archives for March 2010
Wednesday, March 31
Thursday, March 25
Imperfection
The previous entry, about death, brings up an important point about evolution: imperfection. Imperfection is one of the most important ways in which an evolutionary view of the world differs from a creationist one.
An all-powerful God ought to have created a perfect world. This God is supposed to have given humans free will and the ability to choose evil; therefore one might not expect any human utopias. But the natural world is also pervaded by imperfection. The only explanation creationists have for this is that God cursed the whole natural world during Adam’s fall—something that is simply not found in the Bible; it is something creationists just made up. I guess they think God gives them permission to just make stuff up and then proclaim it in God’s name. Stephen Jay Gould was particularly active in pointing out that imperfections are much better evidence for evolution than perfections.
The idea that organisms are perfectly adapted to their environments predates evolutionary theory. This idea was central to natural theology, in which the perfect fit of organism and environment was considered evidence of divine creation. English theologian William Paley, in his famous 1802 book Natural Theology, used perfect adaptation of organisms as evidence that a Supreme Being had created them. At the time Paley wrote his book there was no credible evolutionary theory that could challenge this view.
Natural theology is wrong in one particularly important way. Scientific investigation has found numerous examples of adaptation that are far from being a perfect fit between organism and environment. One example (Gould’s favorite) is the digestive system of the panda. The immediate ancestors of pandas were carnivores, but pandas are herbivores, living exclusively on leaves. Pandas have intestines that are better suited to a carnivore. In well-adapted herbivores, such as sheep, the intestines are up to 35 times as long as the body, while in well-adapted carnivores, the intestines are much shorter, only four to seven times as long as the body. With the help of bacteria, longer intestines allow herbivores more time to digest coarse plant materials, such as cellulose. Meat, in contrast, requires less digestive breakdown. Pandas have intestines that are in the carnivore, not the herbivore, range. The panda’s digestive system is, therefore, not well adapted to its function. Modern evolutionary scientists would attribute this to the recent evolutionary shift from meat to leaves in the diet of the panda’s ancestors: there has not been time for better adaptation in this case. A natural theologian, in contrast, would have a difficult time explaining this example of imperfection. Stephen Jay Gould used the sixth digit, or thumb, of the panda as another example of the imperfection of adaptation produced by the ongoing process of evolution rather than by the Supreme Being invoked by natural theology.
Another example of an imperfect adaptation is pain. The function of pain is to alert an animal to danger or possible damage. Victims of some kinds of strokes, and of leprosy, lose much of their ability to feel pain, and cannot feel the damage that they may do to their extremities. Healthy individuals feel pain and avoid movements that would damage their extremities. Excessive and prolonged pain, however, serves no useful purpose, and much of modern medicine is devoted to the control of excessive pain. Pain is therefore an adaptation, but an imperfect one—a fact to which all of us can attest. Those of us who experience little pain empathize with those who experience much, and if we were in charge the way God supposedly is, we would alleviate some of this pain.
The world is only as good as it has to be for organisms to win in the game of natural selection. It is not perfect, nor is there any evolutionary way for it to be. But at least evolutionists do not have to come up with an excuse for how a God of Infinite Love could allow pervasive and painful imperfections to mar the Creation.
Adapted from the entry “Adaptation” in Encyclopedia of Evolution (Facts on File, 2006, 2007).
An all-powerful God ought to have created a perfect world. This God is supposed to have given humans free will and the ability to choose evil; therefore one might not expect any human utopias. But the natural world is also pervaded by imperfection. The only explanation creationists have for this is that God cursed the whole natural world during Adam’s fall—something that is simply not found in the Bible; it is something creationists just made up. I guess they think God gives them permission to just make stuff up and then proclaim it in God’s name. Stephen Jay Gould was particularly active in pointing out that imperfections are much better evidence for evolution than perfections.
The idea that organisms are perfectly adapted to their environments predates evolutionary theory. This idea was central to natural theology, in which the perfect fit of organism and environment was considered evidence of divine creation. English theologian William Paley, in his famous 1802 book Natural Theology, used perfect adaptation of organisms as evidence that a Supreme Being had created them. At the time Paley wrote his book there was no credible evolutionary theory that could challenge this view.
Natural theology is wrong in one particularly important way. Scientific investigation has found numerous examples of adaptation that are far from being a perfect fit between organism and environment. One example (Gould’s favorite) is the digestive system of the panda. The immediate ancestors of pandas were carnivores, but pandas are herbivores, living exclusively on leaves. Pandas have intestines that are better suited to a carnivore. In well-adapted herbivores, such as sheep, the intestines are up to 35 times as long as the body, while in well-adapted carnivores, the intestines are much shorter, only four to seven times as long as the body. With the help of bacteria, longer intestines allow herbivores more time to digest coarse plant materials, such as cellulose. Meat, in contrast, requires less digestive breakdown. Pandas have intestines that are in the carnivore, not the herbivore, range. The panda’s digestive system is, therefore, not well adapted to its function. Modern evolutionary scientists would attribute this to the recent evolutionary shift from meat to leaves in the diet of the panda’s ancestors: there has not been time for better adaptation in this case. A natural theologian, in contrast, would have a difficult time explaining this example of imperfection. Stephen Jay Gould used the sixth digit, or thumb, of the panda as another example of the imperfection of adaptation produced by the ongoing process of evolution rather than by the Supreme Being invoked by natural theology.
Another example of an imperfect adaptation is pain. The function of pain is to alert an animal to danger or possible damage. Victims of some kinds of strokes, and of leprosy, lose much of their ability to feel pain, and cannot feel the damage that they may do to their extremities. Healthy individuals feel pain and avoid movements that would damage their extremities. Excessive and prolonged pain, however, serves no useful purpose, and much of modern medicine is devoted to the control of excessive pain. Pain is therefore an adaptation, but an imperfect one—a fact to which all of us can attest. Those of us who experience little pain empathize with those who experience much, and if we were in charge the way God supposedly is, we would alleviate some of this pain.
The world is only as good as it has to be for organisms to win in the game of natural selection. It is not perfect, nor is there any evolutionary way for it to be. But at least evolutionists do not have to come up with an excuse for how a God of Infinite Love could allow pervasive and painful imperfections to mar the Creation.
Adapted from the entry “Adaptation” in Encyclopedia of Evolution (Facts on File, 2006, 2007).
Friday, March 19
Darwin and Death
No wonder creationists think that the world is not very old. How could it be, if death is not an integral part of its operation? If plants grew but never died, they would bury the Earth. If animals ate and procreated, but never died, the world would be covered with them. Elephants would be like lemmings and lemmings would be like bacteria. A world without death (which is what the Garden of Eden supposedly was) could not have been designed to last very long. To creationists, death was a curse brought on by the sin of Adam.
To scientists, however, death is the process upon which ecology operates. And evolution too. Evolution has given each organism a built-in process of senescence, which is a gradual shutting-down of the body, which ends in death. Organisms are built to grow up, reproduce, then gradually shut down and die. Death is not a mistake. It has been produced by natural selection.
If an animal lives long enough, it will eventually get killed by an accident, or will get cancer from a mutation caused by—now get this—oxygen. But now consider natural selection. Since accidents and cancer will kill old animals, natural selection does not reward mutations that confer a benefit on old animals. In fact, natural selection favors animals that take risks and pour their resources into reproduction, even though it kills them. Natural selection favors animals that do not hold back for the future, but spend their lives now on reproduction.
The evidence for this is that senescence is genetically programmed. Rather than provide the molecular details of it, let me mention some evolutionary evidence. In animals as diverse as fishes and opossums, populations that live in the presence of predators have shorter life spans than those that live in the absence of predators. If there are predators present, it is better for the opossums to have their offspring as soon as possible, for if they wait, they might be eaten. If predators are absent, the animals can wait a little longer to reproduce, grow a little bit larger first, which allows them to reproduce more. The ideal evolutionary lifespan of an animal depends on the circumstances in which its species has evolved, and is usually pretty short.
Death is not an aberration or mistake. It is not something God inflicted on the world when Eve took a bit of the apple (or, in a creationist movie filmed in southern California, the avocado). It is the product of natural selection. This does not make us any happier as individuals. But at least we can see it as part of a natural world that makes sense.
To scientists, however, death is the process upon which ecology operates. And evolution too. Evolution has given each organism a built-in process of senescence, which is a gradual shutting-down of the body, which ends in death. Organisms are built to grow up, reproduce, then gradually shut down and die. Death is not a mistake. It has been produced by natural selection.
If an animal lives long enough, it will eventually get killed by an accident, or will get cancer from a mutation caused by—now get this—oxygen. But now consider natural selection. Since accidents and cancer will kill old animals, natural selection does not reward mutations that confer a benefit on old animals. In fact, natural selection favors animals that take risks and pour their resources into reproduction, even though it kills them. Natural selection favors animals that do not hold back for the future, but spend their lives now on reproduction.
The evidence for this is that senescence is genetically programmed. Rather than provide the molecular details of it, let me mention some evolutionary evidence. In animals as diverse as fishes and opossums, populations that live in the presence of predators have shorter life spans than those that live in the absence of predators. If there are predators present, it is better for the opossums to have their offspring as soon as possible, for if they wait, they might be eaten. If predators are absent, the animals can wait a little longer to reproduce, grow a little bit larger first, which allows them to reproduce more. The ideal evolutionary lifespan of an animal depends on the circumstances in which its species has evolved, and is usually pretty short.
Death is not an aberration or mistake. It is not something God inflicted on the world when Eve took a bit of the apple (or, in a creationist movie filmed in southern California, the avocado). It is the product of natural selection. This does not make us any happier as individuals. But at least we can see it as part of a natural world that makes sense.
Wednesday, March 10
Darwin and Disenchantment, continued
I have just about finished reading Darwin Loves You, by George Levine. I discussed this book in the previous entry (March 2). Some people worry that science disenchants the world and removes its ability to fill us with awe and wonder. Levine focuses on Darwinian evolution. But I think that evolution disenchants the world a lot less than almost any other branch of science. For example, what is love? According to Helen Fisher in Why We Love, love is several things. Initially it is erotic attraction, stimulated in both men and women by testosterone. Then it is a fixation upon one person, being in love, mediated by dopamine. It is physiologically similar to addiction (and hence I consider it funny that the neurotransmitter is called dopamine—it makes you feel like dope or like you are on dope). Finally, it is long-term satisfaction, mediated by oxytocin and vasopressin. A neurological, biochemical explanation of love—if anything can disenchant the world, it is this. But, of course, it has not. No matter how much we know about the chemistry of love or about evolution, we continue to love one another and love the critters of the Earth. We focus our attention on every detail of the person we love, and every detail of biodiversity of the Earth we love. Neither Darwin nor dopamine have changed this.
Some people think that only religion can keep the world from being merely mechanical. The only choices, they believe, are that the universe has a creator or that it is a heartless, cold machine. But my experience contradicts this. I have found that my creationist students are the ones who are the least interested in looking in the microscope, learning about organisms or about ecology—even from a creationist view. They seem to think that God is simply going to destroy the world anyhow, so it is unimportant. It is the other students—the skeptics and the ones who have a more general religious mindset—who are the most interested in, and feel the most awe for, the natural world. I have found that my creationist students are the ones who will not be quiet and listen to the world; they want to fill it with their own voices, or the noise and fumes of their big pickup trucks. Their faith in God means that the world might as well not exist; it is merely a temporary storage box for ego.
In other words, my creationist students seem to think, regarding the natural world (when they think about it at all), “It’s okay to pour oil on it, it’s okay to drive my truck on it, it’s okay to chop it down or dig it up or pave it with concrete—so long as you don’t believe that it evolved!”
Can anything be more disenchanting than this?
Some people think that only religion can keep the world from being merely mechanical. The only choices, they believe, are that the universe has a creator or that it is a heartless, cold machine. But my experience contradicts this. I have found that my creationist students are the ones who are the least interested in looking in the microscope, learning about organisms or about ecology—even from a creationist view. They seem to think that God is simply going to destroy the world anyhow, so it is unimportant. It is the other students—the skeptics and the ones who have a more general religious mindset—who are the most interested in, and feel the most awe for, the natural world. I have found that my creationist students are the ones who will not be quiet and listen to the world; they want to fill it with their own voices, or the noise and fumes of their big pickup trucks. Their faith in God means that the world might as well not exist; it is merely a temporary storage box for ego.
In other words, my creationist students seem to think, regarding the natural world (when they think about it at all), “It’s okay to pour oil on it, it’s okay to drive my truck on it, it’s okay to chop it down or dig it up or pave it with concrete—so long as you don’t believe that it evolved!”
Can anything be more disenchanting than this?
Tuesday, March 2
Darwin and Disenchantment
Apparently quite a number of people believe that Darwin disenchanted the world. I don’t just mean that he removed all of the supernatural spirits from it, but that he turned it into an impersonal machine. Mutations pop up, natural selection grinds along, producing cactus-machines and cat-machines and little bacterial nanomachines that kill them. It is not just creationists who say this; in fact, creationists say it very little, since to them the world was never enchanted. To creationists, the world is a mere stage for a very brief play prior to God destroying the stage and taking all the actors and actresses to heaven. Without their dogs. It is the philosophical and poetic types who worry that Darwin has removed the magic and wonder of the world.
It had not occurred to me that anyone would think this about evolution. But apparently enough people do that George Levine wrote an entire book (Darwin Loves You) to refute it.
The main answer is that, for Darwin, the natural world was full of beauty and wonder. For him, it was not a machine-like world. He loved everything about it, even the things that looked hideous to some people. A creationist could say that God made barnacles, but would hardly be able to look at one under a microscope without feeling revulsion. Darwin looked at them under a microscope for eight years, and was fascinated by them—although, toward the end, he was getting pretty tired of them as well. In his last research project, Darwin studied the beautiful life of earthworms.
Another problem is the pain and suffering that occurs in the natural world. Some philosophers have tried to deny feelings of empathy and pain in nonhuman animals. This is what Descartes did, and many scientists since his time. But it has clearly been shown that when a mouse observes another mouse in pain, it squirms empathetically—and does so more if the two mice were littermates! There is clearly pain in the natural world.
It is true that the process of natural selection depends upon pain and suffering. But this is not Darwin’s fault. The pain and suffering were there anyway and religious thinkers before Darwin had to come up with an explanation for it. And they tried to do this. First they said that God has subjected us to limitations because our characters can grow only when we cannot have everything we desire, but have to work for it. Fine. But the problem is not limitations but suffering. The problem is that people who work hard have their reward unjustly taken from them. So the religious thinkers next said that God permits suffering because it makes people stronger. What about the people who die before they have a chance to get stronger? Now, the religious thinkers pulled out their big guns. It doesn’t matter what happens here—how many innocent children are killed or enslaved, how many women raped, how many people die of diseases that could be so easily prevented—because when we all get to Heaven everything will be fine.
Darwin had a different solution. He said that the pain and suffering were not caused or prevented by any deity, but was just something that happened. It bothered him. He was a sensitive soul and did not want even an earthworm to suffer. Sort of like St. Francis of Assisi. (See my earlier entry entitled “St. Charles.”) But natural selection, as awful as it sometimes is, is the engine of evolutionary change and has produced the breathtakingly beautiful world of organisms around us.
It had not occurred to me that anyone would think this about evolution. But apparently enough people do that George Levine wrote an entire book (Darwin Loves You) to refute it.
The main answer is that, for Darwin, the natural world was full of beauty and wonder. For him, it was not a machine-like world. He loved everything about it, even the things that looked hideous to some people. A creationist could say that God made barnacles, but would hardly be able to look at one under a microscope without feeling revulsion. Darwin looked at them under a microscope for eight years, and was fascinated by them—although, toward the end, he was getting pretty tired of them as well. In his last research project, Darwin studied the beautiful life of earthworms.
Another problem is the pain and suffering that occurs in the natural world. Some philosophers have tried to deny feelings of empathy and pain in nonhuman animals. This is what Descartes did, and many scientists since his time. But it has clearly been shown that when a mouse observes another mouse in pain, it squirms empathetically—and does so more if the two mice were littermates! There is clearly pain in the natural world.
It is true that the process of natural selection depends upon pain and suffering. But this is not Darwin’s fault. The pain and suffering were there anyway and religious thinkers before Darwin had to come up with an explanation for it. And they tried to do this. First they said that God has subjected us to limitations because our characters can grow only when we cannot have everything we desire, but have to work for it. Fine. But the problem is not limitations but suffering. The problem is that people who work hard have their reward unjustly taken from them. So the religious thinkers next said that God permits suffering because it makes people stronger. What about the people who die before they have a chance to get stronger? Now, the religious thinkers pulled out their big guns. It doesn’t matter what happens here—how many innocent children are killed or enslaved, how many women raped, how many people die of diseases that could be so easily prevented—because when we all get to Heaven everything will be fine.
Darwin had a different solution. He said that the pain and suffering were not caused or prevented by any deity, but was just something that happened. It bothered him. He was a sensitive soul and did not want even an earthworm to suffer. Sort of like St. Francis of Assisi. (See my earlier entry entitled “St. Charles.”) But natural selection, as awful as it sometimes is, is the engine of evolutionary change and has produced the breathtakingly beautiful world of organisms around us.
