Happy Birthday Darwin: Evolution Then and Now
Darwin's theory of evolution has been one of the most robust theories in science. With some modifications, it is still the basis of our understanding of most of biology. Every Feb 12th (Darwin's Birthday) I post an article or series of articles discussing evolution and the evidence for Darwin's theory.
Darwin's theory grew out of an era when a considerable amount of careful observation from around the world was beginning to be formulated into careful scientific ideas. Not all ideas from this era were equally scientific, nor equally valid. Charles Darwin's theory was formulated based on a huge amount of observation both personally made by Darwin and made by correspondents he wrote to from all over the world. It took many years for Darwin to put his ideas into words and his book, Origin of Species, spends a great deal of time addressing criticisms of the theory of Evolution. When Darwin formulated his theory, the Mendelian rules of genetics were unknown, and DNA wasn't even conceived of. So, in essence, the mechanisms and rules that govern evolution were unknown. Darwin defined the patterns of how living things changed and competed, and it was only later that those mechanisms were discovered, giving the statistical and molecular context for Darwin's theory. Those later discoveries have only strengthened Darwin's theory, never contradicting his ideas.
The most fundamental basis for the theory of evolution is the very simple and very evident observation that individuals within a species vary from one another. This may seem so obvious that it seems silly to state it, but it really is the foundation of Darwin's theory and he spends an entire chapter of his book demonstrating variability within species in nature. We now know that this variation is due to genetic differences, differences in the DNA sequence, among individuals. Darwin did not know this. He simply observed that in every species he had any information on, individuals showed clear differences in appearance, in abilities, in behavior and in internal structures. Simple differences like human skin color or our differences in eyesight are examples of this. What is important about individual variation is that such variations can make an individual better or worse able to survive within a given environment and produce children. Since producing children is what contributes to the next generation, differences in an organism's chances of surviving and reproducing can determine whether or not that individual organism contributes to the next generation.
Throughout his book, Darwin compares what happens in nature to the simpler situation of human breeding of domestic animals. Humans select for larger fruit size, higher milk or wool production, less aggressive bulls, etc. and that selection is based on individual differences among individual animals or plants. Over time, our selection of certain traits can make that trait predominant in the animals we have domesticated. As Darwin summarizes it:
In other words, in the same way that humans select for specific traits within the diversity of a population of domesticated plants or animals, natural selection will do the same thing in nature. This natural selection is little different than what humans do when they select particular traits to breed for in our show dogs, our cattle, our wheat plants, etc. The only difference is natural selection is not directed towards a specific goal the way human breeding of domesticated plants and animals is.
What is natural selection? Clearly a plant or animal that can withstand cold better than another, say one rabbit has a thicker coat of fur than another, it is more likely to survive a cold winter or in colder climates than a plant or animal less able to withstand cold. The thicker furred rabbit would be better adapted to the colder conditions, so would be more likely to survive and reproduce. But the thinner furred rabbit may do better in warmer conditions. That is natural selection. Individual traits that favor survival in a particular environment will improve the chances that the genes (to use modern terms) of that individual get passed on to the next generation. Animals that are poorly adapted die before they can produce many offspring, and hence their genes do not remain in the population. A trait that is bad for an individual's survival is unlikely to survive for many generations in a population. That is natural selection: the constraints the environment puts on populations of organisms, favoring the survival and reproduction of some individuals over other individuals.
Darwin came up with this after considering Malthus' theory of population growth. Malthus postulated that populations tend to expand exponentially (geometrically, meaning 2 become 4 become 16 become 256, etc.) while food production expands linearly (2 becomes 4, becomes 8, become 16, etc.). So, Malthus states that over time, populations will expand beyond their ability to find food and hence undergo periods of severe decline due to disease and starvation.
Darwin thought about this and realized that this process of rapid population growth and crash means that there is a selective pressure on a population that favors individuals that can survive the cycles of boom and bust. He recognized that several environmental factors would affect this including competition with other species and within a species. Hence, the basic Malthusian population dynamics would create a situation whereby each individual would be in constant competition with all other individuals of all other species in a given environment for survival. Or, as Darwin put it:
Variation occurs, competition occurs, so variation that improves an individual's chances of surviving the competition is selected for and passed on to offspring.
There is another layer of selection that tends to get ignored when Darwin's theories are considered, and yet Darwin considered it just as important as natural selection. That layer is sexual selection. Natural selection is the selective pressure for certain traits over others due to the fierce competition for survival among all organisms in a given environment. Sexual selection is simply the tendency for certain traits to be favored by other members of a species in a mate. These often are NOT traits that are beneficial in natural selection and can sometimes seem bizarre. Selection by peahens for peacocks with particularly large and dazzling tail feathers is actually selecting for a tail structure that makes the peacock MORE vulnerable to being killed by predators. And yet, the fact that peahens favor it is still a selective pressure for a large and dazzling tail. Many such examples of sexual selection can be seen in nature, as well as in any pick up bar in any city on earth. If a trait, for whatever reason, is favored by the opposite sex, then individuals with that trait will be more likely to reproduce and so pass on the genes for that trait. It is the COMBINATION of natural and sexual selection, acting on individuals of a species that vary from one another, that drive evolution.
As a population changes, it may come to differ from other populations of the same species. This is a well-known phenomenon whereby multiple varieties can exist of the same species. Our domestic dog, cat, cattle, etc. breeds are examples of this. Such varieties can easily be crossed back together to lose their distinct characteristics, but the tendency to have those characteristics will remain. For example, crossing two breeds of dogs will give you a mutt, but that mutt will still have some of the genes of the pure breeds and it's offspring will show similarities to the pure breeds. Varieties occur in nature, often when different populations of a species are separated by a physical barrier: a river, lake, mountain, etc. Darwin spends a great deal of time outlining the observations concerning varieties and how most of those same observations could be applied to species as well. He showed how over time the differences between isolated varieties would increase, producing sub-species and, eventually, new species. This is the final part of Darwin's theory. Variation within a species, acted on by natural and sexual selection, lead to geographical divergences within a species into varieties, then sub-species and finally species. He discusses how the differences between varieties and species can be controversial, leading one scientist to declare two populations mere varieties of one species, and another scientist to declare them two separate species. He suggests that varieties and species are merely opposite ends of one process. I will discuss this in more detail below when I deal with more modern evidence supporting Darwin's theory.
Darwin argues that the way different species are distributed around the world and how they differ from each other is easily explainable by his theory of evolution, where different species branch off of a common progenitor species, as opposed to the then commonly believed theory that each species was separately and independently created. He spends many chapters simply showing how the observed distribution and structures of living things on earth today is more consistent with his theory than with the theory of independent creation of each and every species. For example:
In other words, the similarity of the structure and pattern of the bones of human hands, horse legs, whale fins and bat wings doesn't make sense in terms of similarity of function, because these limbs are used for very different purposes. It seems rather sloppy of a Creator to use this same pattern for all these limb types if independent creation is believed. Independent creation would logically have each limb separately designed for its particular purpose and there is no reason for the identical pattern of bones, including cases where some bones are no longer useful but still remain as vestigial elements in the limb. However, evolution would say that they all have the same pattern because they all were formed from the limbs of a common ancestor by gradual changes over millions of years by natural selection. It is the most logical explanation for all such similarities in structure across classifications of organisms. Now that we understand genes and DNA, we find an even better fit between reality and Darwin's theory.
We now know that the origin of variation is mutation in the DNA. DNA is a molecule with, in essence, a 4-letter alphabet and "words" made up of 3 letters that tell the cell what amino acid goes where in a give protein. DNA also tells the cell under what conditions a protein should be turned on or off. Together, this is what determines what each individual organism looks like, can do and even, to a large degree, how it behaves.
Mutations occur from two sources. Both are, in essence, random. One source is the fact that the molecular machinery for replicating our DNA sometimes makes errors. Most of these errors are edited out. But sometimes the editing machinery fails and a real mutation occurs. The second source is from the environment: many chemicals (natural and artificial) as well as UV and hihger energy radiation cause mutations in our DNA. Again, often these mutations are repaired, but sometimes they slip through.
Mutations in DNA can be silent (have no effect), can alter the structure of a protein, or can alter the way a protein is turned on and off. When a change is made, it often impairs the function of the protein, but occasionally simply alters it in a neutral way or actually improves it. These mutations that lead to changes in the way a protein works or is regulated are the source of variations among individuals. Again, it is by and large a random process.
Natural selection gives the direction to evolution. Changes in a species due only to random mutations with no real selection is called genetic drift, and it can happen in isolated situations. But in general, species change due to natural and sexual selection, directing the changes towards an improved survival or reproductive potential. Changes that produce new varieties and new sub-species are observed all the time in domesticated animals, in nature and in the laboratory. It is the change that leads to the origin of NEW species that is harder to see, though I will show below that it is indeed observable.
The lack of precision that existed during Darwin's day in determining varieties from sub-species to species has been improved thanks to DNA. We can compare the DNA sequences of two groups and tell exactly how closely related they are. This comparison, on a smaller scale, is what we do when we do modern paternity tests or DNA fingerprinting in criminal cases. On a larger scale, this kind of analysis has shown that the wooly mammoth is very closely related to the modern Asian elephant. In essence, the Asian elephant and mammoth are sister species and both are close cousins to the African elephant. The same DNA analysis that can give us DNA fingerprinting can, with greater care, give us relationships among species including an estimate of how long it has been since those two species diverged. These kinds of relationships on the level of DNA are far harder to explain if each species was created independently than if modern species evolved from common ancestor species the way Darwin hypothesized.
So in the more than 100 years since Darwin published Origin of Species, scientific discoveries that Darwin could never have imagined have done nothing but bolster his theory of evolution. It is a theory that was tenuous, though carefully thought out and reasoned, at the time. Further observations at that time all fit well with Darwin's theory. Mendel's laws of genetics fit very well with Darwin's theory and could almost have been devised by Darwin himself since he had done some very similar experiments, but just hadn't done them as extensively as Mendel. Finally, the discovery and understanding of DNA and the entire field of molecular biology, something that Darwin could never have even imagined, fit perfectly into the theory of evolution and in many ways explain aspects of evolution that seemed mysterious to Darwin himself.
Now I wish to discuss some of the objections that have been made to Darwin's theory and show how more than 100 years of research have done nothing but bolster or minorly modify Darwin's theory.
Ever since Charles Darwin first published Origin of Species, many who see his theory as somehow detracting from religion have tried to tear it down. They have pretty much failed from the start and the more we have learned of biology, the more evolution has been supported, if occasionally modified. Interestingly, most objections to Darwin's original theory were recognized and brought up by Darwin himself in Origin of Species. Far from avoiding or denying potential problems, Darwin approached them head on, giving his hypotheses as to how the problems would be solved over time. In general, his hypotheses have proven quite correct.
There are three particular objections that are often brought up to try and discredit evolution. First there is the problem of the gradual evolution of complex organs, such as the eye. How can random variation acted on by natural selection produce an organ as intricate and complex as the eye? Second there is the problem of intermediate species. If evolution is a slow and gradual process, why do we never see the intermediate species, the "missing links" either alive or in the fossil record? These two problems can be called the Problems of Missing Intermediates and can be solved by, in essence, pointing out that a.) intermediates will be rare and rapidly replaced by improved versions, and b.) in reality, intermediates CAN be seen in both instances. I will address these momentarily.
The third problem is, to some degree, more philosophical than scientific. Some argue that it is nothing but a theory of "chance" whereby random events produce wonderful things like eyes and wings. Much like an infinite number of monkeys on typewriters banging out Shakespeare's plays, this sounds very unsatisfying and even impossible. Put in somewhat scientific terms one could say that it violates the Second Law of Thermodynamics that dictates that, in essence, order cannot arise out of disorder without substantial input of energy. Chance cannot bring about order.
This objection is completely based on a false assumption: that evolution involves chance alone. To quote Richard Dawkins from his introduction to the 2003 Everyman's Library edition of Darwin's Origin of Species and Voyage of the Beagle (Darwin):
In other words, although mutation, the basis for variation, is indeed random to a large degree, that is only one part of Darwin's theory. The part that gives evolution its direction, if you want, is selection, both natural and sexual. There is no mere chance to evolution. It is directed by the pressures of the environment in which an organism lives. That is the very essence of Darwin's theory! Those who argue that it is a theory of mere chance have misunderstood the theory.
Now I want to address the first two objections I mentioned. How can something as complex as the eye be formed by small evolutionary changes and where are the intermediate forms of organs like the eye? And, why don't we see many intermediate species, either alive or in the fossil record, showing us the many gradations that take us from varieties within a species to separate species?
I will first address some things that apply to both. First off, the fossil record is not a complete record of evolution. Only a tiny fraction of all living organisms have become fossilized. Eons of the past in vast parts of the world have never survived in the fossil record. Conditions have to be just right for fossils to form. Even once formed, they can be destroyed by erosion and weathering. Finally, for us to ever see fossils they have to be exposed in just the right way at just the right time. Metaphorically, we are looking at a gigantic encyclopedia where most volumes are missing, the volumes we do have are missing pages, and the surviving pages are smudged. We cannot expect to have a complete fossil record wherein we can see every intermediate form through out evolution. Darwin himself went into this in great, almost excruciating, detail.
Another thing that Darwin himself points out about these objections is that imperfect intermediates are likely to be seen only in a small, isolated population for a short period of time. It is the MOST successful forms (either of organs or entire organisms) that compete the best and hence replace earlier, less successful intermediate forms. Intermediates IN GENERAL will be relatively rare and present for a short period of time on a geological scale compared with the more successful complete forms. So we are far more likely to only see the most successful forms of organs and organisms most of the time. As Darwin himself puts it:
In some ways these points eliminate the problems of the missing intermediates. The intermediates will be very rare and the fossil record, by its very nature, will be incomplete. But that is unsatisfying. One wants to say, "Yeah, but why don't we see ANY intermediates?" Even if they are rare and short lived, why aren't any alive TODAY?
The answer is that some ARE alive today and we DO see them. These objections are not only refutable logically as above, but they plain aren't true! Intermediate organs were mentioned by Darwin himself, and intermediate species were observed even in the period immediately after Darwin published and more have been discovered since then.
Taking the question of the evolution of a complex structure like the eye, Darwin points out that in Arthropods (or Articulata as they were called in Darwin's time) such intermediate, imperfect eyes DO exist in living organisms. Again, from Darwin himself (Darwin):
In other words, you CAN see various stages of the evolution of the eye in living animals. From a simple pigmented optic nerve, through various simple structures, up to the full compound eye of the fly, all are seen within the same broad category of animals.
To discuss intermediates between full speciation, I have to go to other sources than Darwin himself. As early as 1863, Henry Walter Bates observed speciation in action. From Janet Browne's biography of Charles Darwin:
Other examples have been observed. One of the most striking and yet mundane are sea gulls. Gulls are an example of a phenomenon called a "ring species" where each neighboring population in a range of an organism can interbreed and represent mere varieties, but the opposite ends of the range have populations that cannot interbreed and are, by all definitions, separate species. From Carl Zimmer's companion book to the 2001 PBS series, Evolution:
Nothing could prove Darwin's hypothesis better than this! Here is speciation IN ACTION with two separate species side by side and a continuous series of gradations that can be followed that takes you from one to the other. Every step of the way is right there for us to see. Ring species like the gull are the most definitive proof of Darwin's theory you can get.
Theories are never "PROVEN" in a definitive sense. They are DISproven if contrary evidence is found. Sometimes a simple change in the theory can be made to accommodate the contrary evidence, preserving the basic theory in a modified form. Other times evidence so contrary is discovered that the entire theory has to be thrown out. If each new piece of evidence is in agreement with the theory, then that theory has support. Evolution has never been disproven. It has frequently been supported by more and more data. Arguably, some new data, such as mass extinctions from comet impacts, has required minor modifications of Darwin's theory, such as "punctuated equilibrium." But no major alterations to the basic design of "variation, selection, speciation" has been required. These foundations of Darwin's theory have ONLY been supported by the extraordinary advances in molecular biology (DNA and protein structure as the bases of variation), ecology (a better understanding of how natural selection and sexual selection work) and the discovery of more fossils and of phenomena like "ring species." The HUGE amount of new discoveries has only strengthened the more than 100-year old theory Darwin came up with. That is an astonishing robustness that has been matched by hardly any other theories in history. Newtonian physics can boast a longer period of dominance, from the 17th to 20th centuries when Einstein had to come up with some modifications. But not much else can boast as great a success as Darwinian evolution.
And 2005 brought even more support for Darwin's theory. From the December 23rd issue of the journal Science:
This is just a sampling of the massive progress that has been made in one year alone, all completely in agreement or modifying in only minor ways Darwin's theory. For a more recent round up of supporting evidence for Darwin's theory, check out 12 Elegant Examples of Evolution from Wired magazine.
Ladies and gentleman, I think we have to recognize that Charles Darwin's theory of evolution is one of the most brilliant and most insightful scientific theories in the history of our species and it is WAY past time for it to be considered controversial. New studies are bound to modify his theory from time to time, but Darwin's theory is something we should cherish as a deep understanding of how life works.
The theory of evolution has itself evolved since Darwin's time. Darwin was convinced that evolution happened only on very long time scales in very slow increments. He believed the evolution of new species as well as the extinction of species would be almost imperceptible events, needing immense time spans. What he didn't know was that evolution can get boosts, not of mutation rates, necessarily, but in the nature of competition, when mass extinctions occur due to meteor or comet impacts, supervolcanoes, or rapid climate change. Such occurrences can lead to extinctions within a very short period of time...within months or years, rather than millions of years. When that happens, the survivors face a greatly reduced population as well as a greatly altered environment, leading to rapid evolution of new species. This is called punctuated equilibrium and is a modern modification of Darwinian evolution. Some people find punctuated equilibrium hard to swallow, but we DO know that mass extinctions occur. We see it in the fossil record and we find the impact craters that correspond in time to some of those mass extinctions. Punctuated equilibrium fits these observations better than any other theory to date.
Bibliography:
Browne, Janet; The Power of Place: Charles Darwin, the Origin and After, Knopf, 2002.
Darwin, Charles; Origin of Species and Voyage of the Beagle, Everyman's Library 2003 edition.
Zimmer, Carl; Evolution: the Triumph of an Idea, companion to the PBS series, Harper Collins, 2001.
Darwin's theory grew out of an era when a considerable amount of careful observation from around the world was beginning to be formulated into careful scientific ideas. Not all ideas from this era were equally scientific, nor equally valid. Charles Darwin's theory was formulated based on a huge amount of observation both personally made by Darwin and made by correspondents he wrote to from all over the world. It took many years for Darwin to put his ideas into words and his book, Origin of Species, spends a great deal of time addressing criticisms of the theory of Evolution. When Darwin formulated his theory, the Mendelian rules of genetics were unknown, and DNA wasn't even conceived of. So, in essence, the mechanisms and rules that govern evolution were unknown. Darwin defined the patterns of how living things changed and competed, and it was only later that those mechanisms were discovered, giving the statistical and molecular context for Darwin's theory. Those later discoveries have only strengthened Darwin's theory, never contradicting his ideas.
The most fundamental basis for the theory of evolution is the very simple and very evident observation that individuals within a species vary from one another. This may seem so obvious that it seems silly to state it, but it really is the foundation of Darwin's theory and he spends an entire chapter of his book demonstrating variability within species in nature. We now know that this variation is due to genetic differences, differences in the DNA sequence, among individuals. Darwin did not know this. He simply observed that in every species he had any information on, individuals showed clear differences in appearance, in abilities, in behavior and in internal structures. Simple differences like human skin color or our differences in eyesight are examples of this. What is important about individual variation is that such variations can make an individual better or worse able to survive within a given environment and produce children. Since producing children is what contributes to the next generation, differences in an organism's chances of surviving and reproducing can determine whether or not that individual organism contributes to the next generation.
Throughout his book, Darwin compares what happens in nature to the simpler situation of human breeding of domestic animals. Humans select for larger fruit size, higher milk or wool production, less aggressive bulls, etc. and that selection is based on individual differences among individual animals or plants. Over time, our selection of certain traits can make that trait predominant in the animals we have domesticated. As Darwin summarizes it:
"No one supposes that all the individuals of the same species are cast in the very same mould. These individual differences are highly important for us, as they afford materials for natural selection to accumulate, in the same manner as man can accumulate in any given direction individual differences in his domesticated productions."
In other words, in the same way that humans select for specific traits within the diversity of a population of domesticated plants or animals, natural selection will do the same thing in nature. This natural selection is little different than what humans do when they select particular traits to breed for in our show dogs, our cattle, our wheat plants, etc. The only difference is natural selection is not directed towards a specific goal the way human breeding of domesticated plants and animals is.
What is natural selection? Clearly a plant or animal that can withstand cold better than another, say one rabbit has a thicker coat of fur than another, it is more likely to survive a cold winter or in colder climates than a plant or animal less able to withstand cold. The thicker furred rabbit would be better adapted to the colder conditions, so would be more likely to survive and reproduce. But the thinner furred rabbit may do better in warmer conditions. That is natural selection. Individual traits that favor survival in a particular environment will improve the chances that the genes (to use modern terms) of that individual get passed on to the next generation. Animals that are poorly adapted die before they can produce many offspring, and hence their genes do not remain in the population. A trait that is bad for an individual's survival is unlikely to survive for many generations in a population. That is natural selection: the constraints the environment puts on populations of organisms, favoring the survival and reproduction of some individuals over other individuals.
Darwin came up with this after considering Malthus' theory of population growth. Malthus postulated that populations tend to expand exponentially (geometrically, meaning 2 become 4 become 16 become 256, etc.) while food production expands linearly (2 becomes 4, becomes 8, become 16, etc.). So, Malthus states that over time, populations will expand beyond their ability to find food and hence undergo periods of severe decline due to disease and starvation.
Darwin thought about this and realized that this process of rapid population growth and crash means that there is a selective pressure on a population that favors individuals that can survive the cycles of boom and bust. He recognized that several environmental factors would affect this including competition with other species and within a species. Hence, the basic Malthusian population dynamics would create a situation whereby each individual would be in constant competition with all other individuals of all other species in a given environment for survival. Or, as Darwin put it:
"If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organization, and I think his cannot be disputed; if there be, owing to the high geometrical powers of increase [reproduction] of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterized will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterized. This principle of preservation, I have called, for the sake of brevity, Natural Selection."
Variation occurs, competition occurs, so variation that improves an individual's chances of surviving the competition is selected for and passed on to offspring.
There is another layer of selection that tends to get ignored when Darwin's theories are considered, and yet Darwin considered it just as important as natural selection. That layer is sexual selection. Natural selection is the selective pressure for certain traits over others due to the fierce competition for survival among all organisms in a given environment. Sexual selection is simply the tendency for certain traits to be favored by other members of a species in a mate. These often are NOT traits that are beneficial in natural selection and can sometimes seem bizarre. Selection by peahens for peacocks with particularly large and dazzling tail feathers is actually selecting for a tail structure that makes the peacock MORE vulnerable to being killed by predators. And yet, the fact that peahens favor it is still a selective pressure for a large and dazzling tail. Many such examples of sexual selection can be seen in nature, as well as in any pick up bar in any city on earth. If a trait, for whatever reason, is favored by the opposite sex, then individuals with that trait will be more likely to reproduce and so pass on the genes for that trait. It is the COMBINATION of natural and sexual selection, acting on individuals of a species that vary from one another, that drive evolution.
As a population changes, it may come to differ from other populations of the same species. This is a well-known phenomenon whereby multiple varieties can exist of the same species. Our domestic dog, cat, cattle, etc. breeds are examples of this. Such varieties can easily be crossed back together to lose their distinct characteristics, but the tendency to have those characteristics will remain. For example, crossing two breeds of dogs will give you a mutt, but that mutt will still have some of the genes of the pure breeds and it's offspring will show similarities to the pure breeds. Varieties occur in nature, often when different populations of a species are separated by a physical barrier: a river, lake, mountain, etc. Darwin spends a great deal of time outlining the observations concerning varieties and how most of those same observations could be applied to species as well. He showed how over time the differences between isolated varieties would increase, producing sub-species and, eventually, new species. This is the final part of Darwin's theory. Variation within a species, acted on by natural and sexual selection, lead to geographical divergences within a species into varieties, then sub-species and finally species. He discusses how the differences between varieties and species can be controversial, leading one scientist to declare two populations mere varieties of one species, and another scientist to declare them two separate species. He suggests that varieties and species are merely opposite ends of one process. I will discuss this in more detail below when I deal with more modern evidence supporting Darwin's theory.
Darwin argues that the way different species are distributed around the world and how they differ from each other is easily explainable by his theory of evolution, where different species branch off of a common progenitor species, as opposed to the then commonly believed theory that each species was separately and independently created. He spends many chapters simply showing how the observed distribution and structures of living things on earth today is more consistent with his theory than with the theory of independent creation of each and every species. For example:
"We have seen that the members of the same class [of organisms], independently of their habits of life, resemble each other in the general plan of their organization...What can be more curious than that the hand of a man, formed for grasping, that of a mole for digging, the leg of the horse, the paddle of the porpoise, and the wing of the bat, should all be constructed on the same pattern, and should include the same bones, in the same relative positions?...
"Nothing can be more hopeless than to attempt to explain this similarity of pattern in members of the same class, by utility or by the doctrine of final causes...On the ordinary view of the independent creation of each being, we can only say that so it is;--that its has so pleased the Creator to construct each animal and plant.
The explanation is manifest on the theory of natural selection of successive slight modifications,--each modification bring profitable in some way to the modified form...In changes of this nature there will be little or no tendency to modify the original pattern or to transpose parts."
In other words, the similarity of the structure and pattern of the bones of human hands, horse legs, whale fins and bat wings doesn't make sense in terms of similarity of function, because these limbs are used for very different purposes. It seems rather sloppy of a Creator to use this same pattern for all these limb types if independent creation is believed. Independent creation would logically have each limb separately designed for its particular purpose and there is no reason for the identical pattern of bones, including cases where some bones are no longer useful but still remain as vestigial elements in the limb. However, evolution would say that they all have the same pattern because they all were formed from the limbs of a common ancestor by gradual changes over millions of years by natural selection. It is the most logical explanation for all such similarities in structure across classifications of organisms. Now that we understand genes and DNA, we find an even better fit between reality and Darwin's theory.
We now know that the origin of variation is mutation in the DNA. DNA is a molecule with, in essence, a 4-letter alphabet and "words" made up of 3 letters that tell the cell what amino acid goes where in a give protein. DNA also tells the cell under what conditions a protein should be turned on or off. Together, this is what determines what each individual organism looks like, can do and even, to a large degree, how it behaves.
Mutations occur from two sources. Both are, in essence, random. One source is the fact that the molecular machinery for replicating our DNA sometimes makes errors. Most of these errors are edited out. But sometimes the editing machinery fails and a real mutation occurs. The second source is from the environment: many chemicals (natural and artificial) as well as UV and hihger energy radiation cause mutations in our DNA. Again, often these mutations are repaired, but sometimes they slip through.
Mutations in DNA can be silent (have no effect), can alter the structure of a protein, or can alter the way a protein is turned on and off. When a change is made, it often impairs the function of the protein, but occasionally simply alters it in a neutral way or actually improves it. These mutations that lead to changes in the way a protein works or is regulated are the source of variations among individuals. Again, it is by and large a random process.
Natural selection gives the direction to evolution. Changes in a species due only to random mutations with no real selection is called genetic drift, and it can happen in isolated situations. But in general, species change due to natural and sexual selection, directing the changes towards an improved survival or reproductive potential. Changes that produce new varieties and new sub-species are observed all the time in domesticated animals, in nature and in the laboratory. It is the change that leads to the origin of NEW species that is harder to see, though I will show below that it is indeed observable.
The lack of precision that existed during Darwin's day in determining varieties from sub-species to species has been improved thanks to DNA. We can compare the DNA sequences of two groups and tell exactly how closely related they are. This comparison, on a smaller scale, is what we do when we do modern paternity tests or DNA fingerprinting in criminal cases. On a larger scale, this kind of analysis has shown that the wooly mammoth is very closely related to the modern Asian elephant. In essence, the Asian elephant and mammoth are sister species and both are close cousins to the African elephant. The same DNA analysis that can give us DNA fingerprinting can, with greater care, give us relationships among species including an estimate of how long it has been since those two species diverged. These kinds of relationships on the level of DNA are far harder to explain if each species was created independently than if modern species evolved from common ancestor species the way Darwin hypothesized.
So in the more than 100 years since Darwin published Origin of Species, scientific discoveries that Darwin could never have imagined have done nothing but bolster his theory of evolution. It is a theory that was tenuous, though carefully thought out and reasoned, at the time. Further observations at that time all fit well with Darwin's theory. Mendel's laws of genetics fit very well with Darwin's theory and could almost have been devised by Darwin himself since he had done some very similar experiments, but just hadn't done them as extensively as Mendel. Finally, the discovery and understanding of DNA and the entire field of molecular biology, something that Darwin could never have even imagined, fit perfectly into the theory of evolution and in many ways explain aspects of evolution that seemed mysterious to Darwin himself.
Now I wish to discuss some of the objections that have been made to Darwin's theory and show how more than 100 years of research have done nothing but bolster or minorly modify Darwin's theory.
Ever since Charles Darwin first published Origin of Species, many who see his theory as somehow detracting from religion have tried to tear it down. They have pretty much failed from the start and the more we have learned of biology, the more evolution has been supported, if occasionally modified. Interestingly, most objections to Darwin's original theory were recognized and brought up by Darwin himself in Origin of Species. Far from avoiding or denying potential problems, Darwin approached them head on, giving his hypotheses as to how the problems would be solved over time. In general, his hypotheses have proven quite correct.
There are three particular objections that are often brought up to try and discredit evolution. First there is the problem of the gradual evolution of complex organs, such as the eye. How can random variation acted on by natural selection produce an organ as intricate and complex as the eye? Second there is the problem of intermediate species. If evolution is a slow and gradual process, why do we never see the intermediate species, the "missing links" either alive or in the fossil record? These two problems can be called the Problems of Missing Intermediates and can be solved by, in essence, pointing out that a.) intermediates will be rare and rapidly replaced by improved versions, and b.) in reality, intermediates CAN be seen in both instances. I will address these momentarily.
The third problem is, to some degree, more philosophical than scientific. Some argue that it is nothing but a theory of "chance" whereby random events produce wonderful things like eyes and wings. Much like an infinite number of monkeys on typewriters banging out Shakespeare's plays, this sounds very unsatisfying and even impossible. Put in somewhat scientific terms one could say that it violates the Second Law of Thermodynamics that dictates that, in essence, order cannot arise out of disorder without substantial input of energy. Chance cannot bring about order.
This objection is completely based on a false assumption: that evolution involves chance alone. To quote Richard Dawkins from his introduction to the 2003 Everyman's Library edition of Darwin's Origin of Species and Voyage of the Beagle (Darwin):
The objection to Darwinism that it is a theory of "chance" is one of the most popular and most foolish of all. Mutation is indeed random, although only in the sense that it is not directed towards improvement. Natural selection is quintessentially the opposite of random. Any fool can see that random chance cannot put together living complexity. That is precisely why Darwinism is necessary. It is a preposterous irony that the statistical improbability of living organization is regularly advanced as though it counted against Darwinism, rather than in favor of it.
In other words, although mutation, the basis for variation, is indeed random to a large degree, that is only one part of Darwin's theory. The part that gives evolution its direction, if you want, is selection, both natural and sexual. There is no mere chance to evolution. It is directed by the pressures of the environment in which an organism lives. That is the very essence of Darwin's theory! Those who argue that it is a theory of mere chance have misunderstood the theory.
Now I want to address the first two objections I mentioned. How can something as complex as the eye be formed by small evolutionary changes and where are the intermediate forms of organs like the eye? And, why don't we see many intermediate species, either alive or in the fossil record, showing us the many gradations that take us from varieties within a species to separate species?
I will first address some things that apply to both. First off, the fossil record is not a complete record of evolution. Only a tiny fraction of all living organisms have become fossilized. Eons of the past in vast parts of the world have never survived in the fossil record. Conditions have to be just right for fossils to form. Even once formed, they can be destroyed by erosion and weathering. Finally, for us to ever see fossils they have to be exposed in just the right way at just the right time. Metaphorically, we are looking at a gigantic encyclopedia where most volumes are missing, the volumes we do have are missing pages, and the surviving pages are smudged. We cannot expect to have a complete fossil record wherein we can see every intermediate form through out evolution. Darwin himself went into this in great, almost excruciating, detail.
Another thing that Darwin himself points out about these objections is that imperfect intermediates are likely to be seen only in a small, isolated population for a short period of time. It is the MOST successful forms (either of organs or entire organisms) that compete the best and hence replace earlier, less successful intermediate forms. Intermediates IN GENERAL will be relatively rare and present for a short period of time on a geological scale compared with the more successful complete forms. So we are far more likely to only see the most successful forms of organs and organisms most of the time. As Darwin himself puts it:
When we see any structure highly perfected for any particular habit, as the wings of a bird for flight, we should bear in mind that animals displaying early transitional grades of the structure will seldom continue to exist to the present day, for they will have been supplanted by the very process of perfection through natural selection.
In some ways these points eliminate the problems of the missing intermediates. The intermediates will be very rare and the fossil record, by its very nature, will be incomplete. But that is unsatisfying. One wants to say, "Yeah, but why don't we see ANY intermediates?" Even if they are rare and short lived, why aren't any alive TODAY?
The answer is that some ARE alive today and we DO see them. These objections are not only refutable logically as above, but they plain aren't true! Intermediate organs were mentioned by Darwin himself, and intermediate species were observed even in the period immediately after Darwin published and more have been discovered since then.
Taking the question of the evolution of a complex structure like the eye, Darwin points out that in Arthropods (or Articulata as they were called in Darwin's time) such intermediate, imperfect eyes DO exist in living organisms. Again, from Darwin himself (Darwin):
In the Articulata we can commence a series with an optic nerve merely coated with pigment, and without any other mechanism; and from this low stage, numerous gradations of structure, branching off in two fundamentally different lines [two different kinds of complex Arthropod eyes], can be shown to exist, until we reach a moderately high stage of perfection. In certain crustaceans, for instance, there is a double cornea, the inner one divided into facets, within each of which there is a lens-shaped swelling. In other crustaceans the transparent cones which are coated by pigment, and which properly act only by excluding lateral pencils of light, are convex at their upper ends and must act by convergence; and at the lower ends there seems to be an imperfect vitreous substance.
In other words, you CAN see various stages of the evolution of the eye in living animals. From a simple pigmented optic nerve, through various simple structures, up to the full compound eye of the fly, all are seen within the same broad category of animals.
To discuss intermediates between full speciation, I have to go to other sources than Darwin himself. As early as 1863, Henry Walter Bates observed speciation in action. From Janet Browne's biography of Charles Darwin:
Bates gave an eyewitness account of the origin of species in nature. Two Amazonian butterflies, the black-and-crimson-spotted Heliconius melpomene and the Heliconius thelixope, if taken separately, were perfectly distinct species. Bates discovered four or five transitional forms living in particular geographical areas in between the two, each connected by a chain of gradations. The intermediate forms were not hybrids. They were geographical races, each on their way to becoming a separate species.
Other examples have been observed. One of the most striking and yet mundane are sea gulls. Gulls are an example of a phenomenon called a "ring species" where each neighboring population in a range of an organism can interbreed and represent mere varieties, but the opposite ends of the range have populations that cannot interbreed and are, by all definitions, separate species. From Carl Zimmer's companion book to the 2001 PBS series, Evolution:
In the North Sea, for instance, there is a species of bird known as the herring gull. It has a grey mantle and pink legs. If you move west through its range, you come across more herring gulls...which look essentially the same as the ones in the North Sea, except for a few minor differences in their coloring. But by the time you reach Canada, the differences become stark, and in Siberia, the gulls have a dark grey mantle and legs that are less pink than yellow. Yet despite these differences, they are still scientifically classified as the herring gull (although their common name is the vega gull). Keep moving through Asia and into Europe, and the gulls continue to get darker and more yellow-legged...all the way to the North Sea where your journey began. Here these gulls, known as the lesser black-backed gulls, live alongside the gray-mantled, pink-legged herring gulls.
Because the two groups of birds look so different and do not mate, they are treated as two separate species. Yet lesser black-backed gulls and herring gulls live at two ends of a continuous ring, inside of which all the birds can mate with their immediate neighbors. A ring species is exactly what you'd expect given the way mutations arise and spread.
Nothing could prove Darwin's hypothesis better than this! Here is speciation IN ACTION with two separate species side by side and a continuous series of gradations that can be followed that takes you from one to the other. Every step of the way is right there for us to see. Ring species like the gull are the most definitive proof of Darwin's theory you can get.
Theories are never "PROVEN" in a definitive sense. They are DISproven if contrary evidence is found. Sometimes a simple change in the theory can be made to accommodate the contrary evidence, preserving the basic theory in a modified form. Other times evidence so contrary is discovered that the entire theory has to be thrown out. If each new piece of evidence is in agreement with the theory, then that theory has support. Evolution has never been disproven. It has frequently been supported by more and more data. Arguably, some new data, such as mass extinctions from comet impacts, has required minor modifications of Darwin's theory, such as "punctuated equilibrium." But no major alterations to the basic design of "variation, selection, speciation" has been required. These foundations of Darwin's theory have ONLY been supported by the extraordinary advances in molecular biology (DNA and protein structure as the bases of variation), ecology (a better understanding of how natural selection and sexual selection work) and the discovery of more fossils and of phenomena like "ring species." The HUGE amount of new discoveries has only strengthened the more than 100-year old theory Darwin came up with. That is an astonishing robustness that has been matched by hardly any other theories in history. Newtonian physics can boast a longer period of dominance, from the 17th to 20th centuries when Einstein had to come up with some modifications. But not much else can boast as great a success as Darwinian evolution.
And 2005 brought even more support for Darwin's theory. From the December 23rd issue of the journal Science:
BREAKTHROUGH OF THE YEAR:
Evolution in Action
Elizabeth Culotta and Elizabeth Pennisi
Amid this outpouring of results, 2005 stands out as a banner year for uncovering the intricacies of how evolution actually proceeds. Concrete genome data allowed researchers to start pinning down the molecular modifications that drive evolutionary change in organisms from viruses to primates. Painstaking field observations shed new light on how populations diverge to form new species--the mystery of mysteries that baffled Darwin himself. Ironically, also this year some segments of American society fought to dilute the teaching of even the basic facts of evolution. With all this in mind, Science has decided to put Darwin in the spotlight by saluting several dramatic discoveries, each of which reveals the laws of evolution in action...
Probing how species split
2005 was also a standout year for researchers studying the emergence of new species, or speciation. A new species can form when populations of an existing species begin to adapt in different ways and eventually stop interbreeding. It's easy to see how that can happen when populations wind up on opposite sides of oceans or mountain ranges, for example. But sometimes a single, contiguous population splits into two. Evolutionary theory predicts that this splitting begins when some individuals in a population stop mating with others, but empirical evidence has been scanty. This year field biologists recorded compelling examples of that process, some of which featured surprisingly rapid evolution in organisms' shape and behavior.
For example, birds called European blackcaps sharing breeding grounds in southern Germany and Austria are going their own ways--literally and f iguratively. Sightings over the decades have shown that ever more of these warblers migrate to northerly grounds in the winter rather than heading south. Isotopic data revealed that northerly migrants reach the common breeding ground earlier and mate with one another before southerly migrants arrive. This difference in timing may one day drive the two populations to become two species.
Two races of European corn borers sharing the same field may also be splitting up. The caterpillars have come to prefer different plants as they grow--one sticks to corn, and the other eats hops and mugwort--and they emit different pheromones, ensuring that they attract only their own kind.
Biologists have also predicted that these kinds of behavioral traits may keep incipient species separate even when geographically isolated populations somehow wind up back in the same place. Again, examples have been few. But this year, researchers found that simple differences in male wing color, plus rapid changes in the numbers of chromosomes, were enough to maintain separate identities in reunited species of butterflies, and that Hawaiian crickets needed only unique songs to stay separate. In each case, the number of species observed today suggests that these traits have also led to rapid speciation, at a rate previously seen only in African cichlids
This is just a sampling of the massive progress that has been made in one year alone, all completely in agreement or modifying in only minor ways Darwin's theory. For a more recent round up of supporting evidence for Darwin's theory, check out 12 Elegant Examples of Evolution from Wired magazine.
Ladies and gentleman, I think we have to recognize that Charles Darwin's theory of evolution is one of the most brilliant and most insightful scientific theories in the history of our species and it is WAY past time for it to be considered controversial. New studies are bound to modify his theory from time to time, but Darwin's theory is something we should cherish as a deep understanding of how life works.
The theory of evolution has itself evolved since Darwin's time. Darwin was convinced that evolution happened only on very long time scales in very slow increments. He believed the evolution of new species as well as the extinction of species would be almost imperceptible events, needing immense time spans. What he didn't know was that evolution can get boosts, not of mutation rates, necessarily, but in the nature of competition, when mass extinctions occur due to meteor or comet impacts, supervolcanoes, or rapid climate change. Such occurrences can lead to extinctions within a very short period of time...within months or years, rather than millions of years. When that happens, the survivors face a greatly reduced population as well as a greatly altered environment, leading to rapid evolution of new species. This is called punctuated equilibrium and is a modern modification of Darwinian evolution. Some people find punctuated equilibrium hard to swallow, but we DO know that mass extinctions occur. We see it in the fossil record and we find the impact craters that correspond in time to some of those mass extinctions. Punctuated equilibrium fits these observations better than any other theory to date.
Bibliography:
Browne, Janet; The Power of Place: Charles Darwin, the Origin and After, Knopf, 2002.
Darwin, Charles; Origin of Species and Voyage of the Beagle, Everyman's Library 2003 edition.
Zimmer, Carl; Evolution: the Triumph of an Idea, companion to the PBS series, Harper Collins, 2001.
posted by mole333 | 10:05 PM
|
0 comments
0 Comments:
Post a Comment
<< Home