Darwin Day at the University of Tennessee
| Darwin Day Home | Events | <Archives> | Resources & Links |
Dr. Douglas Futuyma
First Annual Darwin Day Celebration
University of Tennessee, Knoxville
Feb. 12, 1997
I am pleased and honored to be asked to talk about evolution today, when we celebrate the 188th anniversary of the birth of the man who made one of the most important contributions to western thought. Since 1859, when The Origin of Species was published, evolution has been recognized to have enormous implications for philosophy and for our perception of humanity's nature and our place in nature, a subject on which I don't claim any special knowledge. But in the last century, as the study of evolutionary biology has grown, evolution has shed light on subjects throughout the biological sciences, ranging from molecular biology to behavior and ecology. Evolution, in fact, is one of the two most important unifying themes of biology as a whole. Moreover, the study of evolution, in the broad sense, has had and will continue to have many practical, bottom-line applications to human welfare, in areas such as human health, agriculture, biotechnology, and the use of natural resources.
Evolution consists of descent of different lineages of organisms from common ancestors, and modification of these lineages. That is, all forms of life ultimately have a common ancestor, but some are more closely related to each other than others, meaning that they branched from an ancestor more recently - as this diagram of a phylogenetic tree indicates. (It illustrates relationships among certain beetles that I study.) Moreover, over the course of vast spans of time, they have become more and more different from each other in many of their features. (That is indicated by different shadings, indicating different feeding habits of these beetles.) Many of these divergent features are adaptations to different environments or different ways of life.
The study of evolution consists of three major areas:
One is describing or inferring evolutionary history. That is, what are the relationships among different organisms; what is the phylogenetic tree of life? And what were the steps by which their characteristics have changed? These topics are addressed by paleontologists and by systematists, who infer relationships among living species from similarities and differences in various characteristics, including DNA.
A second major area is elucidating the processes of change: the causes of changes in characteristics and of the origin of new species. This is accomplished partly by studying evolutionary ecology -- how the environment impinges on different species and so favors various characteristics; and partly by evolutionary genetics, since evolution consists of changes that are based in the genes. Evolutionary geneticists use experiments and measurements in the laboratory and in natural populations to study genetic variation - the variation that can be molded into alterations of characteristics. They also study the processes of mutation of genes, genetic drift, natural selection, and other factors that affect genetic change. Increasingly, evolutionary geneticists use molecular tools for these purposes.
A third area is the study of particular groups of organisms and particular kinds of characteristics. For example, some evolutionary biologists specialize in the evolution of plants, or of bacteria. or of the human species. Others study the evolution of biochemical characteristics, of developmental processes, of physiology, behavior, or modes of reproduction that differ among species. In doing so, they attempt to understand how these differences are adaptive -- for instance, what the adaptive advantages are of sexual compared to asexual reproduction, or of warm-bloodedness compared to cold-bloodedness.
Much of what we do in evolutionary biology is fundamental research. Some of it will forever be just of intellectual interest: knowledge is its own reward. But much of it, directly or indirectly, has social implications and applications. These may arise from directly tackling problems in medicine, agriculture, and other areas from an evolutionary point of view. More often, the knowledge, methods, and principles gained from studying model species such as fruit flies or sea urchins can be transferred to solving other problems. Also, evolutionary analyses and perspectives contribute to progress in other areas of biology, and in fact outside of biology.
For example, evolutionary principles can be used to identify the functionally important versus less important parts of DNA sequences -- that is, genes -- and of the proteins that the genes encode. These principles are routinely used by molecular biologists in their work. In developmental biology, the study of how a single cell develops into an adult organism, evolutionary studies of development and of the genes that are responsible for development have provided insights into developmental processes. For instance, developmental biologists are using comparisons among different organisms to work out which genes determine which end of the embryo becomes the head end versus the tail end.
Outside of biology, people who work on artificial intelligence are using programs that are based on models of evolutionary processes, such as the interaction of mutation, recombination, and natural selection. In a related area, computer scientists use such evolutionary models to develop algorithms for solving complex problems efficiently.
Turning to direct social applications: In the health sciences, principles derived from evolutionary genetics help in locating genes for inherited diseases such as cystic fibrosis, and evolutionary genetic principles promise to be useful in locating the precise mutations in these genes that are responsible for such diseases. It is through evolutionary principles that we understand why sickle-cell disease and thalassemia are so common in certain populations. We use evolutionary methods to trace the origin and spread of pathogens such as HIV, the virus that causes AIDS. We have observed many pathogens, such are those that cause malaria and AIDS, evolve resistance to antibiotic drugs, and by understanding this process, evolutionary biologists can help to design ways to retard such evolutionary changes. We use methods of evolutionary systematics and genetics to identify particular species of mosquitoes and other disease vectors, and to trace their rates of movement.
In agriculture, the selective breeding of improved crops and domestic animals is evolution, directed by humans, and it relies on principles developed by individuals who have contributed to both selective breeding and to evolutionary biology. Evolutionary geneticists and systematists who study the evolution and diversity of wild species of plants have contributed to finding genes for pest resistance and other features, that can be transplanted by genetic engineering into crops. In 1970, the southern corn leaf blight caused a loss of more than a billion dollars, because more than 85% of corn acreage had been planted in a genetically uniform strain, to which the blight fungus became adapted. Any evolutionary biologist could have warned -- and I think some did -- that such genetic uniformity was a prescription for disaster, for we are keenly aware of the importance of genetic diversity: it is one of the subjects we study most. Entomologists today, trained in evolution, are working on how to prevent insect pests from evolving insecticide resistance, which adds an estimated $1.4 billion to the annual cost of crop and forest protection in the United States , and they are using evolutionary principles to find natural enemies that could replace or supplement synthetic pesticides.
Fisheries biologists throughout the world use genetic methods to distinguish fish populations and trace their migration routes, which has enormous economic and political implications, especially when these routes cross international borders. These methods were worked out by evolutionary geneticists interested in explaining why species contain genetic variation. If DNA can be used to determine the outcome of paternity suits, or to identify criminals, as in a certain infamous trial, it is only because of the methods and data that have been developed by evolutionary scientists. We use penicillin, quinine, aspirin, taxol, and thousands of other natural chemicals originally discovered in wild plants and other organisms, and the chemical and drug industries are now engaged in searches for other useful natural products. They are aided by individuals trained in evolutionary systematics or evolutionary ecology: systematics because knowing the evolutionary relationships among species provides a guide to which species might have compounds similar to those already discovered, and evolutionary ecology because this field includes the study of how such products are adaptive for the organisms that produce them. The field of conservation biology relies on evolutionary principles in everything from preventing inbreeding depression in rare species to locating areas that should be afforded the highest conservation priority.
Now in view of the fact that evolutionary principles are important throughout biology;
-- of the fact that evolutionary biology has already contributed greatly, and promises to contribute even more, to solving a vast variety of practical and environmental problems;
-- of the fact that biology will shape the 21st century as physics shaped the 20th;
-- of the fact that no educated person can afford to be ignorant of science and technology;
-- of the fact that a recent survey of school children in 43 industrialized nations ranked the United States 18th in science and 23rd in mathematics; and that federal and educational commissions have called for major improvements in science education;
in view of these facts, you would think that everyone would agree on the need for the best possible education for our children in biology -- education that will be profoundly inadequate and misguided unless it includes the fundamental principles of evolution - which almost all biologists agree is one of the two most important unifying principles of biological science.
And so it is disheartening and astonishing that in an age in which we prepare to retrieve samples from the surface of Mars and to repair genetic diseases by gene therapy, a large proportion of Americans - perhaps 40% - do not know the most elementary principles of evolution or any of the evidence for evolution and do not believe in evolution ; and that a smaller but still large minority actively campaigns against teaching the next generation about one of the most important principles in modern science. I understand that the state of Tennessee has recently narrowly escaped renewing the embarrassing reputation it earned 72 years ago, by repudiating a bill designed to diminish public school education on the subject.
And so it seems appropriate to devote the few remaining minutes of this talk to why virtually all biological scientists consider evolution not only a theory, but also a fact.
Evolution is indeed a theory, IF we use that term in the narrow sense in which it is used in science. When physicists or chemists speak of quantum theory or atomic theory, they do not mean speculative or unproven hypotheses. They mean what the Oxford English Dictionary provides as its definition of theory as it is used in science: "a hypothesis that has been confirmed or established by observation or experiment, and is propounded or accepted as accounting for the known facts; a statement of what are known to be the general laws, principles, or causes of something known or observed." Just as atomic theory is a complex, well-substantiated body of statements about orbitals, charge, and so on, that explain the properties and behavior of matter, so the theory of evolution is a complex body of statements about mutation, natural selection, genetic drift, and other processes that are well documented and explain the alterations that life has undergone in the past 3.5 billion years.
But that life has undergone such changes -- that all organisms we know, living and extinct, are descended from simple, one-celled ancestors, is not an unproven hypothesis. That is, it is not a theory as the word is used in everyday speech. The descent of different organisms from common ancestors is a much a fact as the atomic constitution of matter. No scientist has published a paper on "new evidence for evolution" for more than a century, simply because it has not been an issue. You'd might as well ask the editors of an astronomy journal to publish a paper showing that the earth revolves around the sun.
The fact of evolution is based on evidence from every realm of biology. One immediately thinks of the fossil record, and indeed this is important evidence. Creationists sometimes claim that there are no fossil intermediates between very different ancestors and descendants, but this claim is flatly false. Just to cite 2 examples:
Biologists have known for at least a century that whales are descended from land-living mammals, because whales are definitely mammals, with milk, 3 middle ear bones, and other features that are diagnostic of mammals, and we know that the earliest mammals were terrestrial and evolved from terrestrial reptiles. Molecular evidence and some anatomical features indicated that whales are related to hoofed mammals, particularly to the group that includes pigs and cows. Until recently, the earliest known fossil whales were fully aquatic animals, whale-like in every respect. Within the last few years, that situation has changed. The artist has put some flesh on the bones in this picture, but the bones show intermediates, such as Ambulocetus and Rodhocetus, that were structurally like this condylarth, a member of the group that gave rise to hoofed mammals. These animals had partly functional hind legs and could move on land rather like seals, but they also propelled themselves as whales do, by moving the tail up and down. A somewhat later form was Basilosaurus, a fully aquatic early whale. Specimens of Basilosaurus have now been found with legs that were structurally complete, but too small to function.
I mentioned that we know that mammals evolved from reptiles. This is beautifully documented by intermediate fossils. Here are a few of the many intermediates that show the stepwise evolution of mammals from synapsid reptiles over the course of about 80 million years. In the skull alone, these fossil forms show stepwise changes in the evolution of the brain cas e; in the secondary palate, which separates the mouth cavity of mammals from the respiratory passage, but which is lacking in reptiles; the transition from simple, single-cusped teeth to the complex teeth of mammals, with multiple cusps on the molars and premolars; and especially the evolution of the lower jaw. In reptiles, this consisted of several bones, one of which, the articular, formed the joint with the quadrate bone of the skull. In the transitional forms, the dentary bone of the lower jaw became bigger and bigger, the other lower jaw bones became smaller and looser, and the dentary formed a second joint with the skull, articulating with the squamosal bone. Finally, the dentary/squamosal became the new joint, and the articular and quadrate bones became detached and modified into the hammer and anvil bones of the middle ear, where they serve for sound transmission. This transformation of these bones can be seen, repeated today, in the embryos of primitive mammals such as opossums.
But the fossil record is only one source of evidence, and by no means the most important one, for evolution.
We have observed evolution occurring before our eyes - mostly rather small changes to be sure, but at rates that if sustained for a few thousand years would produce major transformations equal to what we see in the fossil record. We have seen a major crisis in health care develop because organisms that cause malaria, tuberculosis, gonorrrhea, AIDS, and other diseases evolve resistance to antibiotics. We have seen almost 400 species of insect pests evolve resistance to chemical insecticides, and evolve the ability to attack strains of crops that had been bred for insect resistance. We have documented changes within a few decades in the diet of several insects, such as the apple maggot, which didn't attack apples until about a century ago. We have seen evolutionary change in the life histories of codling moths and other insects, in the direction that bird species migrate, and in the skeletal dimensions and coloration of birds such as the house sparrow. We have seen plants adapt to herbicides and toxic soils, and have seen new species of plants arise.
Finally, the molecular revolution in biology has furnished us with mountains of information that not only attests to the history of evolution, but also sheds ever more light on evolutionary processes. From the similarity of DNA sequences among different species, we obtain entirely new data on their phylogenetic relationships. Here, for example, is a small excerpt from some DNA sequence data, showing how the phylogeny of 4 species can be inferred. Notice that the DNA sequences range from about 96% to more than 98% identical. These species happen to be human (1), chimpanzee (2), gorilla (3), and orangutan (4). The data imply that humans are most closely related to chimpanzees and least closely related to orangutans among the great apes. Here, from the same DNA study, is a phylogeny of a broader range of primates: it portrays the same relationships as those we infer from anatomy and from the DNA sequences of other genes. That is, independent sources of information point to the same answer -- a consilience of evidence that throughout science is taken as one of the strongest indications that the hypothesis is true. The particular significance of this study is that the DNA sequence is a pseudogene: a formerly functional gene that has been silenced and serves no function at all. The similarities between humans and chimpanzees in this DNA sequence are due to mutations that occurred since these species separated from their common ancestor -- mutations that have no effect whatever on the organism, since the gene is inoperative.
From molecular studies of evolution, we are learning an immense amount about the evolutionary process. Where, for example, did humans and other organisms get all their thousands of genes, which are responsible for the intricate biochemical and developmental processes in an organism?
We know of several mechanisms. For example, the protein product of a single gene sometimes serves several different functions. The protein that constitutes the eye lens of a bird, for instance, is the same as arginosuccinate dehydrogenase, an enzyme that serves an entirely different biochemical function elsewhere in the body.
We also know that whole genes or parts of genes are sometimes duplicated, so that a chromosome then bears a greater number of copies. This process is well understood by molecular biologists, and has been observed in laboratory organisms. The copies then can diverge by mutation and natural selection, so that they become specialized for different functions - such as the two functions served by the eye lens protein. By knowing the phylogenetic relationships among different organisms, we can trace the history of gene duplication and divergence. For instance, the lamprey, a primitive, jawless fishlike creature, has only a single form of hemoglobin, encoded by a single gene. In the ancestor of vertebrates with jaws, the gene duplicated, forming the so-called alpha and beta chains that together form the actual hemoglobin in our red blood cells. In mammals, each of these has been duplicated several times, so that humans carry 9 different hemoglobin genes, encoding functionally different hemoglobins, some of which are active in the fetus, and others after birth. Thus the number and diversity of genes has increased by gene duplication and divergence.
A third process that accounts for genetic and biochemical complexity is mixing and matching. Duplicated genes or parts of genes with different functions have been joined together like Lego blocks into new combinations, giving rise to a tremendous variety of new genes with different functions. For example, this is a diagram of 5 of the different proteins involved in blood clotting. Each of them is made up of different numbers and combinations of modules, shown by different symbols. Each of these modules also is part of entirely different proteins not shown here. For instance, this linear module is also part of the protein-digesting enzyme trypsin.
What is occurring in biology, in fact, is that molecular biology and evolutionary biology are becoming inseparable. Molecular biologists work out the mechanisms of life by studying the DNA and enzymes of a few convenient organisms such as yeast and fruit flies. They provide the fundamental understanding that evolutionary biologists then use for comparing a great variety of different species, in order to learn how the molecular foundations have been acted on by mutation and natural selection and have given rise to the marvellous diversity of living things. By comparing different organisms, evolutionary biologists become able to say how important various molecular mechanisms such as gene duplication have been in making organisms what they are, and they sometimes find patterns of molecular variation that inform molecular biologists about processes that they hadn't known about.
The outcome of this synergism is that biology as a whole is on a steeper learning curve than ever before, and biological knowledge in the next 10 to 20 years will increasingly change our lives. We owe it to those who will live through this change to make available the best possible education in biology - and this includes education in the process of biological change - of evolution, the single most comprehensive explanation of the living world.
Dr. Douglas Futuyma
Department of Ecology and Evolution
State University of New York at Stony Brook
This site is maintained by Steve Furches (furches<at>gmail<dot>com). This page last updated 22-June-2007.