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Evolution3

Evolution3 The world around us changes. This simple fact is obvious everywhere we look. Streams wash dirt and stones from higher places to lower places. Untended gardens fill with weeds. Other changes are more gradual but much more dramatic when viewed over long time scales. Powerful telescopes reveal new stars coalescing from galactic dust, just as our sun did more than 4.5 billion years ago. The earth itself formed shortly thereafter, when rock, dust, and gas circling the sun condensed into the planets of our solar system. Fossils of primitive microorganisms show that life had emerged on earth by about 3.8 billion years ago. Similarly, the fossil record reveals profound changes in the kinds of living things that have inhabited our planet over its long history. Trilobites that populated the seas hundreds of millions of years ago no longer crawl about. Mammals now live in a world that was once dominated by reptilian giants such as Tyrannosaurus rex. More than 99 percent of the species that have ever lived on the earth are now extinct, either because all of the members of the species died, the species evolved into a new species, or it split into two or more new species. Many kinds of cumulative change through time have been described by the term "evolution," and the term is used in astronomy, geology, biology, anthropology, and other sciences. This document focuses on the changes in living things during the long history of life on earth—on what is called biological evolution. The ancient Greeks were already speculating about the origins of life and changes in species over time. More than 2,500 years ago, the Greek philosopher Anaximander thought that a gradual evolution had created the world's organic coherence from a formless condition, and he had a fairly modern view of the transformation of aquatic species into terrestrial ones. Following the rise of Christianity, Westerners generally accepted the explanation provided in Genesis, the first book of the Judeo-Christian-Muslim Bible, that God created everything in its present form over the course of six days. However, other explanations existed even then. Among Christian theologians, for example, Saint Thomas Aquinas (1225 to 1274) stated that the earth had received the power to produce organisms and criticized the idea that species had originated in accordance with the timetables in Genesis.1 -------------------------------------------------------------------------------- Charles Darwin, Alfred Russel Wallace, and Gregor Mendel laid the foundations of modern evolutionary theory. During the early 1800s, many naturalists speculated about changes in organisms, especially as geological investigations revealed the rich story laid out in the fossilized remains of extinct creatures. But although ideas about evolution were proposed, they never gained wide acceptance because no one was able to propose a plausible mechanism for how the form of an organism might change from one generation to another. Then, in 1858, two English naturalists—Charles Darwin and Alfred Russel Wallace—simultaneously issued papers proposing such a mechanism. Both men observed that the individual members of a particular species are not identical but can differ in many ways. For example, some will be able to run a little faster, have a different color, or respond to the same circumstance in different ways. (Humans— including any class of high school students—have many such differences.) Both men further observed that many of these differences are inherited and can be passed on to offspring. This conclusion was evident from the experiences of plant and animal breeders. -------------------------------------------------------------------------------- Darwin and Wallace were both deeply influenced by the realization that, even though most species produce an abundance of offspring, the size of the overall population usually remains about the same. Thus, an oak tree might produce many thousands of acorns each year, but few, if any, will survive to become full-grown trees. Darwin—who conceived of his ideas in the 1830s but did not publish them until Wallace came to similar conclusions—presented the case for evolution in detail in his 1859 book On the Origin of Species by Natural Selection. Darwin proposed that there will be differences between offspring that survive and reproduce and those that do not. In particular, individuals that have heritable characteristics making them more likely to survive and reproduce in their particular environment will, on average, have a better chance of passing those characteristics on to their own offspring. In this way, as many generations pass, nature would select those individuals best suited to particular environments, a process Darwin called natural selection. Over very long times, Darwin argued, natural selection acting on varying individuals within a population of organisms could account for all of the great variety of organisms we see today, as well as for the species found as fossils. -------------------------------------------------------------------------------- If the central requirement of natural selection is variation within populations, what is the ultimate source of this variation? This problem plagued Darwin, and he never found the answer, although he proposed some hypotheses. Darwin did not know that a contemporary, Gregor Mendel, had provided an important part of the solution. In his classic 1865 paper describing crossbreeding of varieties of peas, Mendel demonstrated that organisms acquire traits through discrete units of heredity which later came to be known as genes. The variation produced through these inherited traits is the raw material on which natural selection acts. Mendel's paper was all but forgotten until 1890, when it was rediscovered and contributed to a growing wave of interest and research in genetics. But it was not immediately clear how to reconcile new findings about the mechanisms of inheritance with evolution through natural selection. Then, in the 1930s, a group of biologists demonstrated how the results of genetics research could both buttress and extend evolutionary theory. They showed that all variations, both slight and dramatic, arose through changes, or mutations, in genes. If a mutation enabled an organism to survive or reproduce more effectively, that mutation would tend to be preserved and spread in a population through natural selection. Evolution was thus seen to depend both on genetic mutations and on natural selection. Mutations provided abundant genetic variation, and natural selection sorted out the useful changes from the deleterious ones. Selection by natural processes of favored variants explained many observations on the geography of species differences—why, for example, members of the same bird species might be larger and darker in the northern part of their range, and smaller and paler in the southern part. In this case, differences might be explained by the advantages of large size and dark coloration in forested, cold regions. And, if the species occupied the entire range continuously, genes favoring light color and small size would be able to flow into the northern population, and vice versa—prohibiting their separation into distinct species that are reproductively isolated from one another. Glossary of Terms Used in Teaching About Evolution Evolution: Change in the hereditary characteristics of groups of organisms over the course of generations. (Darwin referred to this process as "descent with modification.") Species: In general, a group of organisms that can potentially breed with each other to produce fertile offspring and cannot breed with the members of other such groups. Variation: Genetically determined differences in the characteristics of members of the same species. Natural selection: Greater reproductive success among particular members of a species arising from genetically determined characteristics that confer an advantage in a particular environment. How new species are formed was a mystery that eluded biologists until information about genetics and the geographical distribution of animals and plants could be put together. As a result, it became clear that the most important source of new species is the process of geographical isolation—through which barriers to gene flow can be created. In the earlier example, the interposition of a major mountain barrier, or the origin of an intermediate desert, might create the needed isolation. Other situations also encourage the formation of new species. Consider fish in a river that, over time, changes course so as to isolate a tributary. Or think of a set of oceanic islands, distant from the mainland and just far enough from one another that interchange among their populations is rare. These are ideal circumstances for creating reproductive barriers and allowing populations of the same species to diverge from one another under the influence of natural selection. After a time, the species become sufficiently different that even when reunited they remain reproductively isolated. They have become so different that they are unable to interbreed. In the 1950s, the study of evolution entered a new phase. Biologists began to be able to determine the exact molecular structure of the proteins in living things—that is, the actual sequences of the amino acids that make up each protein. Almost immediately, it became clear that certain proteins that serve the same function in different species have very similar amino acid sequences. The protein evidence was completely consistent with the idea of a common evolutionary history for the planet's living things. Even more important, this knowledge provided important clues about the history of evolution that could not be obtained through the fossil record. As a zoologist I have discovered many phenomena that can be rationally explained only as products of evolution, but none so striking as the ancestor of the ants. Prior to 1967 the fossil record had yielded no specimens of wasps or other Hymenopterous insects that might be interpreted as the ancestors of the ants. This hypothetical form was a missing link of major importance in the study of evolution. We did have many fossils of ants dating back 50 million years. These were different species from those existing today, but their bodies still possessed the basic body form of modern ants. The missing link of ant evolution was often cited by creationists as evidence against evolution. Other ant specialists and I were convinced that the linking fossils would be found, and that most likely they would be associated with the late Mesozoic era, a time when many dinosaur and other vertebrate bones were fossilized but few insects. And that is exactly what happened. In 1967 I had the pleasure of studying two specimens collected in amber (fossilized resin) from New Jersey, and dating to the late Mesozoic about 90 million years ago. They were nearly exact intermediates between solitary wasps and the highly social modern ants, and so I gave them the scientific name Sphecomyrma, meaning "wasp ant." Since that time many more Sphecomyrma specimens of similar age have been found in the United States, Canada, and Siberia, but none belonging to the modern type. With each passing year, such fossils and other kinds of evidence tighten our conception of the evolutionary origin of this important group of insects. The discovery of the structure of DNA by Francis Crick and James Watson in 1953 extended the study of evolution to the most fundamental level. The sequence of the chemical bases in DNA both specifies the order of amino acids in proteins and determines which proteins are synthesized in which cells. In this way, DNA is the ultimate source of both change and continuity in evolution. The modification of DNA through occasional changes or rearrangements in the base sequences underlies the emergence of new traits, and thus of new species, in evolution. At the same time, all organisms use the same molecular codes to translate DNA base sequences into protein amino acid sequences. This uniformity in the genetic code is powerful evidence for the interrelatedness of living things, suggesting that all organisms presently alive share a common ancestor that can be traced back to the origins of life on earth. One common misconception among students is that individual organisms change their characteristics in response to the environment. In other words, students often think that the environment acts on individual organisms to generate physical characteristics that can then be passed on genetically to offspring. But selection can work only on the genetic variation that already is present in any new generation, and genetic variation occurs randomly, not in response to the needs of a population or organism. In this sense, as Francois Jacob has written, evolution is a "tinkerer, not an engineer."2 Evolution does not design new organisms; rather, new organisms emerge from the inherent genetic variation that occurs in organisms. Genetic variation is random, but natural selection is not. Natural selection tests the combinations of genes represented in the members of a species and allows to proliferate those that confer the greatest ability to survive and reproduce. In this sense, evolution is not the simple product of random chance. The booklet Science and Creationism: A View from the National Academy of Sciences3 summarizes several compelling lines of evidence that demonstrate beyond any reasonable doubt that evolution occurred as a historical process and continues today. In brief: Fossils found in rocks of increasing age attest to the interrelated lineage of living things, from the single-celled organisms that lived billions of years ago to Homo sapiens. The most recent fossils closely resemble the organisms alive today, whereas increasingly older fossils are progressively different, providing compelling evidence of change through time. Even a casual look at different kinds of organisms reveals striking similarities among species, and anatomists have discovered that these similarities are more than skin deep. All vertebrates, for example, from fish to humans, have a common body plan characterized by a segmented body and a hollow main nerve cord along the back. The best available scientific explanation for these common structures is that all vertebrates are descended from a common ancestor species and that they have diverged through evolution. In the past, evolutionary relationships could be studied only by examining the consequences of genetic information, such as the anatomy, physiology, and embryology of living organisms. But the advent of molecular biology has made it possible to read the history of evolution that is written in every organism's DNA. This information has allowed organisms to be placed into a common evolutionary family tree in a much more detailed way than possible from previous evidence. For example, as described in Chapter 3, comparisons of the differences in DNA sequences among organisms provides evidence for many evolutionary events that cannot be found in the fossil record. Bibliography: Biological Sciences Curriculum Study. 1978. Biology Teachers' Handbook. 3rd ed. William V. Mayer, ed. New York: John Wiley and Sons. Francois Jacob. June 10, 1977. Evolution and tinkering. Science 196:1161-1166. National Academy of Sciences. (in press). Science and Creationism: A View from the National Academy of Sciences. Washington, DC: National Academy Press. (See http://www.nap.edu/) P. Ewald. 1994. The Evolution of Infectious Disease. New York: Oxford University Press. "Evolution, Science, and Society: A White Paper on Behalf of the Field of Evolutionary Biology," Draft, June 4, 1997. Jonathan Weiner. 1994. The Beak of the Finch. New York: Alfred A. Knopf. Peter R. Grant. 1991. Natural selection and Darwin's finches. Scientific American, October, pp. 82-87. James H. Tumlinson, W. Joe Lewis, and Louise E. M. Vet. 1993. How parasitic wasps find their hosts. Scientific American, March, pp. 100-106. F. Fenner and F.N. Ratcliffe. 1965. Myxomatosis. Cambridge: Cambridge University Press.

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