A Kansan's Guide to Science, page 2 of 4
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Science is a way of learning about the universe. It provides a way of understanding and explaining our observations of the natural world. Science deals with observed phenomena and seeks to explain the way the world works by employing natural, orderly relationships. Science pursues truth within very narrow limits. Some of humanity's most profound questions pertain to meaning, purpose, and morality. Their answers lie beyond the reach of science.
A crucial part of the scientific method involves making hypotheses. A hypothesis (boldface words are included in the glossary at the end of the book) is an idea that explains observations about the world. In most areas of science, a useful hypothesis is one that makes predictions about the results of future experiments or of future observations from the natural world. These can then be tested against new observations and the hypothesis modified or rejected if necessary.
A hypothesis that has been tested repeatedly and never found to be false is likely to be accepted. If it survives many tests and if it covers a range of topics, it may be referred to as a theory. Thus, in science the term theory has a precise meaning, different from its use in everyday life. A scientific theory is "a statement of what are held to be the general laws, principles, or causes of something known or observed" (Oxford English Dictionary). The important thing is that it is testable. Some well-known theories are the theory of gravitation and the atomic theory of matter. An idea--a theory or hypothesis--that cannot be tested is not interesting to scientists. They refer to such ideas that are outside the realm of science as being unfalsifiable. For example, many ideas about religion are unfalsifiable by the methods of science and thus lie outside the realm of science.
A common misunderstanding about the work of scientists has to do with proof. Although hypotheses or theories can be disproved by the methods of science, they cannot be proved. No scientific theory can be proved in the sense of a mathematical or logical proof. Any accepted scientific theory is simply the best existing explanation of the observations already made, an explanation that has not been shown to be false. This is as true for astronomy, geology, and evolutionary biology as it is for the nonhistorical sciences of physics and chemistry. It is important to realize that some theories are held in higher regard than others. Those that explain a wide range of observations and that generate many fruitful and testable hypotheses are held with greater confidence. Evolution is one such theory. It has been incredibly fruitful for research in medicine, developmental biology, genetics, ecology, paleontology, and geology.
Besides being testable and falsifiable, a scientific idea is most useful if it answers more questions than it raises. The idea that the Earth revolves around the sun is a good scientific idea because it explains the progression of the seasons upon which much of our economy is based. No one has ever seen the Earth move around the sun, and no one will ever do so because there can be no stationary platform in space from which to make such observations. Similarly, no one has seen an electron, touched a thermometer to the sun to learn that it is very hot, or watched Tyrannosaurus rex stalk its prey. In spite of this, these scientific ideas are considered useful and reasonable interpretations of the evidence.
The general field of science can be divided into two groups--historical science and experimental science. Experimental science includes physics, chemistry, genetics, and some branches of astronomy. Experimental scientists probably fit the popular conception of scientists as people in white lab coats pouring chemicals or watching dials. Experimental scientists are interested in making predictions about the future. Ultimately, their goal is to derive scientific laws from hypotheses and theories. Laws are statements, such as the law of gravity, that always apply.
Geology, paleontology, evolutionary biology, and some branches of astronomy are said to be historical sciences. A useful way of thinking about those sciences is to realize that the experiments have already been run by nature, sometimes millions of years ago. The job of the historical scientist is to determine what happened in the past--what experiment nature conducted. A hypothesis in the historical sciences is an explanation of these past events, an explanation that can be tested.
A historical scientist can make observations and statements about the past and develop hypotheses from evidence left from nature's past experiments. For example, at numerous times in the distant past the temperature at the Earth's surface was much lower than it is today. At each of these episodes of cooling, we can study how the animal and plant life evolved, migrated, and went extinct. This enables us to say how life has been affected by these events. We can use this information to make statements about past events and predictions of the future.
Hypotheses in the historical sciences are continually being tested by new observations. If the ideas in a hypothesis are contradicted by new observations, then that hypothesis is rejected. Like all sciences, the historical sciences proceed by an almost continual process of hypothesis development and testing. Many ideas about the historical sciences that were developed in the past have been discarded with the accumulation of new observations and the development of new ideas that provide better explanations. Evolutionary theory remains powerful because it continues to generate fruitful and testable hypotheses and has not been contradicted by evidence from the natural world and because it best explains our observations.
Evolutionary biologists, from paleontologists to molecular geneticists, try to determine the precise sequence of life's history and which species are most closely related to one another. Thus, the study of evolution involves historical science. Evolutionary biologists, however, also try to determine the mechanisms that govern how life evolves. These mechanisms include natural selection, chance factors, and various constraints on the way life evolves. That is why the study of evolution also involves experimental science. We can test some of the mechanisms of evolution in the lab by studying bacteria, flies, or mice over many generations and watching how they evolve over months or years in controlled experiments. Many similar natural experiments have been conducted over many millions of years, and their results are recorded in the fossil record of ancient life.
In spite of its name, creation science is not science at all but is in fact pseudoscience, much like astrology or alchemy. Pseudoscience does not restrict itself to natural explanations of the physical world but often involves supernatural explanations. Creation science is a branch of creationism that is based on the literal interpretation of the Bible and holds that the origin of living species and the Earth occurred as a single event a few thousand years ago. Creation science, unlike legitimate sciences, does not seek to derive explanations through observations and testing of the natural world but instead begins with a belief system and then seeks to find evidence to support this view. Therefore, by its very nature, creation science is not a science because it is not testable and falsifiable.
Biological evolution is the theory that life on Earth has developed gradually from a common ancestor. This is sometimes described as descent with modification. Since life's origin some 4 billion years ago, life has changed in numerous ways as it branched out into different lineages, with adaptations to different environments and changes in genetic makeup. Some major changes include the evolution of plants and animals and, among the animals, some major groups became extinct. The evolution of life can be thought of as a tree with different lines splitting off. Some branches were cut short by extinction, while others are represented by species living in the modern world. It is inaccurate, for example, to view the evolution of the hominids, as a straight-line transformation from some apelike form into transitional forms and eventually with humans at the pinnacle. Humans did not evolve from chimpanzees or gorillas, but instead humans and modern apes share a common ancestor that lived at some time in the past, probably about 6 million years ago.
Evolution is not just a fact and not just a theory; it is both! Evolution is a fact because there is overwhelming scientific evidence that evolution has happened in the past and continues to happen. No reputable scientist questions evolution. That is, all scientific data support the fact that the Earth is more than 4.5 billion years old, that cellular life appeared before 3.5 billion years ago, and that all modern life is descended from a common ancestor. Sometimes people ask scientists and teachers, in the interest of fairness, to present the evidence against evolution. Scientists and teachers do not present such evidence because there simply is no such evidence against evolution. True, there are things we do not yet know, but no one has found any scientific evidence to suggest that evolution does not occur, or that it does not explain all
When people say that evolution is just a theory, they are partly correct; but they are confused about what it means for an idea to be a scientific theory. The scientific usage of theory is different from its meaning in everyday usage. In everyday language, a theory is a hunch or a guess. In science a theory is defined as "a statement of what are held to be the general laws, principles, or causes of something known or observed" (Oxford English Dictionary). Thus, when scientists refer to evolutionary theory, they are talking about the mechanisms of evolution, not about some vague idea of whether evolution has happened or not. Evolutionary theories are a way of explaining the fact of evolution. Scientists have suggested many mechanisms of evolution. Some we accept; some we no longer accept. For example, the theory of natural selection, put forward by Darwin and Wallace in 1858, is still considered an accurate explanation of this mechanism of evolution. On the other hand, we no longer accept the theory of inheritance of acquired characters, which was proposed early in the 19th century by the French biologist Lamarck. Among other things, for example, Lamarck had the idea that successive generations of giraffes inherited ever longer necks because their parents stretched their necks to reach higher branches.
In all sciences, debate about theories exists, but that does not change the facts. For example, just because we cannot always predict where a thunderstorm will happen, this does not mean that thunderstorms do not occur. Similarly, today biologists and paleontologists sometimes debate the importance of different evolutionary mechanisms such as natural selection. These debates do not detract from the fact of evolution; they only demonstrate that evolution is a complex process, like the weather, and that the work of science is not yet finished.
The theory of evolution is supported by evidence from experimentation and from the fossil record. In the laboratory, scientists have shown how fruit flies, bacteria, and many other types of organisms adapt and change evolutionarily, even over the short time scales of human experimentation. Similarly, scientists have observed populations evolving in the wild, both in Kansas and elsewhere. The development of resistance to antibiotics by various disease-causing bacteria, including the bacteria responsible for tuberculosis, is one well-known and unfortunate example of evolution.
Outside the laboratory, evidence for evolution also comes from the fossil record. The evolution of organisms shown by the geological record is consistent the world over. That is, through time the fossil species follow the same order of appearance everywhere. Moreover, that order of evolution and first appearance of fossil species is consistent with everything we know about the relationships among fossils. For example, the fossil record of vertebrates has the following order of first appearances: jawless fishes fishes with jaws lobe-finned fishes amphibians highly terrestrial amphibians reptiles mammal-like reptiles mammals hoofed mammals horses. Similar sequences can be reconstructed for virtually every other living group of organisms. In addition, numerous examples of transitional fossil species are known. A transitional species possesses a mixture of traits that are characteristic of both ancestral and descendant organisms, indicating that it represents an intermediate form. Some examples include the evolutionary transition between fish and amphibians, reptiles and mammals, terrestrial reptiles and birds, and hoofed mammals and whales (fig. 1). Each of these evolutionary transitions involved several distinct species that were intermediate in character between their ancestors and their descendants.
Fig. 1--Evolutionary sequence depicting the transition between terrestrial mammals and whales (mya = million years ago). After drawings by N. Haver, copyright Sinauer Associates; in, Purves et al. (1998).
Several Kansas fossils are key transitional species. One example is the Pennsylvanian-aged reptile Petrolacosaurus, which has features of early reptilian groups as well as later reptiles like crocodiles and the dinosaurs (figs. 4 and 5 [on next page]). Edaphosaurus (fig. 6 [on next page]), also known from a Pennsylvanian-aged lagoon in Kansas, is a transitional link between early reptiles and other groups more closely related to modern mammals. The mosasaurs, which must have terrified the occupants of Kansas seaways in the Cretaceous, are a link between lizardlike groups and snakes (fig. 8 [on next page]). The Cretaceous-aged Parahesperornis, an early bird, is another important evolutionary transitional form between reptiles and modern birds (fig. 11 [on next page]). Like extinct and modern reptiles, Parahesperornis had bones that were not hollow and a mouth filled with teeth, yet it had a birdlike skull, wings, feet, and body, making it a transitional form. Finally, a more recent example is Pliohippus, a relative of the modern horse that roamed the grassy plains of Kansas within the last few million years. Like the modern horse that served the native peoples and early settlers of Kansas so well, Pliohippus was well adapted to the ancient environments of Kansas. About the size of a modern pony, Pliohippus had a single toe or hoof, and is a transitional link between the older, small horses with many toes, which are extinct and only known as fossils, and the modern large draft horse that populates Kansas.
Many people wonder whether the small-scale evolution that scientists observe in the laboratory and in the wild (microevolution) is distinct from the large-scale evolutionary trends that scientists observe, say, in the fossil record (macroevolution). What seems to separate microevolution from macroevolution is the act of speciation--that is, the process in which a new species is formed. Changes can occur within a single species as it adapts to its local environment; changes that are referred to as microevolution. Antibiotic resistance in bacteria is a good example of microevolutionary change. In adapting to an environment of antibiotics through natural selection, bacteria have evolved resistance to the drugs. If there is microevolutionary change within a species but that species does not split off a new descendent species, then there is unlikely to be any visible evolutionary change over long time scales.
Major or macroevolutionary changes are tied to speciation, the origin of new species, which usually occurs when populations of a species are isolated from one another over long periods of time. A small, isolated population can then undergo its own independent microevolutionary changes as it adapts to local environments. With sufficient evolutionary divergence, a new species evolves, and microevolutionary changes are translated to macroevolution. Thus, microevolution and macroevolution are not distinct processes. They are related, and macroevolution results when such situations as the geographical isolation of small populations occur. This makes it more likely that microevolutionary changes will add up to macroevolution.
The evolution of horses provides an example of how new species form following geographic isolation. North America was home to the earliest known horse, Hyracotherium, which was about the size of a small dog, with a curved back, unspecialized teeth, and three and four toes on the back and front hooves respectively, whereas modern horses have only one toe on each of their four hoofs. In the Miocene, about 35 million years ago, as extensive grassy prairies evolved for the first time, Merychippus, a three-toed horse that descended from Hyracotherium, evolved high-crowned teeth that allowed it to feed on grass. Its large size enabled it to run fast to escape the predators that were evolving at the time, especially sabertoothed cats and wolves.
Descendants of Merychippus migrated from North America to the Old World where, in isolated populations, new species evolved that soon went extinct. Meanwhile, in North America isolated populations of Merychippus evolved into the one-toed Pliohippus in the Pliocene and into Equus in the Pleistocene. Shortly after early humans came to North America, Equus went extinct on this continent, but the populations of Equus that had migrated to the Old World thrived and again, as isolated populations, evolved into the six species of Equus we know today as well as several that are known only from the fossil record. Today's species include three separate species of zebra, the onager of Asia, the donkey, and Equus caballus, the modern horse. Domestication of the modern horse began some 2,500 years ago, and they were taken by humans to North America some time around 1500 AD.
Natural selection happens when some individuals in a population survive and reproduce more successfully than others because of their inherited traits. Because these traits enable the individuals to reproduce more successfully, over time the traits will appear more frequently and begin to accumulate in the population. The outcome of this microevolutionary process is adaptation. That is, over time through natural selection, individuals in a population will come to possess traits that lead to their being well adapted to their environment. In fact, Darwin's ideas on how natural selection works are really rather simple, and they are exactly the same as the procedure animal breeders have used for thousands of years to develop new breeds of animals. Animals vary, and some of the variation is inherited. Animals have more offspring than can possibly survive. On average, other things being equal, animals will tend to survive if they have inherited traits that are favored by the environment to lead to survival and reproduction. In this way, favorable, adaptive traits accumulate in a population of organisms, and traits that are not adaptive are weeded out. Such principles are being applied today to Kansas livestock and crop species.
Natural selection is often described as the best explanation of how evolution happened, and numerous examples demonstrate that it is an important force controlling evolution. Polar bears offer one example. They hunt in the snowy arctic; they sneak up on their prey; and they are, after all, white. Thus they appear to be well adapted to their environment. Many features in the history of life, however, suggest that chance has played a very important role in evolution, too. For example, several times in the history of life, mass extinctions have occurred--events so severe that they eliminated most of the species on Earth. Such a mass extinction ended the dinosaurs' reign at the end of the Cretaceous Period. Prior to the mass extinction, dinosaurs were large and dominated the world, whereas mammals were small and nocturnal, sneaking around in the dinosaurs' world. It was not until the dinosaurs went extinct that the mammals could take over and flourish. The mechanism that caused this extinction was a massive asteroid striking the Earth. This example points out that natural selection alone is insufficient to explain the history of life. No matter how well adapted an animal is, it may not survive a catastrophe like a collision with a five-mile-wide asteroid.
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