Unit Three Background
The concept of evolution has an importance in education that goes beyond its power as a scientific explanation. All of us live in a world where the pace of change is accelerating. Today’s children will face more new experiences and different conditions than their parents or teachers have had to face in their lives. The story of evolution is one chapter -- perhaps the most important one – in a scientific revolution that has occupied much of the past four centuries. The central feature of this revolution has been the abandonment of one notion about stability after another: that the earth was the center of the universe, that the world’s living things are unchangeable, that the continents of the earth are held rigidly in place, and so on. Fluidity and change have become central to our understanding of the world around us. To accept the probability of change – and to see change as an agent of opportunity rather than as a threat – is a silent message and challenge in the lesson evolution.
Evolution means a process of change, an unfolding. The world around us changes. This is a 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 form 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 emerged on earth by about 3.8 billion years ago. 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. The ancient Greeks were already speculating about the origins of life and the 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. Charles Darwin and Alfred Russel 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, and oak tree might produce many thousands of acorns each year, but few, if any, will survive to become full-grown trees.
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 the great variety of organisms we see today, as well as for the species found as fossils.
One common misconception among students is that individual organisms change their characteristics in response to the environment. In other works, 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 needs of a population or organism. In this sense, as Francois Jacob has written, evolution is a “tinkerer, not an engineer.” 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 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 form the National Academy of Sciences 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 similarity 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 form a common ancestor species and that they have diverged through evolution.
- In the past, evolution 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 comparisons of the differences in DNA sequences among organisms provide evidence for many evolutionary events that cannot be found in the fossil record.
Evolution is the only plausible scientific explanation that accounts for the extensive array of observations summarized above. The concept of evolution through random genetic variation and natural selection makes sense of what would otherwise be a huge body of unconnected observations. From the cumulative evidence presented by scientists, it is no longer possible to sustain scientifically the view that the living things we see today did not evolve from earlier forms or that the human species was not produced by the same evolutionary mechanisms that apply to the rest of the living world.
National Science Foundation, Teaching About Evolution and the Nature of Science, National Academy Press, 1998, ISBN 0-309-06364-7