Studies of evolution include both fact and theory. It is a fact that groups of organisms evolve, that is, they change over time. The evidence for the fact of evolution is everywhere. Many people think of the fossils that show the history of life on Earth. However, Darwin discovered evidence for evolution from his observations of animals living on islands. They represented different species, yet they all seemed related. He concluded they were related, that they evolved on the islands. Today perhaps the best evidence comes from genes. All organisms—bacteria, yeast, insects, reptiles, birds, and mammals (including us)—share some genes. About 3.8 billion years ago, a population of organisms lived that carried these genes. Every living thing possesses them.

Evolution is also a theory about causes. The main cause of evolution is natural selection. Natural selection is a force in nature that preserves some organisms and destroys others. Natural selection may be environmental, like drought that kills all the fish in a disappearing lake. It may also consist of interactions among species, like that between cat and mouse. Natural selection is also well-supported with evidence. The evidence comes from experiments with living organisms in laboratories and from observations of nature.

This book is about major discoveries in biology and their impact on our understanding of evolution since Darwin published On the Origin of Species in 1859. At the same time, it is also about evolution as science, about whether it is good science, whether its new discoveries constitute good science, and whether they support Darwin’s original concepts.

Darwin’s work on evolution produced four major accomplishments. First, it provided a mechanism (natural selection) that explains how evolution occurs. Second, it united in one synthesis information from fields of biology that seemed separate. Third, it offered testable predictions. Fourth, it inspired new research in every field it touched, creating new publications, techniques, and institutions. It continues to inspire research today.

Most new discoveries after Darwin tended to support Darwin’s work. One, however, threatened to undo his synthesis. This was the rediscovery in 1900 of Mendel’s work on genes that proved inheritance discontinuous, whereas Darwin’s concept of gradual change seemed to imply that inheritance is continuous. However, biologists successfully combined Mendel’s discoveries with Darwin’s gradual evolution and declared Darwin’s synthesis restored under the name New Synthesis (or neo-Darwinism) in 1947. Nonetheless, the New Synthesis remained incomplete. A third synthesis called sociobiology, begun in 1964, restored two other elements from Darwin’s original synthesis—the importance to evolution of relatives and of sexual selection. Now only developmental biology remains excluded. As of the 1980s, biologists began a forth synthesis under the name of evo-devo—evolutionary developmental biology. It is a work in progress.

Science accumulates knowledge

Science describes the physical world and discloses its history. The physical world is composed of matter and energy. Science limits its descriptions to matter and energy, their composition, interactions, and development.

Science cannot describe the physical world exactly as it is, for science is ignorant of many details. Moreover, an exact description would be too cumbersome to use. Therefore, science builds simplified models of the physical world. In many ways, this is similar to creating a map. Maps are models. A city street map, for example, may show the main roads and the location of the central government buildings and places of historical interest. However, it cannot show all the details because these would clutter the map, making it impossible to use. Moreover, the map’s purposes are restricted. Some of the features are irrelevant to the purpose of the map. Others may be unknown to the mapmakers. Thus, the map ignores the minor streets, the shops, apartment complexes, private homes, telephone poles, electric lines, and sewers.

Scientific knowledge changes. However, it does not change randomly. Rather, it develops, accumulating facts, asking new questions, making fresh predictions, and altering and enlarging theories. Scientists may know certain facts A, B, and C and have a theory that ties them together. However, the theory is limited because it fails to cover other known facts, D and E, which may support a different theory. Through painstaking work, scientists develop a new theory that not only explains A, B, and C, but also explains D and E. This is a better theory because it is bigger. It encompasses more knowledge. At the same time, it may raise new questions, leading scientists to look for new information, to discover new facts. Later, these may be accommodated in the old theory, or they may require a theory that differs from the old one. Thus, scientific knowledge accumulates, and theories expand and change to encompass the new information.

For example, Charles Darwin, the founder of modern biology, knew several books that described evolution, one written by his own grandfather. The idea that species transformed into other species was popular, yet scientists rejected it because no one could provide a mechanism to explain how species change. During many years of observation and experimentation, Darwin discovered facts no one had known before, then added knowledge that others accumulated from geology, embryology, anatomy, and biogeography. (Biogeography is the study of the location on Earth of existing and fossil organisms.) Then he offered a mechanism that explained the facts. He called the mechanism that drove the transformation of species natural selection. Discovery of this mechanism for species change transformed the study of biology forever.

Darwin’s work raised new questions. This is one sign of a major scientific theory. Major theories are fertile. They inspire new questions, open up new areas of research, generate new publications, and sometimes create new disciplines and new institutions. One of the major questions Darwin’s work raised was about heredity. No one knew about the role genes play in heredity because no one knew about genes. Almost 50 years after Darwin’s work, biologists discovered genes. At first, some biologists thought they raised problems for Darwin’s theory of evolution by natural selection, but after several decades of theoretical work and new discoveries, biologists incorporated genes into the theory of evolution. The enlarged theory is called neo-Darwinism or the New Synthesis. It supports Darwin’s mechanism of natural selection so well that for almost 75 years now, biologists have considered natural selection the main mechanism driving evolution, just as Darwin’s original theory said.

A scientific theory, then, is not some wooly idea invented for the occasion. Rather it is a carefully worked out concept, based on evidence, used to integrate as much data as possible under one explanation.