can occur that will be passed on to offspring. Over time, the members of a population of flies adapted to a new environment can undergo so many changes that they no longer can, or routinely do, interbreed with flies from the parent population. At that point, the new population can be considered a new species.
The formation of new species occurs through many different mechanisms, and the study of these mechanisms remains one of the most exciting areas of modern evolution
Figure 2
The fly Drosophila substenoptera, which lays its eggs in rotting bark, lives in upland forests on the island of Oahu. (Photograph courtesy of Kevin Kaneshiro.)
ary biology. Evolutionary biologists are investigating why some species have remained largely unchanged for long periods of time while others have undergone rapid diversification into multiple species. They are exploring whether evolutionary change happens gradually or in spurts separated by long periods when outward change is less dramatic. They are examining the molecular mechanisms of evolutionary change. In all of these cases, scientists are studying the details of mechanisms by which biological evolution has occurred.
Science Produces Explanations That Can Be Tested Using Empirical Evidence
Science requires that scientific explanations of phenomena be based on events or mechanisms that can be observed in the natural world. This is how science builds a base of shared observations and ideas to which new knowledge can be added.
For example, scientists studying the characteristics of plants and animals in Hawaii look for natural explanations for those characteristics. They propose hypotheses that explain the evolution of those characteristics through naturally occurring mechanisms. Then they gather additional information to test their hypotheses. Because hypotheses are based on phenomena that can be measured or observed, other scientists can test the hypotheses by gathering their own data. Based on the evidence gathered, the hypothesis can be accepted or rejected and new, more refined hypotheses can be developed.
One potential source of confusion in discussing the theory of evolution is the meaning of the words “theory,” “hypothesis,” and “fact.” In popular usage, a “theory” is something that is not known for sure. But the word “theory” has a very different meaning in science than it does in everyday use. In science, “theory” refers to an explanation of some aspect of the natural world that is held with great confidence because it is supported by overwhelming evidence. The theory of gravitation holds that all objects are attracted to each other in proportion to their mass. The cell theory says that all living things are composed of cells.
Scientists use the word “hypothesis” to describe an idea or model that has not yet been fully tested. For example, in studying
