Evolution  (Page 6/9)

In summary, natural selection is a powerful agent, and recognition of this process was a powerful insight. Darwin proposed his theory in 1859, and elaborated on it in other books and other editions of the Origin of Species . Since Darwin’s time, other scientists have identified other agents, in addition to natural selection, that result in changes in the characteristics of a population, and you will learn more about those in later chapters. Additionally, other scientists made many predictions based on this explanatory framework, and did many experiments to test those predictions. Scientists are still engaged in that process today, and Darwin’s ideas have been confirmed many times over, and even extended so that we understand how the process works in much more detail than Darwin did. That is, as you learned in the previous chapter, one hallmark of a great theory.

Evidence for evolution

There are multiple lines of evidence, many of which were unimagined in the time of Darwin, that support his explanation for the diversity of life. The following is not meant to be an exhaustive cataloguing of that evidence. Indeed, more evidence accumulates every day, making it impossible to point out all of the threads in that fabric. It is also important to recognize that the evidence doesn’t come just from biology. For example, as noted above, Darwin’s explanation would require a lot of time and many generations. If the earth was too young, none of this could have happened. The sciences of physics and geology confirm that the earth is over 4.5 billion years old, which is plenty long enough for evolution to occur. The fossil record, the research subject of geology and paleontology, also provides substantial supporting evidence for Darwin’s big idea. The discovery of continental drift, and the development of plate tectonic theory, made sense of a lot of observations about both the fossil record and about populations of living organisms. Let’s look at a few of the lines of evidence, and see how they all weave together to make the coherent and elegant fabric that is the hallmark of a good scientific theory.

The fossil record

In science, radically new explanations can only be successful when the conventional explanations no longer explain all the observations. In the history of biology, this was the situation in the early part of the nineteenth century, when many interesting fossils were being discovered and carefully scrutinized. It soon became apparent that fossils were indeed the remains of once-living organisms, and that fossils in geologically younger strata seemed to be both similar and different from those in older strata. The fossil record showed that whole groups of organisms appeared and disappeared during the history of the earth. Others seemed to be much the same in rocks of different ages. Familiar organisms, particularly marine mollusks such as clams and snails, could be found in older rocks, but in many cases these organisms were not identical to the current organisms. Plant fossils told the same story. The reigning explanation for the diversity of life, creation of all these creatures at the same time and place, clearly did not explain these new observations. Evolutionary theory was a much more satisfying scientific explanation, and the development of that theory by Darwin and others started at that time.