Inferring and Explaining

69 Analogy would lead me one step further, namely, in the belief that all animals and plants are descended from some one prototype. 2 Darwinworried that “analogymay be a deceitful guide,” but contemporary facts about themolec- ular structure of DNAmake his speculation look even stronger. If the history of life is as descent with modi- fcation outlines, the obvious question is what brings about all this change? If new species arise from ancestors, what is the origin of spe- cies ? Darwin’s stroke of genius was an original answer to this question. Breeders “select” in order to improve the stock. Nature, by analogy, also selects, but not consciously, nor with a pur- pose. Natural selection is the engine that drives biological history. More individuals in every generation are born than will survive, so there is a “struggle for existence.” Some individuals are lucky enough to be bornwith slight advantages in this struggle, and these advantages improve their chances of surviving and passing along these advantages to their ofspring. So over time, there is inevitable changewithin the species, and given enough time, these changes accumulate to result in the start of a new species. Rival Explanations to Common Descent In the rest of this chapter, we will spend most of our time focusing on the least scientifcally controversial of Darwin’s two theories but the one that is clearly the most controversial in the popular culture—descent withmodifcation. I want to try to convince you that evidence Dar- win presented for this view of biological history is quite overwhelming. More recent additional evidence has only further strengthened his orig- inal argument. We should candidly address the sources of this cultural controversy. Te primary source, of course, is that Darwin is widely believed to be antireligious. Te theory of descent is seen by many as not only denying the literal truth of the Old Testament but fat-out denying the existence of God. Tis latter interpretation is surprising in light of the fact that many contem- porary scientists see themselves as conventional religious believers yet accept the scientifc truth of common descent and natural selection. My students are constantly surprised to hear that Darwin nowhere denies the existence of God, nor does On the Origin of Species say anything about either the creation of the universe or the origins of life. Many sincere theists have seen complete consistency between what traditional religion teaches us and what our best natural sciences, including biology, teach. Te second source of controversy regards the place of our own species in Darwin’s picture. Although he tried to sof-pedal it in Origin , it was perfectly obvious to his contemporaries, both opponents and supporters, that humans were just as much a part of descent with modifca- tion and natural selection as every other part of the biological world. Tis is troubling to many, I realize. I personally think it explains a lot of what I know about myself, my friends, and the social world I live in. Before On the Origin of Species , the most widely accepted rival account of biological his- tory assumed that species were permanent, unchanging, and the individual products of divine creation. Tis view was not just a reli- gious one but a scientifc one as well. Te theory darwIn and Common desCent