Inferring and Explaining

68 InferrIng and exPlaInIng fIgure 2. Simple cladogram Retrieved from https://www.geol .umd.edu/~jmerck/honr219d/notes/ 06.html. experiment. Darwin was able to articulate gen- eral principles that have unifed biology formore than a century and a half. Andmost remarkably, ft perfectlywith biological discoveries that Dar- win himself could never have imagined. The Two Theories Te “theory of evolution,” what Darwin called a “theory of descent with modifcation by natural selection,” is really three distinct theories, two of which owe a great deal to Darwin. We can translate the term evolution as simply meaning “biological change.” Te biological world we see today, including the species now in existence, is diferent from the biological world at diferent historical periods. Many theorists before Dar- win, including his own grandfather, Erasmus Darwin, had proposed theories of biological change. Te problem was that, although (as we shall see directly) these theories nicely explained many known facts, no one before Darwin had any good ideas as to the causes of this change. Descent with modifcation , or equivalently, common descent , is Darwin’s theory about the patterns in, and the history of, this biological change. Darwin cannot really be given credit for originating the theory of descent with modif- cation, but one of the great achievements of On the Origin of Species is that he laid out the evi- dence for this theory so powerfully that within less than a generation almost every biologist accepted the truth of this view of biological history. Te above very simple graphic gives us the heart of descent with modifcation. Te verti- cal axis represents time, and the horizontal axis represents the present or past picture of biologi- cal diversity. So in this little snippet, we learn about the history of three related species, A, B, and C. Tat they are related is indicated by the common ancestor at the bottom (i.e., earlier in history). We also learn that species B and C are more recent evolutionary arrivals than species A, since the common ancestor that begins their history is higher (later). Descent with modif- cation says that current species are related to one another through a series of ever-narrowing common ancestors (thus common descent). Te logical extension of this line of reasoning is, as Darwin saw, that all life can be traced back to a single common ancestor. I cannot doubt that the theory of descent with modi- fcation embraces all the members of the same great class or kingdom. I believe that animals are descended from at most only four or fve progenitors, and plants from an equal or lesser number.