Biological evolution is a subset of the general idea that the universe has changed through time. In the nineteenth century, Charles Darwin spoke of "descent with modification," and that phrase still nicely communicates the essence of biological evolution. Descent connotes heredity, and indeed, members of species pass genes from generation to generation. Modification connotes change, and indeed, the composition of species may change through time. Descent with modification refers to a genealogical relationship of species through time. Just as an individual's genealogy can be traced back through time, so too can the genealogy of a species. And just as an individual's genealogy has missing links—ancestors whose names or other details are uncertain— so too the history of a species is understandably incomplete. Evolutionary biologists are concerned both with the history of life—the tracing of life's genealogy—and with the processes and mechanisms that produced the tree of life. This distinction between the patterns of evolution and the processes of evolution is relevant to the evaluation of some of the criticisms of evolution that will emerge later in this book. First, let's look briefly at the history of life.
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