Answer the following question given this paragraph: 

Biology has only a few over arching theories. One of these, the Cell Theory of Life, explains the historic continuity of organisms, while the Theory of Evolution by Natural Selection (and other processes), explains both the diversity of organisms and how populations of organisms change over time. Finally, the Physicochemical Theory of Life explains how it is that organisms can display their remarkable properties without violating the laws that govern all physical and chemical systems.40 What is life, exactly? Clearly, if we are going to talk about biology, and organisms and cells and such, we have to define exactly what we mean by life. This raises a problem peculiar to biology as a science. We cannot define life generically because we know of only one type of life. We do not know whether this type of life is the only type of life possible or whether radically different forms of life exist elsewhere in the universe or even on Earth, in as yet to be recognized forms. While you might think that we know of many different types of life, from mushrooms to whales, from humans to the bacterial communities growing on the surfaces of our teeth (that is what dental plaque is, after all), we will discover that the closer we look the more these different “types of life” are in fact all versions of a common underlying motif, they represent versions of a single type of life. Based on their common chemistry, molecular composition, cellular structure, and the way that they encode, read, and use hereditary information in the form of molecules of deoxyribonucleic acid (DNA), all topics we will consider in depth later on, there is no reasonable doubt that all organisms are related, they are descended from a common ancestor. We cannot currently answer the question of whether the origin of life is a simple, likely, and predictable event given the conditions that existed on the Earth when life first arose, or whether it is an extremely rare and unlikely event. In the absence of empirical data, one can question whether scientists...


Q: The theory of evolution by what (and other processes) explains both the diversity of organisms and how populations of organisms change over time?


A:
natural selection