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What is Life?

Biology is the study of life. But, what is life? We all recognize it, but defining it can be a challenge and there is no single correct definition. Some of the usual definitions that you may have encountered in the past may be unsatisfactory. For example, a common definition is, "the ability to self-replicate." This works most of the time, but what about viruses? They don't replicate (on their own), but they behave in many other ways as living entities. One definition that is particularly satisfying deals with how life behaves over successive generations. That is, life is not static; rather, it changes with time. In this course we will define life as "that which undergoes, or is subject to, natural selection." This definition sounds simple, but as you will learn, it involves complex interactions.

Natural Selection and Evolution

The title of this course is Concepts of Biology and Biodiversity. Evolution is an important process in biology, and it explains the diversity of life that exists in our world. We now know that all life is related and can be traced back to a common ancestor that existed some 3.5 billion years ago. In the vast time since life first appeared, evolution has resulted in a continuous branching of new species that can be described as a tree of life. Rooted firmly at the bottom is life's common ancestor, whereas modern species reside at the tree's tips; the vast branching that exists between the two describes the history of life on the planet.

In this class, evolution will be a reoccurring theme. Whenever appropriate, we will discuss how evolutionary processes have shaped life's intricacies. As you will learn, evolution has many facets that have contributed to the diversity of life. However, only one process adapts life to the environment; namely, natural selection.

What is Natural Selection?

Natural selection is a process by which a population becomes adapted to an environment over time. At the most fundamental level, natural selection results in changes in the frequency of certain genes and/or alleles over successive generations within a population. As you might gather, genetics plays an important role in understanding natural selection; therefore, this course will begin by discussing the basic tenets of genetics. In the next few tutorials you will learn how heritable traits are passed between generations and how these traits behave in a population.