The title of this course is “Concepts of Biology and Biodiversity”. Biological evolution (defined most succinctly as changes in the genetic structure of a population over time) is, perhaps, the most important concept in biology, and it explains the diversity of life that exists in our world (the images below illustrate our current knowledge of the evolution of major life forms through geologic time – you do not need to know the details of the different eras – rather these are included to illustrate some of the changes in biological diversity since the origin of life ).
Currently there are approximately 1.5 million described species on earth (and most of these species are insects – sometimes referred to, by famed biologist E.O. Wilson, as “the little things that run the world” because insects play such important roles in the world’s ecology). And while scientists know that the amount of biological diversity on earth is much greater than these 1.5 million described species – there has not been agreement on how many species are likely to exist on earth. In 2017, researchers from the University of Arizona published a paper claiming that there are likely 2 billion unique species on earth!
This is an incredible amount of biological diversity. While we don’t know the total number of species on earth, we do know that all life is related and can be traced back to a common ancestor that existed approximately 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. In fact, the father of evolution, Charles Darwin, wrote:
The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth
The image below shows a sketch from one of Darwin's notebooks. Rooted firmly at the bottom of the tree is life's common ancestor, while modern species are found at the tree's tips; the vast branching that exists between the two describes the history of life on the planet.
The tree of life below is a modern tree created by researchers using ribosomal protein sequences. It shows a much greater diversity of species than Darwin's simple drawing. In this class, evolution will be a reoccurring theme. Whenever appropriate, we will discuss how evolutionary processes have shaped life's diversity and adaptations (adaptations are traits that help an organism survive in its current environment). 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; this process is natural selection.
The video below further discusses the information you have just read. Click on the arrows in the bottom right of the screen to expand this video to full screen: