The Core Theme: Evolution Accounts for the Unity and Diversity of Life


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The Core Theme: Evolution Accounts for the Unity and Diversity of Life (Campbell Biology)

Evolution is the one idea that makes logical sense of everything we know about living organisms. As the fossil record clearly shows, life has been evolving on Earth for billions of years, resulting in a vast diversity of past and present organisms. But along with the diversity there is also unity, in the form of shared features. For example, while sea horses, jackrabbits, hummingbirds, and giraffes all look very different, their skeletons are organized in the same basic way.

The scientific explanation for the unity and diversity of organisms—as well as for the adaptation of organisms to their particular environments—is evolution: the concept that the organisms living on Earth today are the modified descendants of common ancestors. As a result of descent with modification, two species share certain traits (unity) simply because they have descended from a common ancestor. Furthermore, we can account for differences between two species (diversity) with the idea that certain heritable changes occurred after the two species diverged from their common ancestor. An abundance of evidence of different types supports the occurrence of evolution and the theory that describes how it takes place. To quote one of the founders of modern evolutionary theory, Theodosius Dobzhansky, “Nothing in biology makes sense except in the light of evolution.” To understand Dobzhansky’s statement, we need to discuss how biologists think about the vast diversity of life on the planet.


Urry, Lisa A.. Campbell Biology. Pearson Education. Kindle Edition.


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