References Cited

Individuals change throughout their lifetimes; they grow, receive injuries, color their hair, or pierce their eyebrows. These changes are not evolutionary, because they cannot be inherited by the next generation. The changes are lost when the individual possessing them dies. Individuals do not evolve, only populations evolve. Species evolve over successive generations as their local populations interbreed and change. The biological definition of a species embodies this concept: a species is a group of naturally occurring populations that can interbreed and produce offspring that can interbreed. This point is very important: species always consist of changing and interbreeding populations. There never was a first ‘saber-toothed cat,’ ‘first mastodon,’ or ‘first dinosaur.’ Instead, there was a first population of interbreeding individuals that we call ‘saber-toothed cats,’ or ‘mastodons,’ or ‘dinosaurs.’ At any given time in the past, members of populations of a species were capable of interbreeding. It is only with ‘20/20 hindsight,’ the perspective of time, that we designate the breaks between ancestor and descendant species at a particular point.

Although we can often test the biological definition of species directly when studying populations of living organisms, we cannot do the same with fossils. No matter how long we watch, no two fossils will ever breed. Therefore, we must look for other ways to determine relatedness among fossil organisms. Because genetically similar organisms produce similar physical features, paleontologists can use the bones, shells, and other preserved body parts to help us recognize species in the fossil record.