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.