For the previous installments: part 1, 2.
If common ancestry is true, we would expect the evolutionary tree of life (TOL) based on animal morphology to line up with the evolutionary TOL based on molecules, but they don’t. In fact, there is no one TOL based on morphology or one TOL based on molecules. There are multiple TOLs.
Interestingly, a comparison of different genes from the same organism can result in different TOLs for that organism. The same is true of morphology. For example, when the TOL is constructed based on germ-cell formation (which is basic to the evolutionary process because it underlies reproduction, and we would expect for all organisms in a branch of the TOL to have the same germ-cell formation) it leads to one TOL, but this TOL differs radically from TOLs constructed based on body-play symmetry, the number of primary tissues, or the mode of development.
Looking at the Cambrian pyla specifically, there is no one TOL showing what the ancestry looked like leading up to the Cambrian.
Convergent evolution is a word to describe how similar morphological features develop in different species, where that feature was not shared by their common ancestor. There are many examples of so-called convergent evolution. But convergent evolution undermines the presupposition underlying the theory of common descent: similar homology indicates similar ancestry. Convergent evolution demonstrates that homological similarities do not necessarily imply a common ancestry. Convergent evolution negates the logic of the argument for common descent from homology.
Chapter 7 (more…)