Creationists are fond of the "it can't happen" argument: they like to point to things like the complexity of the eye or intricate cell lineages and invent bogus rules like "irreducible complexity" so they can claim evolution is impossible. In particular, it's easy for them to take any single organism in isolation and go oooh, aaah over its elaborate detail, and then segue into the argument from personal incredulity.
Two things, one natural and one artificial, help them do this. Organisms are incredibly complicated, there is no denying it. This should be no solace to the anti-evolutionists, though, because one thing natural processes are very good at is building up complexity. The other situation that has helped them is our current reliance on model systems.
We use a few model systems extensively to study development—Drosophila, C. elegans, Danio come to mind—and they give us an unfortunately rigid view of how developmental processes occur. The model systems that are favored for laboratory work are those that have rapid, streamlined development with a great deal of consistency to the pattern—variability is avoided, and we tend to look for reproducible rules. We get a false impression of the rigidity and inflexibility of developmental systems.
How to correct that? We use the model systems as a starting-off point, and look at related organisms. As we start to accumulate information about diverse species, the variability in the patterns of development becomes more prominent, and we see that the evolutionary pathways aren't difficult to see at all. The worm vulva is a great example of how phylogenetic studies of development can inform our understanding of evolution.
Continue reading How to evolve a vulva (on Pharyngula)