Another way to test hypotheses about adaptation is to consider trait variation across a group of species instead of focusing on the trait of a single species. Rather than seeking to explain why polar bears have fur of a certain thickness, one tries to explain why bears in colder climates have thicker fur than bears in warmer climates. The former problem is hard to solve, since it is hard to say exactly what fur thickness polar bears should have if natural selection guided the evolution of that trait.

Darwin repeatedly used the hypothesis of common ancestry as a platform on which to build his various ideas about testing hypotheses concerning natural selection. He also argued that adaptive similarities provide little or no evidence for common ancestry. Although this second claim needs to be fine-tuned, Darwin was right that ample evidence for common ancestry can exist even if none of the characteristics we observe were caused to evolve by natural selection.

This is not to deny that there are versions of theism that do conflict with evolutionary biology. Young Earth Creationism is an example; it claims that God created life on earth within the past 10,000 to 50,000 years. But other types of theism are different.

Methodological naturalism gives advice to scientists about what they should include in their theories. There is a second type of methodological naturalism that gives advice to philosophers, which I call "methodological naturalismp." It says that the methods that philosophers should use in assessing philosophical theories are limited to the methods that scientists ought to use in assessing scientific theories.

Group selection and individual selection are just two of the selection processes that have played important roles in evolution. There also is selection within individual organisms (intragenomic conflict), and selection among multi-species communities (an idea that now is getting attention in work on the human microbiome). All four of these levels of selection find a place in multi-level selection theory.

Some philosophical arguments (e.g., in connection with the mind/body problem) look pretty good, while others (e.g., those that criticize moral realism) do not.

In many contexts, simplicity is not an aesthetic frill.