Yesterday, I claimed that a major reason large evolutionary
changes often don’t happen is that competition from the
creatures around you stops you from changing. In other words, in
environments that are already rich in different species, natural
selection often prevents large changes. My piece of evidence for
this was a claim that, when you take other organisms away — when
you reduce competition for food, or space — evolution explodes.
Today, I want to examine the truth of these claims more closely.
I want to start with a question: Why aren’t there more insects
in the sea? Insects, after all, make up the bulk of all known
animal species — most animals are insects — yet hardly any
insects live in the sea. It’s not because of the water. Many
insects live in freshwater for at least part of their lives.
Think of mayfly nymphs, or caddis fly larvae — these are the
ones that often build themselves tubular houses out of stones
and gravel. (Caddis fly larvae can make amusing pets. If you
keep them in an aquarium, and