How Parasitism Evolves

A few rules show up again and again:

• From free-living ancestors: harmless hitchhikers gradually turn into exploiters.
• Over and over: every major branch of life evolved parasitism on its own.
• By subtraction: tapeworms drop their gut, plants drop chlorophyll, Sacculina drops everything.
• Under the Red Queen: host and parasite must keep evolving just to hold their ground.
• Fast where it counts: immune genes race to keep up with constant attack.