Daphnia

Daphnia (water fleas) provide the chapter's primary example of phenotypic plasticity - the ability to express different physical forms from the same genetic code based on environmental signals. When young Daphnia detect kairomones (chemical signals) from predatory fish, they grow a literal helmet: a spiky protrusion that makes them harder to swallow.

This isn't evolution across generations - it's adaptation within a lifetime. Same genes, different body, triggered by environmental cues. Offspring raised in predator-rich water also grow helmets; move them to predator-free water and they develop normally. The molecular mechanism involves gene regulation: environmental signals affect which transcription factors are active, changing gene expression without changing DNA.

Daphnia illustrate why phenotypic plasticity evolved: environments change faster than genomes. Waiting for the right mutation is too slow. Better to have genetic capacity for multiple phenotypes and switch between them based on current conditions.