Acactus with long spines protects itself from animals better than a cactus with short spines. according to natural selection,

1. short-spined cacti will survive longer and reproduce more often than
2. long-spined cacti long-spined cacti will survive longer and reproduce more often than short-spined cacti
3. over time, the population will become entirely made of short-spined cact
4. over time, cacti with long spines will completely die out