What makes some animals venomous and others not?

Venom is produced by specialized glands that evolved independently many times across the animal kingdom, meaning there's no single origin. Animals become venomous through genetic mutations that repurpose ordinary proteins, often digestive enzymes or hormones, into toxic compounds. Over generations, natural selection favors individuals whose venom helps them capture prey, defend against predators, or compete with rivals, so the trait spreads through the population. Whether a species evolves venom depends largely on its ecological niche and lifestyle. Predators that hunt fast or dangerous prey benefit enormously from immobilizing toxins, while animals with other effective strategies like speed, armor, or camouflage face less selective pressure to develop venom. It's essentially a cost-benefit calculation driven by survival and reproduction over evolutionary time.