How repeatable is evolutionary history?
Some clover species have two forms, one of which releases cyanide to discourage nibbling by snails and insects and the other of which does not. A scientist at Washington University in St. Louis found that this “polymorphism” has evolved independently in six different species of clover, each time by the wholesale deletion of a gene. The clover species are in a sense predisposed to develop this trait, suggesting that evolution is not entirely free form but instead bumps up against constraints.