There are many examples where species share common plastic traits. What appears to be convergence may just be the plastic response of the organism to its environment. Examples include the following: Limbs that protrude from an animal’s body have more surface area per unit mass than the rest of the body. In cold weather the animal loses more heat per unit mass from these limbs than from other parts of the body. In many species the tails and legs are shorter for those living in colder climates and larger for those in warmer climates. Gulls’ wings are shorter in cold climates than in warm. Hares and foxes also have shorter ears in cold climates than in warm. Eskimos have shorter arms and legs than do people living in warmer climates. . . . Jodan’s rule: Many species of fish tend to have more vertebrae when they live in cold water than do the same species living in warm water. These differences have been shown to depend on the temperature at which the fish have been reared.
What these rules show is not convergence. They show that different species adopt the same anatomical strategies when they have to cope with the same environmental conditions. We have seen that these strategies cannot come from random mutations. It is much more reasonable to say they come from environmental cues acting on the genetic program.
Lee Spetner