Constraints are problems that natural selection can't seem to solve. As a result of how an organism is put together, some types of variation just aren't expected to appear. All vertebrates, for example, have (at most) four limbs. Think about it: You never see mice with six legs. And based on human understanding of mammalian development, it's pretty unlikely that Pegasus, with his four legs and two wings, would ever evolve. Being a mammal and having six limbs is just not a variant you ever see. So even if having a few extra limbs conferred a fitness advantage, natural selection can't get there because there is no heritable variation for extra limbs.
Trade-offs, the balance between fitness benefits and fitness costs, represent another key concept in evolutionary biology. You can think of trade-offs as being the "jack of all trades, master of none" phenomenon. Consider all the different kinds of birds in nature. Why isn't there a superbird with the talons of an eagle, the webbed feet of a duck, and all the other avian parts you can think of rolled up into one bird? Well, you can start to see the problem right away: It would be hard for an eagle to sink its talons into a poor little bunny if the eagle had webbed feet. A pretty obvious trade-off exists between swimming feet and grasping feet. The evolution of life histories is rife with examples of trade-offs; head to Chapter 10 for details.
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