Let’s say that you want to send a postcard to a friend, but your only means of doing so is to throw the card into the air and hope the wind carries it to your friend’s front door. How many cards would you have to loft in order to achieve a high probability of success? The answer, of course, is many, and this is precisely the reason that pines generate huge numbers of pollen grains each year. Suppose instead that you can give your card to the letter carrier whose route goes by your friend’s house—your odds of success are now much greater. This analogy gives some idea of a key evolutionary innovation that appeared in the angiosperms. The “letter carriers” in angiosperm fertilization are animals, and the evolution of flowers is what allowed angiosperms to use animals to transport pollen.

Animal pollination is far more efficient than pollination by wind—that is, a higher proportion of pollen reaches an egg. Thus, a plant that relies on animal pollination can afford to produce less pollen. Flowers initially evolved to make animal pollination possible in dense tropical forests. However, as angiosperms diversified into many habitats, some species reverted to wind pollination. Today, angiosperms make use of a wide range of pollination strategies. Here, we examine how angiosperms reproduce, beginning with flowers, which when fertilized develop into fruits.