The evolutionary histories of the angiosperms and their animal pollinators are closely intertwined. A rapid increase in angiosperm diversity occurred between 100 and 65 million years ago, about the same time as the diversification of bees (Hymenoptera) and butterflies (Lepidoptera). The evolution of flowers allowed animals to specialize on new food resources. At the same time, animal pollinators greatly facilitated the movement of pollen between plants within the same population.

Flowers communicate their presence through both scent and color. For pollination to be reliable, however, it must be in the interest of the animal pollinator to move repeatedly between flowers of the same species. Flowers earn fidelity from their pollinators by providing rewards, frequently in the form of food (Fig. 30.14). Many insects consume some of the pollen itself, while many flowers also produce nectar, a sugar-rich solution attractive to bats and birds as well as to insects.

Some species provide rewards other than food. For example, many orchids secrete chemicals that male bees need to make their own sexual pheromones. Other flowers provide an enclosed and sometimes heated chamber in which insects can aggregate. But some flowers do not provide rewards at all: Instead, they “trick” their pollinators into visiting. For example, flowers that look and smell like rotting flesh attract flies, and orchids that look like a female bee even emit similar pheromones to attract male bees (Fig. 30.15). Male bees find these orchids irresistible and, as they attempt to copulate with the flower, deliver and receive pollen.

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Producing rewards and attractants such as nectar and petals requires resources. Thus, while animals can provide more efficient pollination than can the wind, the resources spent may be wasted if nonpollinating visitors “steal” pollen or nectar. For this reason, many flowers have mechanisms to protect their investments. For example, tubular cardinal flowers provide rewards to hummingbirds able to probe their depths, and snapdragons provide rewards only to insects that are strong enough to push apart their petals.

Many plants have evolved flowers that match rewards and attractants to the metabolic needs and sensory capabilities of their pollinators. For example, flowers pollinated by larger pollinators such as bats and birds produce copious amounts of nectar. Bat-pollinated flowers tend to be large, white or cream colored, and strongly scented, all traits appropriate to the size and nocturnal habits of their pollinators. The flowers are positioned among branches and leaves in such a way that they can be found by echolocation. In contrast, bird-pollinated flowers are often red, a color most insects cannot see, and have no scent. Because interactions between plants and pollinators affect gene flow, they can affect rates of speciation. A recent radiation in columbine occurred as the plants shifted from bumblebees to hummingbirds to hawkmoths as their primary pollinator (Fig. 30.16).

Angiosperms are thought to have evolved in tropical forests. Pollen cannot move freely through the air in the high density of foliage that is present year round in these forests, favoring the evolution of animal pollination. However, as angiosperms diversified and spread around the globe, some evolved wind pollination once again. The reversal to wind pollination is associated with habitats where pollinators are not reliably abundant, such as dry regions.

Wind pollination is most effective in species that are locally abundant. For example, many grasses rely on wind for pollination. Wind pollination is also common in temperate-zone trees, which produce flowers in the early spring, before leaves expand and before pollinators are active. Wind-pollinated species produce large amounts of pollen, and their petals tend to be tiny. Hay fever is an allergic reaction to pollen, virtually all of which is from wind-pollinated species.