Pollination

Botanists theorize that seed plants with morphologically distinct pollen (male) and ovules (female) evolved from ancestors with free-sporing heterospory, where the male and the female spores are also morphologically distinct.

The evolution of pollination coincided with the evolution of seed. Fossilized pollen grains of the seed ferns, an extinct group of seed-producing plants with fern-like leaves, have been dated to the late Carboniferous period (about 300 million years ago). These early seed plants relied upon wind to transport their pollen to the ovule. This was an advance over free-sporing plants, which were dependent upon water, as their sperm had to swim to reach the egg. The evolution of pollination therefore allowed seed plants to colonize terrestrial habitats.

It was once widely believed that insect pollination was the driving force in the evolutionary origin of angiosperms. A cross section of the anther of a lily, showing open pollen sacs and the release of pollen grains. JLM Visuals. Reproduced by permission. However, paleobotanists have recently discovered pollen grains of early gymnosperms, which were too large to have been transported by wind. This and other evidence indicates that certain species of early gymnosperms were pollinated by insects millions of years before the angiosperms had originated.

Once the angiosperms had evolved, insect pollination became an important factor in their evolutionary diversification. By the late Cretaceous period (about 70 million years ago), the angiosperms had evolved flowers with complex and specific adaptations for pollination by insects and other animals. Furthermore, many flowers were clearly designed to ensure cross-pollination, exchange of pollen between different individuals. Cross-pollination is often beneficial because it produces offspring which have greater genetic heterogeneity, and are better able to endure environmental changes. This important point was also recognized by Darwin in his studies of pollination biology.