Evolution Of Flowers
The flower originated as a structure adapted to protect ovules, which are borne naked and unprotected in the Gymnosperms, ancestors of the Angiosperms. Botanists are uncertain about which group of Gymnosperms is most closely related to the Angiosperms. Recently, examination of sexual fertilization in the different groups of Gymnosperms suggests that Angiosperms may be most closely related to the Gnetophyta, a small phylum with three genera (Ephedra, Gnetum, and Welwitschia) and about 70 species.
The Angiosperms first appeared in the fossil record in the early Cretaceous period (about 130 million years ago) and rapidly increased in diversity. Once the flowering plants had evolved, natural selection for efficient pollination by insects and other animals was important in their diversification. By the mid-Cretaceous, species with flowers of many different designs had evolved. These varying designs evolved as a consequence of the close association of the flowers and their animal pollinators, a process referred to as coevolution. In addition, many flowers became self-incompatible, in that they relied upon cross-pollination, that is, pollination by another individual of the same species. Cross-pollination increases the genetic diversity of the offspring, in general making them more fit.
Today, flowering plants are the dominant terrestrial plants in the world. There are more than 300 different families of flowering plants. The Asteraceae, with over 15,000 species, is one of the largest and most diverse families of Angiosperms, and their flowers are highly evolved. The dandelion, daisy, and sunflower are familiar species of the Asteraceae. In these species, many small individual flowers are packed closely together into a dense inflorescence called a head, which appears rather superficially like a single large flower. Each individual flower in the head has a single ovule, which produces a single seed upon fertilization. The flowers of many species of the Asteraceae, such as the dandelion, evolved highly specialized sepals, which are scale-like and are referred to as a pappus. In summer, the pappus of the dandelion expands into the furry white structure which aids the tiny attached seeds in their dispersal by the wind.