Evolution of Angiosperm (Flowering Plants)


It is, without a doubt, the angiosperm division of land plants that is best-adapted and suited for Earth's surface. It is ultimately pure mutative descent from the ancestral green algae which has allowed for a set of derived characters to surface in angiosperms. First, however, it should be noted that flowering plants diverged from ancestral gymnosperms around 250 mya (million years ago), while the earliest flowering plants to date consist of those from roughly 150 mya. This tells us that flowers were not a punctuated form of equilibria. Rather, an ancestral form of angiosperm, one lacking a seed coating and dispersal methods.


Derived Characters


Photosynthetic Optimization - To ensure that the plant organism is able to reach an abundant supply of the needed CO2 and H2O. What facilitated this increase in photosynthetic rate was an increase in vascularity of plant leafs, as well as gradual development of stomata. This would ensure the proper exchange for gases with a minimal loss of water when the conditions were not favorable. Which essentially, the whole new surface was a new foreign environment for plants to thrive, so it was a natural process for selection to occur.


Embryo Protection - Though it is unclear whether endosperm formation occurred gradually or instantaneously (probably the latter, through a mutation in genomic count) in angiosperms, it is also certain that the smaller the size of the embryos, the more vast the endosperm is. The logical evolutionary advantage for this is the fact that when the embryo is left more vulnerable by its size, the more nutrition it needs to thrive. The endosperm composition is enriched with nutrients needed for the embryo inside to develop, including proteins, oils, and, respectively, starch. It is also important to note the embryo as a separate entity/organism from the embryo altogether. This occurs in double fertilization, when the zygote develops from one of the two sperm nuclei of the pollen tube and reaches the egg within the gametophyte. The other sperm nucleus merges with two polar nuclei cells, forming the triploid (3n) organism. It is most certain that such an evolution has an advantage for the seed embryo, that it is unlikely that the plant formed this by chance. Pollen tubes actually act as a replacement mechanism for the scarcity of water to disperse gametes, which is found in mosses and liverworts.


Chemical Toxins - One of the more advanced and sporadic forms of protection and adaptations of angiosperms include the chemical "shielding" which protects and warns nearby predatory herbivores. The chemical diversity and intensity these have are like no other, such as geranium which has a distinct host fighting chemical that targets only Japanese beetles. This chemical paralyzes the beetle within 30 minutes of consumption. There are also non-direct chemicals which help to disperse predatory species. For instance, while some chemicals, such as toxalbumin, make a direct impact on the predator by harming them directly, others act by disabling metabolic functions and enzymes to work on the plant.



Selected Pollinators and Pollination - Certain angiosperm have developed colorful petals, stamens, and carpels as a method of carrying out pollination. This is specially true of those plants which rely on an outside source of pollinators, such as bees or hummingbirds. When this is the case, flowers will 99% of times be brightly colored to attract them. In an exchange of pollen for the mutualist symbiont, the gametes in the pollen grains are dispersed to other flowers which will later fertilize the zygote.