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Phylogeny and Evolution of Flowering Plants

Home | Theories of Evolution | Phylogentic Tree & Dichotomous Key | Monocots & Dicots | Cyanobacteria | Green Algae | Psilophyta | Lycophyta | Ginkgophyta | Coniferophyta | Gnetophyta | Anthophyta | Sphenophyta | Bryophyta | Hepatophyta | Anthocerophyta | Pterophyta | Cycadophyta | Angiosperm Evolution Essay | Work Cited

Theories of Evolution

Endosymbiosis

Biologists generally accept the first step, or rather organism, of angiosperm evolution as being cyanobacteria. Cyanobacteria are aquatic, photosynthetic bacteria that are sometimes called by the more deceptive name of blue-green algae. As they are bacteria, they are also prokaryotic and usually unicellular organisms. Unlike most bacteria, however, cyanobacteria often grow in large colonies that can actually be large enough to be visible to the unaided human eye.

The process of endosymbiosis supports the concept of cyanobacteria. The theory behind endosymbiosis is that cyanobacteria evolved into chloroplasts after other eukaryotic cells absorbed them as endosymbionts (the technical name for any organism that lives with the body or cells of another organism). Now that the cyanobacteria are no longer individual cells and are instead chloroplasts within another organism, it is time to introduce the next organism that is part of this proposed sequence for angiosperm evolution: chlorophyta.

Cyanobacteria Page

Mostly Male

The origin of flowering plants has been one of the most famous and contentious issues in evolutionary biology. Based on data from living plants, the “Mostly Male” theory was developed. This theory serves to explain that developmental control of flower organization derives more from systems active in the male reproductive structures of the gymnosperm ancestor, rather than from the female, with ovules being ectopic in the original flower:
    The mostly-male theory is based on observations of the duplication, in seed plants, of a nuclear gene associated with reproductive morphogenesis and the subsequent loss, in flowering plants, of one of the two copies of this gene. Simplifying the original theory greatly, the argument can be outlined as follows: ovules, usually sterile ones, may be produced ectopically, i.e. in positions where they are not expected to occur. Ectopic ovules on microsporangiate strobili could have replaced the function of female strobili, if these were lost as a result of the ancestor of flowering plants losing the duplicate gene referred to above. As suggested at the right, an angiosperm pistil could have resulted from an ovule-bearing but otherwise sterile microsporophyll (the placenta) becoming enclosed by another sterile microsporophyll (the ovary wall). Perianth members might have developed by sterilization of the most basal microsporophylls. (Dickinson)
The idea behind the evolution of gnetophyta to angiosperms is that the two distinct leaves that are present within gnetophyta are comparable to the two cotyledons that are present within the dicotylodonea group of angiosperms. This explanation of the evolution of angiosperms does not yet totally answer the question of the way in which angiosperms have evolved into the way that they appear today. To take this into account, the theory that this explanation is based off of states that through various mutations that dicotylodonea group of angiosperms could eventually diverge to form the lilliopsida group of angiosperms which has only a single cotyledon. The converse of this theory is also held to be plausible; that lilliopsida evolved first and then mutated to become dicotylodonea.

By Brittany, Katelyn, Kyle, and Robbie