Difference between parallel evolution and convergent evolution
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When two species are similar in a particular character, evolution is defined as parallel if the ancestors shared that similarity; if they did not, the evolution of that character in those species is defined as convergent. However, this distinction is not clear-cut.
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Sometimes evolutionary change follows a common pathway in two or more unrelated or distantly-related organisms because of similar environmental pressures. It culminates in unrelated organisms with similar morphological characteristics even though they did not have a common ancestor. This phenomenon is called parallel evolution. There are many examples of parallel evolution in plants, including distantly-related plant families that have evolved from an autotrophic to a parasitic mode of existence. Some plants have evolved independently into a mycotrophic mode of existence where they obtain nutrients from mycorrhizal soil fungi, which in turn, are parasitic on the roots of nearby forest trees and shrubs. Photosynthetic pathways, such as CAM (crassulacean acid metabolism) and C-4 photosynthesis, have also evolved independently in distantly-related plant families.
C-4 Photosynthesis
CAM Photosynthesis
Parasitic Flowering Plants
Mycotrophic Flowering Plants
Another example of parallel evolution is the appearance of xylem vessels in the vascular tissues of very distantly-related plants, such as Ephedra in the gymnospermous division Gnetophyta and flowering plants in the angiospermous division Anthophyta (Magnoliophyta). In addition, species of Ephedra have double fertilization, where two sperm are involved in the fertilization process. Double fertilization was once thought to be a strictly angiosperm characteristic. Some older references have suggested that the Gnetophyta may represent a "missing link" in the evolution of flowering plants, but others say that vessels and double fertilization are examples of parallel evolution, and the Gnetophyta are more closely related to conifers than angiosperms. The current consensus among authorities (as of 2010) is that Amborella trichopoda (a primitive angiosperm without vessels) and all other flowering plants represent sister clades derived from an common ancester without vessels. This latter conclusion supports the independent (parallel) evolution of vessels in the Gnetophyta.
Cladograms showing four different hypotheses for the monophyletic origin of flowering plants.
Although there are different hypotheses for the monophyletc origin of flowering plants (angiosperms), vessels appear in two distantly related groups: the Gnetopytes and Angiosperms. This is an example of parallel evolution (Homoplasy).
The Remarkable Gymnosperms: Ephedra & Welwitschia
Stem & Root Anatomy Of Angiosperms & Gymnosperms
When parallel evolution under similar environmental conditions in distantly-related organisms results in plants and animals that are morphologically very similar in overall appearance, this is called convergent evolution. It should be noted here that some authors use these two terms interchangeably. North American cactuses (family Cactaceae) and South African euphorbias (family Euphorbiaceae) belong to different plant families and are distant relatives in the phylogeny of flowering plants; however, they both have succulent, thick stems that store water, they both have spines for protection, and the both are adapted for survival in arid desert regions with low rainfall. Without flowers, some African euphorbias are practically indistinguishable from their North American counterparts.
Which of these xerophytes is a cactus and which one is a euphorbia?
The Diverse Euphorbia Family
Why Differentiate Parallel From Convergent Evolution?
The following paragraph is from an article by Jeff Arendt and David Reznick in Trends in Ecology and Evolution Vol. 23 (1): 26-32, 2007:
"Biologists often distinguish 'convergent' from 'parallel' evolution. This distinction usually assumes that when a given phenotype evolves, the underlying genetic mechanisms are different in distantly related species (convergent) but similar in closely related species (parallel). However, several examples show that the same phenotype might evolve among populations within a species by changes in different genes. Conversely, similar phenotypes might evolve in distantly related species by changes in the same gene. We thus argue that the distinction between 'convergent' and 'parallel' evolution is a false dichotomy, at best representing ends of a continuum. We can simplify our vocabulary; all instances of the independent evolution of a given phenotype can be described with a single term - convergent."
C-4 Photosynthesis
CAM Photosynthesis
Parasitic Flowering Plants
Mycotrophic Flowering Plants
Another example of parallel evolution is the appearance of xylem vessels in the vascular tissues of very distantly-related plants, such as Ephedra in the gymnospermous division Gnetophyta and flowering plants in the angiospermous division Anthophyta (Magnoliophyta). In addition, species of Ephedra have double fertilization, where two sperm are involved in the fertilization process. Double fertilization was once thought to be a strictly angiosperm characteristic. Some older references have suggested that the Gnetophyta may represent a "missing link" in the evolution of flowering plants, but others say that vessels and double fertilization are examples of parallel evolution, and the Gnetophyta are more closely related to conifers than angiosperms. The current consensus among authorities (as of 2010) is that Amborella trichopoda (a primitive angiosperm without vessels) and all other flowering plants represent sister clades derived from an common ancester without vessels. This latter conclusion supports the independent (parallel) evolution of vessels in the Gnetophyta.
Cladograms showing four different hypotheses for the monophyletic origin of flowering plants.
Although there are different hypotheses for the monophyletc origin of flowering plants (angiosperms), vessels appear in two distantly related groups: the Gnetopytes and Angiosperms. This is an example of parallel evolution (Homoplasy).
The Remarkable Gymnosperms: Ephedra & Welwitschia
Stem & Root Anatomy Of Angiosperms & Gymnosperms
When parallel evolution under similar environmental conditions in distantly-related organisms results in plants and animals that are morphologically very similar in overall appearance, this is called convergent evolution. It should be noted here that some authors use these two terms interchangeably. North American cactuses (family Cactaceae) and South African euphorbias (family Euphorbiaceae) belong to different plant families and are distant relatives in the phylogeny of flowering plants; however, they both have succulent, thick stems that store water, they both have spines for protection, and the both are adapted for survival in arid desert regions with low rainfall. Without flowers, some African euphorbias are practically indistinguishable from their North American counterparts.
Which of these xerophytes is a cactus and which one is a euphorbia?
The Diverse Euphorbia Family
Why Differentiate Parallel From Convergent Evolution?
The following paragraph is from an article by Jeff Arendt and David Reznick in Trends in Ecology and Evolution Vol. 23 (1): 26-32, 2007:
"Biologists often distinguish 'convergent' from 'parallel' evolution. This distinction usually assumes that when a given phenotype evolves, the underlying genetic mechanisms are different in distantly related species (convergent) but similar in closely related species (parallel). However, several examples show that the same phenotype might evolve among populations within a species by changes in different genes. Conversely, similar phenotypes might evolve in distantly related species by changes in the same gene. We thus argue that the distinction between 'convergent' and 'parallel' evolution is a false dichotomy, at best representing ends of a continuum. We can simplify our vocabulary; all instances of the independent evolution of a given phenotype can be described with a single term - convergent."
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