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Section 26_2: Phylogenetic Trees

Page history last edited by Bobby St. Pierre 10 years, 12 months ago

Learning Objectives

• Understand what a phylogeny represents.

• Describe the difference between shared primitive and derived characters.

• Differentiate among monophyletic, paraphyletic, and polyphyletic groups.

• Interpret or construct a phylogenetic tree, including cladogenetic or anagenetic events.

Section 26.2 Summary

Section 26.2 focus's on phylogenetic trees. First off, phylogeny is the study of the evolutionary relationships between species. Therefore, a phylogenetic tree, is a visual representation of these relationships. They are constantly changing as new research and data showing the relationships is discovered. Ideally this will correspond to taxonomy, and species will be grouped based on common ancestry. The two ways that new species come into existence, are recorded differently on a phylogenetic tree. Anagenesis, in which a single species evolves into one other species, is represented by a species with a line connecting it directly to one other specices. Cladogenesis, in which a single species evolves into two or more species. The point at which these branches occur are known as nodes, and help show the relative time periods that the changes that differentiate between the two species occurred.

The next part of my section discusses the various groupings that are used in phylogeny. These groups are monophyletic, paraphyletic, and polyphyletic. A monophyletic group is a group comprising of the most recent common ancestor species, and all of it's descendents, this would be the majority of evolutionary groups. A paraphyletic group is similar, except that it excludes certain species. An example of a paraphyletic group would be birds and dinosaurs, although birds have a common ancestor with dinosaurs, they are excluded in referencing dinosaurs as a whole. A polyphyletic group is one which is comprised several species without a common ancestor. This groups are caused by misconceptions in previous phylogeny, no current groups are classified as such, and this term is primarily used to challenge a phylogeny.

The final point I wish to summarize is the difference between primitive and derived characters. Primitive characters are homologous traits shared by two or more species, that were acquired prior to their most recent common ancestor. Derived characters are those which were acquired from the most recent common ancestor. These traits are very useful in differentiating between species as they evolve over time.

Vocabulary

Systematics- The study of biological diversity and evolutionary relationships.

Phylogeny- The evolutionary history of a species or group of species.

Phylogenetic tree- A hypothetical representation of the evolutionary relationships between species.

Anagenesis- When a single species evolves directly into one other species.

Cladogenesis- When a single species evolves into two different species.

Clade- A common ancestral species and all of it's descendents.

Monophyletic group- A group of species (taxon) consisting of the most recent common ancestor and all of its ancestors.

Paraphyletic group-A group of species that contains a common ancestor and some, but not all of its descendants. Example- Dinosaurs and birds (see powerpoint/lecture)

Polyphyletic group- A group of several evolutionary lines but does not include the most recent common ancestor of the lineages.

Homology- Similarities among various species that occur because the species are derived from a common ancestor.

Shared Primitive Characteristic (symplesiomorphy)- Shared traits (homologous) acquired earlier in a species' evolution, before their last common ancestor.

Shared Derived Characteristic (synapomorphy)- Shared traits that acquired from their last common ancestor.

Useful Materials (x4)

	This video is a very good explanation on how to read phylogenetic trees. The narrator explains how new additions would be placed on the tree. She explains that the relatedness of species can be determined based on how far up in the tree the branch separating them is. This video also explains how to differentiate between extinct and living species and evolutionary lines. She makes a key point in stating that the majority of species that have ever lived are now extinct, and despite the fact that many phylogenetic trees will not show this, there are many more branches that have been discontinued than there are those which follow to the present. There is a minor misconception at 4:20 in the video, where the narrator says that the x-axis represents relatedness, the relatedness of a species is directly determined by reading the phylogenetic tree's branches, and reading it by solely the x-axis will not always be accurate.
	This video is from Bozeman Biology, a channel on youtube well known for their many videos explaining concepts of Biology. The actual video is very informative, and it gives the viewer a lot of general knowledge about the subject, and also helps explain differences between similar terminologies. He does an excellent job of differentiating between monophyletic, paraphyletic, and polyphyletic groups, giving examples of each and explaining in a bit of detail why they are placed in groups as such. He goes a little bit too in-depth as far as individual species at times, but overall a very good. Important information at 9:40-12:45
http://evolution.berkeley.edu/evolibrary/article/phylogenetics_02	This source from UC Berkeley explains several of the vocabulary terms I discussed above, in relation to an actual phylogenetic tree. Reading this would be good for visual learners who didn't pick up everything from my other explanations.
http://evolution.berkeley.edu/evosite/evo101/IICTreebuilding.shtml	This source helps explain the process of creating a phylogenetic tree. Offers great information on shared derived and shared primitive characteristics. It explains the characteristics and shows practical application on how to use said characteristics to differentiate on a phylogenetic tree. I suggest reading all of the page linked, and then clicking on the heading that says "Phylogenies" at the top of the page, and reading the section on phylogenetic tree interpretation. Both are short and easy reads.