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DNAPhylo - Lecture 4 Building the Tree of Life Cladograms...

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Lecture 4: Building the Tree of Life: Cladograms, DNA, and Molecular Clocks Besides figuring out the mechanism, Darwin’s other great contribution to evolutionary studies was the implication that ALL living things are descended from one common ancestor. The very process of evolution by speciation that he described implies a tree-like structure, with species that diverged as branches close together coming out from one branching point. The only figure that Darwin included in his 500+ page opus The Origin of the Species was a diagram of this structure to the tree of life. Darwin’s only figure in The Origin of the Species . He knew that if you traced these tree-like branches back far enough they would all meet at one point, the ultimate common ancestor. One of the major tasks of evolutionary biology today is the reconstruction of this tree for all the living things alive today, as well as the fossil groups we have found. Early attempts at this were done through thorough examination of the groups involved. Those that looked the most similar were put next to each other on the tree of life, and less similar groups were farther away. This is basically what Linneaus did in his classification scheme, and the tree of life lends itself to this kind of intuitive classification, but we’ll talk more about that later. Nowadays the building of the tree is much more formalized, through a process called cladistics . First some terminology. The groups that we are trying to classify are called taxa (sing. taxon ). Usually these are species, the most fundamental grouping of animals, although you could use similar techniques for larger groupings like genera, smaller ones like populations, and even individual genes (again, more later). What we are building in this formalized method is called a cladogram , an example of which is below. Taxa show up as endpoints on branches which stem off from nodes . In a complete cladogram each node has only two branches coming off. The group of all the taxa that are derived from one node is called a clade (thus the name cladogram; these clades will become important later when we use cladograms to make classification schemes, called taxonomies ). A cladogram shows only relative relationships; who is related to whom. There is no sense of absolute time implied; fossil groups would show up on the top of the cladogram right next to their living relatives. This means there are no lineages shown (ancestor-descendent relationships). Cladograms do have some neat properties though. They can be swiveled around any node and still show the same relationships. They can also be
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used to show how evolutionary traits have been acquired in different groups. To do this, we first need to talk about how these cladograms are built.
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