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Phylogenetics

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    Phylogenetics lecture notes

    Example- Constructing a Tree

    In this example, we will construct a phylogenetic tree based on the characteristics of four different species- a dog, frog, horse, and snail.

    There are two possible ways to arrange the branches of a tree with four species.

                       

    There are 15 different ways that 4 species can be arranged on these two trees. With more than 4 species, there are many possible ways to construct the tree. For example, there are more than 282 million ways to arrange 10 species on a tree.

    The table lists six characteristics for four the four species in our example.

     

    Dog

    Frog

    Horse

    Snail

    Backbone

      Yes  

      Yes  

      Yes  

    No

    Brain

    yes

    yes

    yes

      yes  

    Four legs

    yes

    yes

    yes

    no

    Hair

    yes

    no

    yes

    no

    Shell

    no

    no

    no

    yes

    An examination of the table above shows that dogs and horses share four characteristics. We conclude that these four characteristics were present in the ancestor of these two species. The frog shares three characteristics with the dog and also three characteristics with the horse. We conclude that these three characteristics were present in the ancestor to these three species. The snail shares only one characteristic with any of the other animals. The tree below was constructed based on this information. The numbers on the tree below indicate the number of characteristics that are shared by all of the species in the group. For example, the dog, horse, and frog share 3 characteristics- a backbone, a brain, and four legs. Based on the table, the snail is the least related because it shares only one characteristic with the others- a brain.

    There are other ways to construct this tree but they require more evolutionary changes. For example, the tree below (left) shows the frog and dog sharing a more recent ancestor with each other than they do with the other two species. The dog has hair but the frog does not. Therefore, we assume that the common ancestor did not have hair. The presence of hair must have evolved in dogs after the dog and frog separated. However, the horse also has hair. In this tree, the presence of hair evolved two times- once in dogs and once in horse.

    The tree on the right (below) is more parsimonious because hair evolved only one time.