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Biology LibreTexts

Phylogenetics

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.