5.2: Phylogeny and Cladistics

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Can two different species be related?

Of course they can. For example, there are many different species of mammals, or of one type of mammal, such as mice. And they are all related. In other words, how close or how far apart did they separate from a common ancestor during evolution? Determining how different species are evolutionarily related can be a tremendous task.

Phylogenetic Classification

Linnaeus classified organisms based on obvious physical traits. Basically, organisms were grouped together if they looked alike. After Darwin published his theory of evolution in the 1800s, scientists looked for a way to classify organisms that showed phylogeny. Phylogeny is the evolutionary history of a group of related organisms. It is represented by a phylogenetic tree, like the one in Figure below.

One way of classifying organisms that shows phylogeny is by using the clade. A clade is a group of organisms that includes an ancestor and all of its descendants. Clades are based on cladistics. This is a method of comparing traits in related species to determine ancestor-descendant relationships. Clades are represented by cladograms, like the one in Figure below. This cladogram represents the mammal and reptile clades. The reptile clade includes birds. It shows that birds evolved from reptiles. Linnaeus classified mammals, reptiles, and birds in separate classes. This masks their evolutionary relationships.

Summary

Phylogeny is the evolutionary history of group of related organisms. It is represented by a phylogenetic tree that shows how species are related to each other through common ancestors.
A clade is a group of organisms that includes an ancestor and all of its descendants. It is a phylogenetic classification, based on evolutionary relationships.

Review

What is a clade?
What is cladistics, and what is it used for?
Explain why reptiles and birds are placed in the same clade.
Dogs and wolves are more closely related to each other than either is to cats. Draw a phylogenetic tree to show these relationships.

Image	Reference	Attributions
	[Figure 1]	License: CC BY-NC
	[Figure 2]	Credit: Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation Source: CK-12 Foundation License: CC BY-NC 3.0
	[Figure 3]	Credit: Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation Source: CK-12 Foundation License: CC BY-NC 3.0