Interpreting patterns of relatedness

The key to interpreting patterns of relatedness in an evolutionary context is to trace back to the point in the tree where taxa share their most recent common ancestor (MRCA). You can practice this by finding two species of interest in a tree and tracing back along their branches to identify the node where they meet – this is their MRCA.

Below we will explore examples from the vertebrate phylogeny to see how we can describe evolutionary relationships (Figure 14).

Looking at Figure 14, we can make the following statements:

• Humans (red) are more closely related to mice (bright blue) than they are to lizards (green)

This is because humans share a common ancestor more recently with mice (ancestor = purple spot) than they do with lizards (ancestor = dark blue spot).

• Frogs (purple) are more closely related to lizards (green) then they are to fish (pink)

This is because frogs share a common ancestor with lizards more recently (ancestor = orange spot) than they do with fish (ancestor = black spot).

• Fish (pink) are equally related to mice (bright blue) as they are to frogs (purple)

This is less intuitive, but if you trace back to the MRCAs you will see why: mice and frogs share the same common ancestor (black spot) with fish, so neither species is more closely related to fish.

Remember:

• These interpretations rely on our tree being rooted because the root is needed to define the direction of evolution and therefore what is ‘more recent’ in evolutionary time.
• If we rotate the branches to change the topology of the tree then the tree still has the same biological meaning and evolutionary relationships.

You can read more about common misconceptions when interpreting patterns of relatedness in this tutorial (3).