Question

Biology 2 Lab test review.

Explain the easier way of how to read Phylogenetic trees. Please provide easy to follow explanation and please include a diagram/ or pictures giving a visual explanation.

Answer 1

In a phylogenetic tree, the species or groups of interest are found at the tips of lines referred to as the tree's branches. For example, the phylogenetic tree below represents relationships between five species, A, B, C, D, and E, which are positioned at the ends of the branches:

The pattern in which the branches connect represents our understanding of how the species in the tree evolved from a series of common ancestors. Each branch point (also called an internal node) represents a divergence event, or splitting apart of a single group into two descendant groups.

At each branch point lies the most recent common ancestor of all the groups descended from that branch point. For instance, at the branch point giving rise to species A and B, we would find the most recent common ancestor of those two species. At the branch point right above the root of the tree, we would find the most recent common ancestor of all the species in the tree (A, B, C, D, E).

Each horizontal line in our tree represents a series of ancestors, leading up to the species at its end. For instance, the line leading up to species E represents the species' ancestors since it diverged from the other species in the tree. Similarly, the root represents a series of ancestors leading up to the most recent common ancestor of all the species in the tree.

Which species are more related?

In a phylogenetic tree, the relatedness of two species has a very specific meaning. Two species are more related if they have a more recent common ancestor, and less related if they have a less recent common ancestor.

In this method, we start at the branch ends carrying the two species of interest and “walk backwards” in the tree until we find the point where the species’ lines converge.

For instance, suppose that we wanted to say whether A and B or B and C are more closely related. To do so, we would follow the lines of both pairs of species backward in the tree. Since A and B converge at a common ancestor first as we move backwards, and B only converges with C after its junction point with A, we can say that A and B are more related than B and C.

Importantly, there are some species whose relatedness we can't compare using this method. For instance, we can't say whether A and B are more closely related than C and D. That’s because, by default, the horizontal axis of the tree doesn't represent time in a direct way. So, we can only compare the timing of branching events that occur on the same lineage (same direct line from the root of the tree), and not those that occur on different lineages.

Some tips for reading phylogenetic trees

You may see phylogenetic trees drawn in many different formats. Some are blocky, like the tree at left below. Others use diagonal lines, like the tree at right below. You may also see trees of either kind oriented vertically or flipped on their sides, as shown for the blocky tree.

The three trees above represent identical relationships among species A, B, C, D, and E. You may want to take a moment to convince yourself that this is really the case – that is, that no branching patterns or recent-ness of common ancestors are different between the two trees. The identical information in these different-looking trees reminds us that it's the branching pattern (and not the lengths of branches) that's meaningful in a typical tree.