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High school biology

Course: High school biology > Unit 7

Lesson 3: Phylogeny
Science
High school biology
Evolution
Phylogeny
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Phylogeny review

Google Classroom

Key terms

TermMeaning
PhylogenyThe study of evolutionary lineages of a species, or group of species
Common ancestorAn ancestor shared by two or more descendant species
Phylogenetic tree (cladogram)A diagram that represents evolutionary relationships among organisms

Phylogeny

Phylogeny helps scientists organize species or other groups in ways that represent our understanding of how they evolved from common ancestors.

Phylogenetic trees

In a phylogenetic tree, or cladogram, 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:
Phylogenetic tree with labeled branching points
Phylogenetic tree. Image modified from OpenStax, CC BY 4.0.
How the branches connect represents our understanding of how the species in the tree evolved from a series of common ancestors. At each branch point lies the most recent common ancestor of all the groups descended from that branch point.
Phylogenetic tree with most recent common ancestors labeled
Phylogenetic tree and most recent common ancestors. Image modified from OpenStax, CC BY 4.0.
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 furthest left branch point, we would find the most recent common ancestor of all the species in the tree.
Not all phylogenetic trees look the same. 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 orientation of the phylogenetic tree does not change the information in the tree.
Types of phylogenetic trees. Image modified from OpenStax, CC BY 4.0.

Relatedness

In a phylogenetic tree, two species are more related if they have a more recent common ancestor, and less related if they have a less recent common ancestor.
To find the most recent common ancestor of any pair or group of species, start at the branch ends carrying the two species of interest and “walk backwards” in the tree until you find the point where the species’ lines meet.
Species A and B share a more recent common ancestor than species B and C because they converge sooner in the lineage.
Comparing relatedness of species using phylogenetic tree. Image modified from OpenStax, CC BY 4.0.
For example, to determine whether species A and B are more related than species B and C, we would follow the lines of both pairs of species backward in the tree. A and B meet first, indicating they have a more recent common ancestor and are more related than B and C.

Common mistakes and misconceptions

  • Phylogenetic trees are hypotheses of relatedness. Although we know that modern organisms evolved from ancient organisms, the pathway of this evolution is sometimes a best guess based on the amount of evidence available at the time. The more we uncover about the lineage of a set of organisms, the more accurate the phylogenetic trees become.
  • Phylogenetic trees are not just based on physical traits. To create a phylogenetic tree, scientists often compare and analyze many characteristics of the species or other groups involved. Although this may include internal and external physical traits, it can also include other factors like behavior or DNA sequences.

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