Evolutionary trees are a method used by scientists to show how they…

The idea

An evolutionary tree, also called a phylogenetic tree, is a branching diagram showing how scientists currently believe organisms are related through common ancestors. The branching points represent common ancestors, and the tips represent present-day or extinct species. Closely related organisms share a more recent common ancestor and sit on branches that split more recently. For example, a tree might show humans and chimpanzees sharing a recent common ancestor, while both are more distantly related to fish. The word believe matters: evolutionary trees are models based on the best available evidence, so they can be redrawn when new data appear. A tree does not show a species turning into another living species today; it shows shared ancestry over evolutionary time.

What earns the marks

• An evolutionary tree is a branching diagram showing proposed relationships between organisms.
• Branching points represent common ancestors from which later groups diverged.
• Organisms sharing a more recent common ancestor are shown as more closely related.
• Evolutionary trees are models based on evidence and may be revised when new data become available.
• The tips of the tree represent species, which may be living or extinct.

Common mistakes and how to put them right

• Reading a tree as a sequence of one species turning into another.

  Put it right by explaining that branches show shared ancestry, not a chain of transformation.

• Assuming the tree is fixed truth.

  Put it right by stating that trees are evidence-based models that can be revised.

• Ignoring branch points: correct this by identifying branching points as common ancestors.

Exam tips

• Describe what the branch points and tips represent before interpreting relationships.
• Use the phrase common ancestor when explaining why two organisms are closely related.
• State that evolutionary trees are models based on evidence, so they can change.