Evolutionary trees and cladograms — Grade 12 Life Sciences

What a phylogenetic tree shows

A phylogenetic tree shows the evolutionary relationships between species. Each node (branch point) is a common ancestor; each tip is a living or extinct species. Time runs from the root (oldest, at the base or left) towards the tips (most recent).

Most recent common ancestor (MRCA) — the last ancestor that two species share. The lower on the tree the two lineages join, the more closely related the species.
Clade — an ancestor together with all of its descendants. A valid clade is a single node and everything beneath it.
Sister species — two species that join at the same node.

Reading relationships from a tree

To decide how closely two species are related, trace each tip back until the two lineages meet. The node where they meet is their MRCA.

✏️ Worked example

1Species W and X join at node 1. Species Y joins that pair at node 2. Species Z joins everything at node 3 (the root).

2W and X share the most recent ancestor (node 1), so they are the closest relatives.

3Y is more closely related to W and X than to Z, because their lineages join at node 2, which is more recent than node 3.

Common mistake: the order of the tips from left to right does not matter. You can flip any node (swap two branches) without changing the relationships. Only the joining points matter.

Cladograms and derived traits

A cladogram is a tree built from shared derived traits: new characteristics that appeared in an ancestor and were passed to its descendants. Each trait is placed at the node where it first appeared. Groups that share more derived traits are more closely related.

✏️ Building a cladogram

1Species A (an insect): no backbone. Species B (a fish): backbone. Species C (a lizard): backbone and amniotic egg. Species D (a mouse): backbone, amniotic egg and hair.

2The backbone appears first and is shared by B, C and D. The amniotic egg appears next and is shared by C and D. Hair is found only in D.

3Place the traits at nodes: backbone at the first node, amniotic egg at the second, hair at the third. C and D are sister species.

Shared homologous structures (such as the forelimbs of humans, bats and whales) support common ancestry. Shared traits that evolved separately (such as wings in birds and insects) are analogous and do not show close relationship.

Divergence and convergence

Divergence happens when one population splits and the groups evolve in different directions; this creates the branches of a tree. Convergent evolution happens when unrelated species develop similar traits because they live in similar environments. Dolphins (mammals) and sharks (fish) both have streamlined bodies, but they are not closely related.

Key point: similar appearance does not prove close relationship. Trees are built from shared derived traits and from DNA evidence, not from looks alone.

Answer in your exercise book, then open each memo to mark your work.

Question 1 · (4 marks)

Study a tree with tips A, B, C, D. A and B join at the node closest to the tips; C joins A and B at a lower node; D is the outgroup at the root. (a) Which two species are most closely related? (b) Explain your answer using the term most recent common ancestor.

Show memo

(a) A and B ✓
(b) A and B join at the node closest to the tips ✓, so they share the most recent common ancestor ✓. C and D share only older ancestors with them ✓.

Question 2 · (4 marks)

Define the term clade and state whether the group formed by species B, C and D (from the tree in question 1) is a valid clade. Justify your answer.

Show memo

A clade is an ancestor together with all of its descendants ✓✓.
B, C and D is not a valid clade ✓: the ancestor of B and C also gave rise to A, so a group that leaves out A does not contain all the descendants of one ancestor ✓.

Question 3 · (6 marks)

A cladogram shows: a backbone at the first node (shared by fish, lizards and mice); an amniotic egg at the second node (shared by lizards and mice); hair at the third node (mice only). (a) Name the sister species. (b) Which derived trait separates mice from lizards? (c) Explain why hair is a derived trait and not an ancestral one.

Show memo

(a) The lizard and the mouse ✓✓ (they share the amniotic egg node).
(b) Hair ✓✓
(c) Hair appeared in the ancestor of mice only, after their lineage split from the lizards ✓, so it is not shared with the older groups (fish, lizards) ✓.

Question 4 · (3 marks)

Explain why the wings of bats and birds are an example of convergent evolution, and why the forelimbs of bats and humans are homologous.

Show memo

Bat and bird wings have the same function (flight) ✓ but evolved separately in unrelated lineages, with different structures (skin membrane vs feathers), so they show convergent evolution ✓.
Bat and human forelimbs share the same basic bone plan (humerus, radius and ulna, carpals, phalanges) inherited from a common ancestor, so they are homologous ✓.