Biology · Heredity, Variation & Evolution
Common ancestry, branching evolution and evidence
A branching tree asks who shares which ancestry. It does not line organisms up from worse to better. Start with three present-day lineages called A, B and C, then follow their branches backward before interpreting where the labels happen to sit.
The earlier lessons on biological scales and cells provide useful background: recognise a cell, its DNA and the difference between an individual and a population. The DNA and nucleotide explanation in Chemistry of Life is optional support. You need fractions such as one half and one quarter; the worked cases explain the calculations before using them. Detailed molecular copying, Mendelian inheritance and evolutionary inference return in later chapters.
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Read a rooted tree by shared ancestors, recognise equivalent branch rotations and distinguish evidence for a relationship from a picture's layout.
Find the shared branch point
In the supplied topology, A and B meet at an ancestral branch point m. Their joint branch meets C at the older root r. A and B therefore share a more recent common ancestor with one another than either shares with C.
The branch points represent ancestral lineages or populations, not automatically identified single individuals. The tips in this diagram all represent present-day lineages. C is not the ancestor of A or B merely because its branch separates earlier in the drawing.
Rotating a branch point can put C above the A–B pair or swap A and B without changing any parent–descendant connections. Follow the connections, not the left-to-right or top-to-bottom order of tip labels.
Root-to-tip movement represents descent, but this schematic has no dated branch scale. A longer drawn line does not mean more years, more mutations or more evolutionary progress.
Rotate branches while keeping the same ancestry
Read the diagram in words
Both layouts give A and B the more recent shared ancestral node m, and join their branch with C at root r. Rotating the branch points changes the tip order while retaining every ancestry connection.
| From | To |
|---|---|
| r: shared ancestral lineage | m: A–B ancestral lineage |
| r: shared ancestral lineage | Lineage C |
| m: A–B ancestral lineage | Lineage A |
| m: A–B ancestral lineage | Lineage B |
| From | To |
|---|---|
| r: shared ancestral lineage | Lineage C |
| r: shared ancestral lineage | m: A–B ancestral lineage |
| m: A–B ancestral lineage | Lineage B |
| m: A–B ancestral lineage | Lineage A |
Older ancestry
Present-day lineages
Compare evidence that can distinguish histories
Scientists infer evolutionary relationships by comparing evidence, including homologous structures, comparable DNA sequences and the fossil record in its geological context. Shared patterns can support common ancestry; the detailed arrangement of a particular tree remains a scientific hypothesis tested against evidence.
Similarity of function alone is insufficient. A bird and an insect can both fly even though the relevant wing structures have different histories. Conversely, inherited structures can become suited to different functions, so relatedness does not require identical outward appearance.
Sequence comparisons require suitable corresponding regions and methods that account for how change occurs. Several kinds of evidence can strengthen an inference. A single short fragment or one superficial resemblance is a limited basis for a detailed history.
New evidence can change a branch placement or its estimated date. Uncertainty about a particular placement is distinct from the extensive evidence connecting organisms through shared descent.
Count differences without inventing a clock
Our invented five-position alignment gives A as ACGTA, B as ACGTT and C as ATATT. A and B differ at one position, A and C at three, and B and C at two. Positions are aligned so that the exercise compares the same numbered locations.
These counts make A and B the most similar pair in this fragment. They give a concrete question to investigate, but five supplied positions do not establish an actual species tree. Mutation processes, repeated changes, sampling and additional regions would matter to a real inference.
The tree above is an assigned reading model, not a tree calculated from this alignment. No date or evolutionary rate can be extracted from the counts alone. A molecular clock requires additional assumptions, calibration and evidence.
Difference counts require comparable positions
Read the diagram in words
A–B differs once, A–C three times and B–C twice in the supplied five-position alignment. A–B is the most similar pair in this fragment.
| Item | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Lineage A | A | C | G | T | A |
| Lineage B | A | C | G | T | T |
| Lineage C | A | T | A | T | T |
| Pair | Number of differing positions | Differing positions |
|---|---|---|
| A–B | 1 | 5 |
| A–C | 3 | 2, 3, 5 |
| B–C | 2 | 2, 3 |
Worked example
Rotate the tree while keeping its history
Write the first layout as ((A, B), C) and a rotated layout as (C, (B, A)). In each, A and B share node m before their joint branch reaches root r with C. The two pictures encode the same relationships. In contrast, ((A, C), B) would pair A with C at the more recent node and would represent a different topology. Parentheses are a compact way to name connections here; they are not a measure of evolutionary distance.
Check your understanding
Does putting C at the top of the rotated diagram make it the closest relative of A?
Show reasoning
No. The unchanged branch connections show A and B share the more recent common ancestor.
Are A and B descendants of the present-day lineage C in the supplied model?
Show reasoning
No. All three are present-day tips, and their common ancestors are represented by internal branches or nodes.
What can the five-position alignment tell us, and what can it not tell us?
Show reasoning
It gives the supplied difference counts and shows A–B as the most similar pair in this fragment. It supplies neither a validated species tree nor a divergence date.
What to carry forward
Read ancestry from connections and evaluate histories using appropriate evidence.
References
Resources supporting this lesson’s explanations and examples.
- Understanding Evolution — Understanding phylogenies (Reviewed and updated June 2020)University of California Museum of Paleontology · University of California, BerkeleyUniversity teaching resource
Mary Ann Clark, Matthew Douglas and Jung Choi · OpenStax2 sources