Biology · Evolutionary Processes & Adaptation

Evidence for evolution from fossils, organisms and genomes

A fossil has a mixture of familiar and unfamiliar structures. A genome contains a striking correspondence with another genome. Each observation is useful, but neither should be read as a complete history on its own. Evolutionary explanations gain strength when different kinds of evidence fit the same account.

The introductory inheritance chapter distinguishes alleles, genotypes and phenotypes. Meiosis explains segregation and crossing over; the DNA chapter explains sequence changes. Fractions and simple multiplication are enough for the constructed examples here. No example represents a measured natural population unless explicitly identified as published research.

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By the end, you can

Distinguish observations of change and shared ancestry from claims about a particular ancestor or evolutionary mechanism.

Fossils locate organisms in a changing history

Fossils preserve remains or traces of past organisms. Their geological setting helps place them in time. In a sedimentary sequence that has not been overturned or otherwise reordered, lower layers were deposited before layers above them. This gives relative order; it does not supply a numerical age by itself. Other geological evidence, including suitable radiometric dates, helps establish numerical ages.

Preservation is uneven. Hard parts, particular environments and accessible rocks are better represented than many soft-bodied organisms or settings. A gap in the record therefore needs interpretation. Fossils reveal past diversity and combinations of characters, rather than a complete list of every organism that lived.

A notable published example is Tiktaalik, described from Late Devonian rocks of Arctic Canada in 2006. Its anatomy combines fish features such as scales and fin rays with features relevant to the tetrapod body plan, including a mobile neck and wrist-like organisation. Such a mosaic helps investigate transitions. It does not establish that this particular specimen was the direct ancestor of living land vertebrates.

A record in layers and a possible branching history

Constructed relative order only; layer thickness is not elapsed time, symbols are not real taxa, and the branching history is a hypothesis rather than an established ancestor sequence.

Read the diagram in words

Left: three invented undisturbed sedimentary layers contain fossil symbols with different character sets. Right: one candidate branching history shows related lineages rather than a direct fossil-to-fossil ladder.

Observed layer	Observed character set
L3 · R, S, T observed	R, S, T
L2 · R, S observed	R, S
L1 · R observed	R

Lineage	Relationship
Shared ancestral lineage → Sampled R lineage	Descent
Shared ancestral lineage → Unsampled branching lineage	Descent
Unsampled branching lineage → Sampled R, S lineage	Descent
Unsampled branching lineage → Sampled R, S, T lineage	Descent

Later time upward

Shared structures and sequences provide different comparisons

Homologous structures correspond because they were inherited from a common ancestor, even when their present functions differ. Similar function alone is weaker evidence: unrelated lineages can evolve similar solutions independently. Anatomical comparisons therefore examine position, connections and development, alongside outward appearance.

Genome comparisons examine corresponding sequences, gene arrangements and other inherited features. Researchers must establish what is being compared: unrelated short sequences can match by chance, and duplicated genes have their own histories. Many characters and explicit methods are more informative than choosing one resemblance that fits a preferred tree.

Human chromosome 2 provides a specific genomic example. A 1991 study identified an internal arrangement of telomere-related repeats and neighbouring sequences consistent with an ancestral end-to-end chromosome fusion. That is evidence of a historical rearrangement. The observation alone is neither a date for the fusion nor a complete account of human ancestry.

An ancestral joining can leave an internal sequence signature

A simplified explanation of a type of fusion evidence, not an actual chromosome map, sequence alignment, dated event or proof from any arbitrary internal repeat. No centromere behaviour is shown.

Read the diagram in words

Two schematic ancestral chromosome segments have terminal repeat regions at their facing ends. A joined descendant segment carries opposing repeat orientations internally, with neighbouring sequence blocks on either side.

Schematic segment	Sequence region	Repeat orientation
Ancestral segment A	Neighbouring sequence L
Ancestral segment A	Facing terminal repeats	Right
Ancestral segment B	Facing terminal repeats	Left
Ancestral segment B	Neighbouring sequence R
Joined descendant segment	Neighbouring sequence L
Joined descendant segment	Internal opposing repeat orientations	Right / Left
Joined descendant segment	Neighbouring sequence R

Evidence supports explanations with stated limits

Descent with modification predicts related patterns across fossils, anatomy and genomes. Researchers compare proposed histories against those observations and against alternatives such as convergence or later gene transfer. Branches represent lineages, not a ladder with a most advanced living species at its top.

Evidence that a lineage changed is separate from evidence explaining each change. A fossil trend might involve selection, population replacement or several processes. A sequence relationship does not by itself reveal the reproductive advantage of every difference. Keeping observation, inference and mechanism separate makes the explanation stronger.

Worked example

Read a constructed fossil column without inventing an ancestor

Imagine three undisturbed model layers: oldest L1 contains a symbol with character R; middle L2 contains a symbol with R and S; youngest L3 contains a symbol with R, S and T. The order supports the statement that these character combinations are represented at different relative times in this model record.

It does not establish three direct parent-to-descendant steps. The fossils could lie on related side branches, and each layer samples only part of past diversity. A branching history is a hypothesis to test with more characters, specimens and geological evidence. The letter symbols are invented, not measurements of Tiktaalik.

Check your understanding

Why does finding R and S in a fossil not prove that the fossil was the direct ancestor of a living organism?
Show reasoning
A shared combination can also occur in a related side branch. Direct ancestry needs more evidence than resemblance and relative age; an individual specimen rarely settles that relationship.
Two animals have streamlined bodies. What additional evidence would help distinguish homology from convergence?
Show reasoning
Compare detailed structures and their developmental relationships, then examine corresponding genomic characters and broader lineage relationships. Similar performance in water alone does not identify a shared origin for every feature.
A lower layer contains no fossil of a particular soft-bodied organism. Does that prove the organism did not exist then?
Show reasoning
No. Absence could reflect preservation, the sampled setting or limited collecting. An absence can become informative with suitable repeated sampling and preservation conditions, but this single gap does not establish nonexistence.

What to carry forward

Different evidence can converge on shared ancestry while leaving particular ancestors, dates and mechanisms open to further tests.

References

Resources supporting this lesson’s explanations and examples.