What Is Molecular Evidence?

Molecular evidence is evidence for evolutionary relationships obtained by comparing biological molecules among organisms.

Important molecules include:

• DNA.
• RNA.
• Proteins.
• Amino acid sequences.

All living organisms use DNA to store genetic information and use broadly shared molecular processes to express that information. These fundamental similarities are important evidence for the common ancestry of life.

Modern molecular biology allows scientists to compare organisms at the level of individual DNA bases and amino acids.

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DNA and Evolution

DNA, or deoxyribonucleic acid, contains genetic information.

A DNA molecule consists of sequences of four bases:

• Adenine (A).
• Thymine (T).
• Cytosine (C).
• Guanine (G).

A short DNA sequence might look like:

A T G C C A T G A A C T

Another organism might have:

A T G C C A T G G A C T

Scientists can compare these sequences to investigate evolutionary relationships.

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Why DNA Provides Evidence for Evolution

DNA is inherited from parents.

When populations separate and evolve independently, changes gradually accumulate in their DNA.

These changes can result from:

• Mutations.
• Natural selection.
• Genetic drift.
• Other evolutionary processes.

Therefore, species that share a relatively recent common ancestor generally have more similar DNA sequences than species whose common ancestor lived much further in the past.

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DNA Is Inherited

The connection between inheritance and molecular evidence is important.

Consider an ancestral population.

Its DNA is passed to its descendants.

If the population separates:

Common ancestral population

→ Population A

→ Population B

The two populations initially have very similar DNA.

As generations pass, mutations and other genetic changes accumulate independently.

Eventually:

Population A DNA

and

Population B DNA

become increasingly different.

The amount and pattern of molecular difference can therefore provide information about evolutionary relationships.

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Mutations Produce DNA Differences

A mutation is a change in DNA.

Mutations can occur when DNA is:

• Copied.
• Damaged.
• Repaired.

Mutations may involve:

• Substitution of one base for another.
• Addition of bases.
• Removal of bases.
• Larger rearrangements of DNA.

Mutations provide new genetic variation.

If mutations are inherited, they can become part of the evolutionary history of a population.

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Comparing DNA Sequences

Scientists can compare corresponding DNA sequences from different organisms.

Consider these simplified sequences:

Species A

A T G C C A T G A A

Species B

A T G C C A T G G A

Species C

A C G T T A C C G A

Species A and B differ at only one position.

Species A and C differ at several positions.

Based only on this short sequence, A and B show greater molecular similarity.

This would be consistent with A and B sharing a more recent common ancestor with each other than either shares with C.

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Worked Example: Counting DNA Differences

Consider the following DNA sequences:

Species A

A T G C C A T A

Species B

A T G C C G T A

Compare each position.

A and B are identical except at one position:

Species A: A

Species B: G

Therefore:

Number of differences = 1

Now compare Species A with Species C:

Species C

G T A C T G T C

There are several differences.

The smaller number of differences between A and B suggests a closer evolutionary relationship, assuming the sequences being compared are homologous and suitable for the comparison.

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Sequence Alignment

DNA sequences cannot always be compared simply by placing them beside each other.

Mutations can add or remove DNA bases.

Scientists therefore perform sequence alignment.

Alignment arranges corresponding parts of sequences so that meaningful comparisons can be made.

For example:

Species A: A T G C C A T G

Species B: A T G - C A T G

The dash represents a gap introduced during alignment.

Computer programs are commonly used to align long DNA sequences.

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Percentage Similarity

Scientists may calculate the percentage of positions that match between sequences.

Suppose two DNA sequences contain 100 comparable positions.

If 96 positions match:

Similarity = 96%

If another pair matches at only 78 positions:

Similarity = 78%

All else being appropriate for the comparison, the first pair shows greater molecular similarity.

However, percentage similarity alone does not tell the complete evolutionary story. Scientists consider which genes are being compared and use statistical evolutionary models.

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Worked Example: DNA Similarity

Suppose scientists compare the same gene in four species.

Species A and B: 98% similarity

Species A and C: 91% similarity

Species A and D: 76% similarity

This evidence suggests:

• A and B have the greatest molecular similarity.
• A and D have the least similarity among these comparisons.

The evidence would support a hypothesis that A and B share a more recent common ancestor than A and D.

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Closely Related Species

Closely related species usually have similar DNA because they inherited much of their DNA from a relatively recent common ancestor.

Consider two populations that separated recently.

Only a limited number of generations have passed.

There has therefore been less time for genetic differences to accumulate.

Compare this with populations that separated millions of years earlier.

More genetic changes may have accumulated in the separate lineages.

The general pattern is:

More recent common ancestor → usually greater DNA similarity

More distant common ancestor → usually greater accumulated molecular differences

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DNA Similarity Does Not Mean DNA Is Identical

Closely related species can still contain many genetic differences.

Even members of the same species have genetic variation.

Therefore, scientists do not expect:

related species = identical DNA

Instead, they examine patterns of similarity and difference across corresponding DNA sequences.

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Shared Genes

Different species can possess related versions of the same genes.

This is especially common for genes controlling fundamental cellular processes.

Examples include genes involved in:

• Cellular respiration.
• DNA replication.
• Protein synthesis.
• Cell division.
• Development.

The widespread presence of related genes across different organisms provides evidence of common ancestry.

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Very Different Organisms Can Share Genes

Humans, plants, fungi, and microorganisms appear extremely different.

Yet many basic cellular processes are shared.

All cellular organisms use:

• DNA.
• RNA.
• Ribosomes.
• Proteins.
• ATP as a major energy-transfer molecule.

This deep molecular similarity is consistent with the idea that living organisms share ancient evolutionary ancestry.

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The Genetic Code

Cells use information in DNA to build proteins.

DNA information is first used to produce RNA, which can then guide protein production.

A simplified sequence is:

DNA → RNA → protein

Most organisms use essentially the same genetic code to translate nucleotide information into amino acids.

The widespread similarity of the genetic code among organisms is important molecular evidence for common ancestry.

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Proteins as Evolutionary Evidence

Proteins are made from chains of amino acids.

The order of amino acids in a protein depends on genetic information.

Therefore:

DNA changes

can sometimes lead to

changes in amino acid sequences.

Scientists can compare the same protein in different species.

Similar protein sequences can provide evidence of evolutionary relationships.

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Comparing Protein Sequences

Imagine a protein containing the following simplified sequences:

Species A

Met – Gly – Val – Leu – Ala

Species B

Met – Gly – Val – Leu – Ser

Species C

Met – Lys – Ile – Phe – Ser

Species A and B differ at only one amino acid.

Species A and C differ at several positions.

The greater similarity between A and B would be consistent with a closer evolutionary relationship.

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Why Protein Comparisons Work

Proteins are encoded by genes.

Related organisms inherit related genes from common ancestors.

As DNA changes through evolutionary time, protein sequences can also change.

Therefore:

Common ancestor

→ inherited gene

→ mutations accumulate

→ DNA sequences diverge

→ protein sequences may diverge.

Scientists can compare these molecular differences when reconstructing evolutionary relationships.

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Not Every DNA Change Changes a Protein

An important point is that a mutation in DNA does not always change the amino acid sequence of a protein.

The genetic code contains some redundancy.

Different DNA codons can sometimes specify the same amino acid.

Therefore:

DNA sequences may differ

while

the resulting protein sequences remain identical.

DNA comparison can therefore reveal evolutionary differences that protein comparison may not detect.

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Conserved Proteins

Some proteins change very slowly during evolution because their functions are extremely important.

These are often described as highly conserved.

Changes that seriously disrupt their function may reduce reproductive success and therefore remain uncommon.

Conserved proteins can be particularly useful for comparing organisms that diverged a long time ago.

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Cytochrome c

Cytochrome c is a protein involved in cellular respiration.

Versions of cytochrome c occur in many organisms.

Scientists can compare its amino acid sequence among species.

Species with fewer sequence differences generally show greater molecular similarity.

Historically, comparisons of proteins such as cytochrome c provided important molecular evidence for evolutionary relationships.

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Hemoglobin

Hemoglobin is another protein that can be compared among vertebrates.

Hemoglobin carries oxygen in red blood cells.

Comparisons of hemoglobin genes and protein sequences can reveal similarities and differences among species.

These patterns can contribute to hypotheses about evolutionary relationships.

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DNA Usually Provides More Information Than Protein Alone

DNA sequences often provide more detailed information than protein sequences.

This is partly because:

• Not all DNA mutations change proteins.
• Large amounts of non-protein-coding DNA can also be compared.
• Many different genes can be analyzed.
• Entire genomes can now be compared.

Modern evolutionary biology therefore relies heavily on DNA and genome sequencing.

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Molecular Phylogenetics

Molecular phylogenetics uses molecular information to investigate evolutionary relationships.

Scientists may compare:

• Individual genes.
• Many genes.
• Whole genomes.
• Protein sequences.

The resulting evidence can be used to construct a phylogenetic tree.

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Reading a Phylogenetic Tree

A phylogenetic tree represents hypotheses about evolutionary relationships.

A branching point represents a common ancestor or divergence of evolutionary lineages.

Suppose a tree shows:

Species A and Species B joining at one recent branching point.

Species C joins their lineage at an earlier branching point.

This suggests:

A and B share a more recent common ancestor with each other than either shares with C.

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Similar DNA Can Help Build Evolutionary Trees

Imagine the following DNA differences:

A and B: 2 differences

A and C: 9 differences

B and C: 8 differences

The molecular evidence suggests that A and B are more similar to each other.

Scientists can combine information from many DNA sequences to determine which branching patterns are best supported by the data.

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Worked Example: Three Species

Scientists compare a gene from three species.

Species A

A T G C C A A T

Species B

A T G C C G A T

Species C

G C A T T G C A

Species A and B differ at one position.

Species A and C differ at many positions.

Species B and C also differ at many positions.

A reasonable interpretation is:

A and B show evidence of a closer evolutionary relationship than either does with C.

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Worked Example: Protein Evidence

Scientists compare a protein consisting of 50 amino acids.

Species X and Y differ at:

2 positions

Species X and Z differ at:

14 positions

Species Y and Z differ at:

13 positions

The molecular evidence suggests that X and Y are more similar.

This would support the hypothesis that X and Y share a more recent common ancestor.

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Mutations Accumulate Over Time

When two populations become separated, their DNA begins accumulating changes independently.

Imagine:

Ancestral population

↓ separation

Population A | Population B

Over thousands or millions of generations, mutations occur in both lineages.

The longer the populations remain separated, the more molecular differences may accumulate on average.

This principle forms part of the basis for using molecular evidence to investigate evolutionary history.

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Molecular Clocks

Scientists sometimes use genetic changes to estimate when evolutionary lineages diverged.

This approach is called a molecular clock.

The basic idea is:

More accumulated molecular differences may indicate more time since divergence.

However, mutation rates are not perfectly constant.

Different:

• Genes.
• Species.
• Genomic regions.
• Time periods.

can have different rates of change.

Molecular clocks therefore require calibration and statistical analysis.

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Fossils Can Help Calibrate Molecular Clocks

Fossil evidence can provide approximate minimum ages for evolutionary events.

Scientists can combine:

Fossil evidence

with

molecular differences

to estimate rates of genetic change.

This allows molecular and fossil evidence to complement each other.

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Molecular Evidence and Comparative Anatomy

Comparative anatomy examines physical structures.

Molecular biology examines DNA and proteins.

These independent sources of evidence often support similar evolutionary relationships.

For example:

Anatomy

may suggest two species share homologous structures.

DNA

may show high sequence similarity.

When independent evidence supports the same relationship, confidence in the evolutionary interpretation increases.

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Molecular Evidence and Embryology

Embryology shows that related organisms can share developmental patterns.

Molecular biology helps explain why.

Development is controlled by genes.

Related organisms can share:

• Similar developmental genes.
• Similar gene sequences.
• Similar patterns of gene activity.

For example, related Hox genes help control body development in many groups of animals.

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Molecular Evidence Can Reveal Unexpected Relationships

Physical appearance can sometimes be misleading.

Convergent evolution can cause distantly related organisms to develop similar structures.

DNA evidence can help scientists distinguish between:

Similarity caused by common ancestry

and

similarity caused by convergent evolution.

This has caused scientists to revise some classifications that were originally based mainly on appearance.

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Example: Whales

Whales are highly adapted to aquatic life.

Their body shape can superficially resemble fish.

However, molecular evidence strongly supports their placement among mammals.

DNA evidence also helps scientists investigate which groups of mammals share the closest evolutionary relationships with whales.

This demonstrates how molecular evidence can reveal relationships that external appearance alone might obscure.

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DNA Barcoding

Scientists can sometimes identify species by examining standardized regions of DNA.

This approach is called DNA barcoding.

DNA barcoding can help:

• Identify organisms.
• Distinguish similar species.
• Study biodiversity.
• Identify unknown biological samples.
• Detect potentially undescribed species.

It demonstrates how characteristic molecular differences can be used to distinguish biological groups.

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Whole-Genome Comparisons

Modern technology allows scientists to sequence entire genomes.

A genome is the complete genetic material of an organism.

Instead of comparing a single gene, scientists can compare millions or billions of DNA bases.

Whole-genome comparisons can reveal:

• Shared genes.
• Mutations.
• Duplicated genes.
• Chromosomal changes.
• Evolutionary relationships.

This provides enormous amounts of evidence for studying evolution.

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Shared Molecular Errors

Some particularly useful evolutionary evidence comes from shared unusual genetic changes.

Suppose two species contain the same unusual mutation at the same position in corresponding DNA.

A reasonable explanation may be that the mutation occurred in a common ancestor and was inherited by both descendant lineages.

Shared molecular characteristics can therefore help scientists identify branches within evolutionary trees.

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Molecular Evidence and Common Ancestry

The molecular evidence for common ancestry includes several observations:

• Organisms use DNA as genetic material.
• Organisms share broadly similar genetic codes.
• Related species have similar DNA sequences.
• Related species share many genes.
• Related proteins have similar amino acid sequences.
• Shared mutations can reveal common ancestry.
• Molecular relationships often agree with fossil and anatomical evidence.

Together, these patterns are consistent with evolutionary descent from common ancestors.

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Interpreting Molecular Evidence Carefully

More DNA similarity generally suggests a closer evolutionary relationship when appropriate homologous sequences are being compared.

However, scientists must consider:

• Which DNA regions are compared.
• Mutation rates.
• Natural selection.
• Sequence length.
• Insertions and deletions.
• Statistical uncertainty.
• Whether the sequences are genuinely homologous.

Professional phylogenetic studies therefore use much more sophisticated methods than simply counting differences.

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Homologous Genes

Homologous genes are genes related through evolutionary descent.

Scientists must compare appropriate homologous sequences when reconstructing evolutionary relationships.

Two sequences may appear somewhat similar for reasons other than recent common ancestry.

Careful sequence analysis helps scientists identify meaningful comparisons.

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Different Genes Evolve at Different Rates

Some genes change rapidly.

Others change slowly.

Fast-changing sequences may be useful for comparing closely related species.

Slow-changing sequences may be useful for investigating much older evolutionary relationships.

Scientists therefore select molecular evidence appropriate to the evolutionary question being studied.

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Worked Example: Choosing the Closest Relationship

Suppose molecular analysis produces the following results:

Species P and Q: 99% similarity

Species P and R: 94% similarity

Species P and S: 81% similarity

Among these comparisons, P and Q have the greatest molecular similarity.

The evidence therefore supports a closer evolutionary relationship between P and Q than between P and R or P and S.

It does not mean P evolved from Q.

Instead, P and Q would be expected to share a relatively recent common ancestor.

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Worked Example: DNA and Anatomy Disagree

Imagine two species look very similar.

Scientists initially classify them as close relatives.

Later, DNA analysis reveals substantial molecular differences.

Further investigation shows that their similar structures evolved independently because they occupy similar environments.

This is an example of how convergent evolution can make anatomy misleading and how molecular evidence can help clarify evolutionary relationships.

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Multiple Lines of Evidence

Scientists obtain the strongest evolutionary conclusions when different evidence agrees.

For example:

Fossils

show changes through geological time.

Comparative anatomy

reveals homologous structures.

Embryology

reveals shared developmental patterns.

DNA

reveals molecular similarities.

Proteins

reveal related amino acid sequences.

When these independent observations support similar evolutionary relationships, the explanation becomes much stronger.

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Common Mistakes

Thinking Closely Related Species Have Identical DNA

Closely related species usually have highly similar DNA, but differences still exist.

Assuming Similar DNA Means One Modern Species Evolved From Another

Closely related modern species generally share a common ancestor.

Thinking Every Mutation Changes a Protein

Some DNA mutations do not alter amino acid sequences.

Assuming Every Gene Evolves at the Same Rate

Different DNA regions can change at very different rates.

Thinking Protein Similarity Is Unrelated to DNA

Protein sequences are ultimately determined by genetic information, so DNA changes can affect protein sequences.

Using Percentage Similarity Alone

Scientists consider many genes, sequence quality, evolutionary models, and other evidence.

Assuming Appearance Always Shows the Closest Relationship

Convergent evolution can produce similar-looking organisms that are not close relatives.

Thinking Molecular Clocks Are Perfect Clocks

Mutation rates vary, so molecular clocks require calibration and careful analysis.

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Check Your Understanding

1. What is molecular evidence?

2. Why can DNA be used to investigate evolutionary relationships?

3. Why do closely related species generally have more similar DNA?

4. What is a mutation?

5. How can mutations contribute to molecular differences between species?

6. What is sequence alignment?

7. Species A and B have 98% DNA similarity, while Species A and C have 82% similarity. What does this suggest?

8. Why should this conclusion be described as evidence rather than absolute proof by itself?

9. How can proteins provide evidence for evolution?

10. Why do similar protein sequences often indicate evolutionary relationships?

11. Why might two organisms have different DNA sequences but identical versions of a particular protein?

12. What is a conserved protein?

13. What is molecular phylogenetics?

14. What does a branching point on a phylogenetic tree represent?

15. Explain why two closely related modern species should be described as sharing a common ancestor rather than one having evolved directly from the other.

16. What is a molecular clock?

17. Why must molecular clocks be calibrated?

18. How can DNA evidence help distinguish common ancestry from convergent evolution?

19. Explain how molecular evidence complements fossil and anatomical evidence.

20. Three species have the following numbers of DNA differences: A–B = 3, A–C = 17, B–C = 16. Which pair shows the closest molecular similarity, and what evolutionary relationship might this suggest?

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Key Terms

• Molecular evidence – evolutionary evidence obtained by comparing biological molecules.
• DNA – molecule containing hereditary genetic information.
• Nucleotide – basic unit of DNA containing a sugar, phosphate, and nitrogenous base.
• DNA sequence – order of nucleotide bases in DNA.
• Mutation – change in a DNA sequence.
• Sequence alignment – arrangement of DNA or protein sequences so corresponding positions can be compared.
• Gene – DNA sequence that contributes to a functional product.
• Protein – molecule made from one or more chains of amino acids.
• Amino acid – building block of proteins.
• Genetic code – system through which nucleotide sequences specify amino acids.
• Conserved sequence – DNA or protein sequence that has changed relatively little through evolution.
• Homologous genes – genes related through evolutionary descent.
• Genome – complete genetic material of an organism.
• Molecular phylogenetics – use of molecular evidence to investigate evolutionary relationships.
• Phylogenetic tree – branching representation of hypotheses about evolutionary relationships.
• Molecular clock – method using molecular differences to help estimate evolutionary divergence times.
• Common ancestor – ancestral population from which different evolutionary lineages descended.
• DNA barcoding – identification of organisms using standardized DNA regions.

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Key Takeaways

• DNA provides powerful evidence for evolutionary relationships.
• DNA is inherited, allowing molecular similarities to persist between related organisms.
• Mutations cause DNA sequences to change over generations.
• After populations separate, molecular differences can accumulate independently.
• Closely related species generally have more similar DNA because they share a more recent common ancestor.
• Scientists can compare corresponding DNA sequences and count or model differences.
• Sequence alignment allows corresponding regions of DNA to be compared accurately.
• Protein sequences can also provide evidence of evolutionary relationships.
• Proteins are made from amino acids, and their sequences are determined by genetic information.
• Similar protein sequences can indicate shared evolutionary ancestry.
• Not every DNA mutation changes a protein.
• Some genes and proteins are highly conserved.
• Different genes evolve at different rates.
• Molecular evidence can be used to construct phylogenetic trees.
• Greater molecular similarity generally supports a more recent common ancestor when appropriate sequences are compared.
• Molecular clocks can help estimate divergence times but require careful calibration.
• DNA evidence can reveal relationships that are difficult to identify from appearance alone.
• Convergent evolution can make unrelated organisms look similar, while molecular evidence can help reveal their different histories.
• Whole-genome comparisons provide enormous amounts of evolutionary information.
• Molecular evidence is strongest when considered alongside fossils, comparative anatomy, embryology, and other independent evidence.