Topic 7.6 Notes – Evidence of Evolution

1. Evolution Is Supported by Multiple Independent Lines of Evidence

Evolution means populations change over time, and all organisms share common ancestors. What makes the case so strong is that geology, anatomy, genetics, and math all independently support that idea.

Five Major Categories of Evidence

a. Geological data

• Rock layers (strata) form over time. Deeper layers are older.
• Fossils appear in a consistent order worldwide.
• Major geological events such as climate shifts and mass extinctions line up with evolutionary changes in organisms.

When species disappear in the rock record and new forms appear afterward, that pattern matches evolutionary predictions.

b. Fossil record

• Fossils are preserved remains or traces of organisms.
• They show species appearing, changing, and going extinct.
• Transitional forms connect major groups.

If species were created independently and never changed, fossils would appear randomly. They don’t.

c. Morphological (anatomical) evidence

• Homologous structures
• Vestigial structures
• Comparative embryology

All of these show structural similarities that are best explained by shared ancestry.

d. Biochemical and genetic evidence

• DNA sequence comparisons
• Protein (amino acid) comparisons
• Universal genetic code

The closer the DNA match, the more recent the common ancestor.

e. Mathematical and population genetics models

• Hardy-Weinberg equilibrium predicts when evolution is not occurring.
• Models track allele frequency changes over time.
• Statistical phylogenetics infers evolutionary relationships.

When math, fossils, and DNA all support the same branching pattern, that convergence is powerful.

2. Fossils and How We Date Them

Fossils show that organisms in the past were different from organisms today. Dating methods tell us when they lived.

Relative Dating and Stratigraphy

The law of superposition states that lower rock layers are older than layers above them.

• Fossils in deeper layers are older.
• This gives relative age, not exact years.
• Fossil sequences are consistent across continents.

This is why dinosaurs never appear above mammals in undisturbed rock layers.

Absolute Dating and Radiometric Methods

Radiometric dating uses predictable radioactive decay.

Each isotope has a half-life, the time required for half of it to decay.

Carbon-14 (C-14)

• Used for recent organic material
• Effective up to about 50,000 years

Other isotopes for older material

• Potassium-40 → Argon-40
• Uranium-238 → Lead-206

These allow scientists to assign actual ages to rocks and fossils.

On exams, you may see a decay graph and need to interpret how many half-lives have passed. Just count the halving pattern.

3. Morphological Evidence of Common Ancestry

Structural similarities tell a story of modification over time.

Homologous vs. Analogous Structures

Homologous structures are especially important. In vertebrate forelimbs, the same set of bones appears again and again, even though the limbs perform very different functions.

Homologous forelimbs in mammals

Notice that the humerus, radius, ulna, carpals, metacarpals, and phalanges are present in the human, cat, whale, and bat. Same bones. Different functions. That pattern supports descent with modification.

Vestigial Structures

These are reduced remnants of ancestral features.

Examples:

• Whale pelvic bones
• Human coccyx (tailbone)
• Wings in flightless birds

They make sense under evolution because ancestors used them fully.

Comparative Embryology

Early embryos of related organisms often look similar. Shared developmental genes suggest shared ancestry.

4. Molecular and Genetic Evidence

DNA gives the most precise measure of relatedness.

DNA and Protein Comparisons

• Compare nucleotide sequences.
• Compare amino acid sequences in proteins.
• More similarity means more recent divergence.

Even small differences accumulate over time through mutation.

You might be given percent similarity between species and asked which are most closely related. Highest similarity = closest branch.

Molecular Clocks

Mutations accumulate at roughly steady rates in some genes.

• Scientists estimate divergence time by counting differences.
• Results often match fossil-based estimates.

Universal Genetic Code

All organisms use the same codon system for amino acids.

That shared code strongly supports a single common ancestor.

Extant and Extinct Organisms

• Extant species → DNA directly compared.
• Extinct species → ancient DNA from fossils.

Ancient DNA has confirmed relationships predicted from morphology alone.

5. Biogeography and Phylogenetic Trees

Biogeography

Biogeography studies species distribution.

Patterns that support evolution:

• Closely related species live near each other.
• Island species resemble nearby mainland species.
• Geographic barriers lead to genetic divergence.

Isolation → different selective pressures → speciation.

Phylogenetic Trees

Phylogenetic trees diagram evolutionary relationships over time. In the example below, a single ancestral lineage splits into two descendant lineages at a branching point.

Basic phylogenetic tree with a most recent common ancestor

Key ideas:

• Nodes represent common ancestors, such as the most recent common ancestor shown at the branching point.
• Species that share a recent node are more closely related.
• Trees are hypotheses built from morphological and molecular data.

Students often misread trees by looking at left-to-right order. What matters is branching points and shared ancestry, not the position of the tips.