7.6 Evidence of Evolution

Evidence for evolution comes from many fields at once: fossils and how they are dated, anatomical clues like homologous and vestigial structures, and molecular comparisons of DNA and protein sequences. When these independent lines of evidence point to the same relationships, they support common ancestry and change over time. For AP Biology, connect each type of evidence to the claim it supports instead of just naming examples.

Evidence of Evolution Summary

Evidence of evolution comes from geographical, geological, physical, biochemical, and mathematical data. AP Biology focuses on how fossils, fossil-dating methods, morphological homologies, vestigial structures, DNA sequences, and protein amino acid sequences support common ancestry and change over time.

The key exam move is to connect the data to the claim. Similar bone patterns suggest shared ancestry, vestigial structures show modified ancestral traits, and fewer DNA or amino acid differences usually indicate a more recent common ancestor.

Why This Matters for the AP Biology Exam

This topic trains you to connect different kinds of data to one conclusion: that organisms have changed over time and share common ancestors. On the AP Biology exam, you may need to describe what counts as evidence for evolution and explain how a specific dataset (fossil ages, a forelimb diagram, a table of DNA or amino acid differences) supports evolutionary relationships. Free-response questions often ask you to interpret a representation and justify a claim with evidence, so being precise about what each type of data shows matters more than just listing examples.

A common scoring trap is using vague language. You want to explain why a shared bone pattern or similar gene sequence points to a common ancestor, not just name it.

Key Takeaways

• Evidence for evolution is multidisciplinary: geographical, geological, physical, biochemical, and mathematical data all support it.
• Fossils can be dated by the age of surrounding rock, isotope decay (such as carbon-14), and geographical data.
• Morphological homologies, including vestigial structures, point to common ancestry.
• Comparing DNA nucleotide sequences and protein amino acid sequences gives molecular evidence of common ancestry.
• More similar sequences generally indicate a more recent common ancestor.
• Homologous structures show shared descent; analogous structures look similar but evolved separately and do not by themselves show close ancestry.

How Many Disciplines Support Evolution

Evolution is supported by scientific evidence from many disciplines. No single observation proves it; instead, separate fields point to the same patterns, which is what makes the case strong.

• Geographical data: The geographic distribution of species reflects shared history. Related species tend to cluster in the same region, and island species often resemble the nearest mainland species. Application: Darwin's finches on the Galapagos diversified from a common ancestor based on each island's conditions.
• Geological data: Rock layers (strata) provide a timeline of Earth's history and record environmental changes that shaped which traits were favored.
• Physical data: Comparative anatomy, embryology, and vestigial structures show patterns explained by shared ancestry. Application: the pentadactyl (five-digit) limb appears in vertebrates from bats to whales.
• Biochemical data: DNA and protein sequence similarities indicate degrees of relatedness, and the near-universal genetic code is consistent with common ancestry.
• Mathematical data: Population genetics and phylogenetic analysis quantify allele frequency changes and evolutionary relationships. Hardy-Weinberg equations can reveal when a population is evolving.

How Fossils Are Dated

Scientists estimate fossil ages using several approaches. For AP Biology, focus on three: the age of the surrounding rock, isotope decay, and geographical data.

• Stratigraphy (rock layers): Analyzing the layers of rock and soil where a fossil is found gives its age relative to other fossils in the same area. Deeper layers are generally older.
• Radiometric dating: Measuring the decay of radioactive isotopes acts like a clock. Different isotopes work for different time scales:
  • Carbon-14: dates organic material up to about 50,000 years old; decays to nitrogen-14 with a half-life of 5,730 years.
  • Potassium-40 to Argon-40: used for older rocks; half-life of about 1.3 billion years.
  • Uranium-238 to Lead-206: used for very ancient rocks; half-life of about 4.5 billion years.
• Geographical data: Comparing a fossil with others found nearby helps place it in a time period and ecosystem.

Other methods such as paleomagnetism and tephrochronology exist as supplementary examples, but the three above are what you should be ready to use.

Together, these methods let scientists reconstruct an organism's evolutionary history and the ancient ecosystems it lived in.

Morphological Homologies and Vestigial Structures

Morphological homologies are anatomical similarities inherited from a common ancestor, even when the structures now serve different functions.

Homologous structures share the same underlying anatomy because of shared descent. The forelimbs of humans, whales, bats, and cats all use the same basic bone arrangement (humerus, radius, ulna) despite being used for grasping, swimming, flying, and walking. That shared pattern, not the function, is the evidence of common ancestry.

Analogous structures, in contrast, perform similar functions but evolved independently. Bird wings and insect wings both allow flight but arose from different ancestral structures. Analogous structures do not by themselves indicate close common ancestry.

Vestigial structures are reduced or non-functional versions of features that were useful in ancestors. They act like biological footprints of evolutionary history.

Additional examples include reduced human body hair (goosebumps are a leftover of fur-raising for insulation) and ear muscles that other mammals use to move their ears.

Molecular Evidence: DNA and Protein Sequences

Comparing DNA nucleotide sequences and protein amino acid sequences provides strong evidence for evolution and common ancestry. The logic is direct: organisms that share more similar sequences tend to share a more recent common ancestor, because fewer changes have accumulated since they diverged.

• Similar sequences across diverse organisms reflect traits inherited from a shared ancestor.
• The near-universal genetic code across life supports common descent.
• Scientists combine sequence similarities with shared traits and fossil evidence to build phylogenetic trees that model these relationships.

Genetic evidence comes from both living (extant) and extinct organisms. Sequences from living species show degrees of relatedness, while ancient DNA from fossils can reveal sequences of extinct species and help fill gaps in evolutionary trees. When this molecular evidence lines up with anatomical and fossil evidence, the overall case becomes more reliable.

How to Use This on the AP Biology Exam

Free Response

• When asked to describe evidence for evolution, name the data type and what it shows. For example: "Homologous forelimb bones share the same arrangement, which indicates a common ancestor."
• When asked to explain, connect cause to effect. Don't stop at "they have similar DNA." Add that more similar sequences mean fewer changes since divergence, so a more recent common ancestor.
• If given a table of amino acid or nucleotide differences, the species with the fewest differences from a reference are the most closely related.

Data Analysis

• For fossil-dating items, match the isotope to the time scale. Carbon-14 is for relatively recent organic material; potassium-argon and uranium-lead are for much older rocks.
• In rock strata, treat deeper layers as older unless told otherwise.

Common Trap

• Do not call analogous structures evidence of close common ancestry. They show convergent evolution, not shared descent.
• Avoid Lamarckian wording. Traits are not gained because an organism "needs" or "tries"; favorable existing variations are passed on more often.

Common Misconceptions

• "One piece of evidence proves evolution." The strength comes from many independent fields agreeing, not from a single fossil or gene.
• "Vestigial means completely useless." Vestigial structures have reduced or changed function compared to ancestors, but some still do something (the appendix houses some bacteria; the coccyx anchors muscles).
• "Similar function means closely related." Function can be convergent. Shared underlying structure or sequence is the real signal of common ancestry.
• "Carbon-14 dates everything, including dinosaurs and rocks." Carbon-14 only works on organic material up to roughly 50,000 years old. Older material needs isotopes with longer half-lives.
• "More DNA differences mean a species is more evolved." Sequence differences track how long ago lineages diverged, not how advanced an organism is.
• "Homologous and analogous are the same idea." Homologous structures come from shared ancestry; analogous structures evolved separately to do similar jobs.

Related AP Biology Guides

• 7.1 Introduction to Natural Selection
• Unit 7 Overview: Natural Selection
• 7.2 Natural Selection
• 7.4 Population Genetics
• 7.3 Artificial Selection
• 7.5 Hardy-Weinberg Equilibrium