# Evidence of Evolution

> AP Biology · Unit 7: Natural Selection
> Source: https://www.owlsprep.com/study/ap-biology-u7-evidence-of-evolution/

This guide covers all major lines of evidence for evolution for AP Biology Unit 7, including fossil, biogeographic, comparative anatomy, and molecular evidence. You will learn to connect evidence to claims of common ancestry and descent with modification.

**Prerequisites:** [Descent with modification and core evolutionary theory](https://www.owlsprep.com/study/ap-biology-u7-overview/); Basic DNA and protein structure; Speciation and common ancestry definitions

## Learning objectives

- Identify different lines of evidence for evolution
- Distinguish between homologous, analogous, and vestigial structures
- Use radiometric dating and molecular sequence data to estimate relatedness and divergence time
- Justify claims about common ancestry using multiple lines of evidence

## Core Overview of Evidence for Evolution

Evidence of evolution consists of repeatable, testable observations from multiple independent fields that confirm the core claims of evolution: all living organisms on Earth share common ancestry, and populations change over generations via descent with modification.

This topic makes up approximately 12% of Unit 7 (Natural Selection), which itself accounts for 13-20% of the total AP exam score. Questions about evidence of evolution appear in both multiple-choice (MCQ) and free-response (FRQ) sections, most commonly asking to identify the correct line of evidence for a scenario or justify claims about common ancestry.

## Fossil and Geologic Evidence

**Fossil Evidence** — Preserved remains or traces of organisms from past geologic eras, embedded in sedimentary rock, ice, or amber, used to infer evolutionary relationships over time.

Geologic strata (layers of rock) form sequentially, with older layers deposited deeper underground than younger layers, allowing researchers to order fossils by relative age. Radiometric dating uses the known constant half-life of radioactive isotopes to calculate the absolute age of a sample: carbon-14 dating is used for fossils younger than ~50,000 years, while uranium-lead dating is used for older rock layers.

$$N(t) = N_0 \left( \frac{1}{2} \right)^{\frac{t}{t_{1/2}}}$$

Where $N(t)$ is the remaining quantity of the parent isotope, $N_0$ is the original quantity, $t$ is time elapsed, and $t_{1/2}$ is the half-life of the isotope. Key predictions of evolution confirmed by fossil evidence include transitional fossils (intermediate forms between major taxonomic groups) and the order of appearance of organism groups matching predictions from common ancestry.

**Worked example:** A fossilized bison bone found in permafrost has 6.25% of its original carbon-14 remaining. Carbon-14 has a half-life of 5730 years. What is the approximate age of the fossil?

1. Convert the remaining percentage to a power of one-half

   $$6.25\% = 0.0625 = \frac{1}{16} = \left(\frac{1}{2}\right)^4$$
2. From the half-life formula, the exponent equals $\frac{t}{t_{1/2}}$, so:

   $$\frac{t}{t_{1/2}} = 4$$
3. Substitute the known half-life to solve for $t$

   $$t = 4 \times 5730 = 22920$$
4. Verify: 1 half-life = 50% remaining, 2 = 25%, 3 = 12.5%, 4 = 6.25%, which matches the given value.

> **Exam tip:** You will not need to complete complex half-life calculations on the AP exam, but you must recognize that $1/2^n$ remaining means $n$ half-lives have passed, and connect fossil age to evolutionary position.

## Comparative Anatomy and Biogeography

Comparative anatomy studies similarities and differences in body structure between species, while biogeography studies the geographic distribution of species around the world. Both fields provide key evidence for common ancestry.

**Structural Homologies** — Similar body structures inherited from a common ancestor, even if they serve different functions in modern species. Distinct from analogous structures, which have similar function but evolved independently via convergent evolution, so do not indicate close common ancestry.

*Example:* The forelimbs of humans, bats, whales, and cheetahs are homologous (same bone structure, different functions), while the wings of bats and bees are analogous (same function, different origin).

A key type of homology is vestigial structures: structures that have lost their original adaptive function from an ancestor, even if they have a new minor function in the modern organism. Biogeography confirms evolution by showing that closely related species are clustered in adjacent geographic regions, and endemic island species are most closely related to species from the nearest mainland, matching predictions of colonization and subsequent divergence.

**Worked example:** Researchers studying flightless emus on Australia and ostriches on Africa find that both have small, non-functional wings that cannot be used for flight. Both are large, running birds closely related to flying birds. Is the non-functional wing of emus and ostriches an example of a vestigial structure, and how does this support evolution?

1. Categorize the structure: non-functional wings in flightless birds are vestigial structures, a type of structural homology.
2. Confirm that the wings are homologous to the functional wings of flying bird ancestors: they share the same embryonic origin and bone structure as functional wings.
3. The presence of a non-functional wing structure makes sense only if emus and ostriches descended from flying bird ancestors that lost the ability to fly as they adapted to a terrestrial running niche.
4. This is not convergent evolution of analogous traits: the shared homologous structure confirms common descent from flying ancestors, rather than independent evolution of flightlessness from non-winged ancestors.

> **Exam tip:** Always explicitly state the difference between homologous and analogous structures when asked to justify a claim about common ancestry; AP exam readers require this distinction for full credit.

## Molecular Biology Evidence for Evolution

Molecular evidence is the most quantifiable and concrete line of evidence for common ancestry, generated by comparing DNA nucleotide sequences and amino acid sequences of homologous proteins across species. The core principle is: if two species share a more recent common ancestor, they will have fewer genetic differences, because there has been less time for neutral mutations to accumulate in each lineage since divergence.

This principle forms the basis of the molecular clock hypothesis, which assumes a roughly constant rate of neutral mutation fixation over time, allowing researchers to estimate the time since two species diverged. To compare sequences, scientists align homologous genes (genes derived from the same ancestral gene) and count the number of nucleotide or amino acid differences between sequences.

**Worked example:** The table below shows the number of amino acid differences in the conserved heat-shock protein HSP90 between humans and four other vertebrate species:

| Species | Number of differences from human HSP90 |
|---------|-----------------------------------------|
| Chimpanzee | 0 |
| Gorilla | 1 |
| Mouse | 7 |
| Chicken | 14 |

Which species shares the most recent common ancestor with humans, and which is most distantly related? Justify your answer.

1. Recall the molecular clock principle: fewer sequence differences correspond to less time for mutations to accumulate since divergence from a common ancestor.
2. Chimpanzees have zero differences from human HSP90, which means no mutations have accumulated in this protein since the two lineages diverged, so they share the most recent common ancestor.
3. Chickens have 14 differences, the largest number, so they are the most distantly related to humans.
4. This pattern is consistent with the known fossil record: human and chimpanzee lineages diverged ~6 million years ago, while the human and chicken lineages diverged ~300 million years ago, giving much more time for mutations to accumulate.

**Check your understanding**

Test your understanding of molecular relatedness with this AP-style multiple choice question:

1. The conserved protein cytochrome c is used to compare relatedness between eukaryotes. Researchers compare the cytochrome c amino acid sequence of a domestic dog (Canis lupus familiaris) to four other species: gray wolf (Canis lupus) (2 differences), coyote (Canis latrans) (6 differences), red fox (Vulpes vulpes) (13 differences), domestic cat (Felis catus) (18 differences). Based on this evidence, which of the following cladograms correctly represents the relationships between these species?

   - A) Dog branches first at the base of the tree, followed by gray wolf, then coyote, then red fox, with cat as the most recent taxon
   - B) Cat is the outgroup, then red fox branches off, then coyote, then dog and gray wolf are sister taxa
   - C) Cat is the outgroup, then dog branches off first, then gray wolf, coyote, and red fox form a monophyletic clade
   - D) Dog and cat are sister taxa, gray wolf and coyote are sister taxa, red fox is the outgroup

   *Why:* Fewer amino acid differences between two species indicate a more recent common ancestor, so dog and gray wolf (only 2 differences) must be sister taxa. Cat has the most differences, so it is the outgroup. This matches option B.

> **Exam tip:** When asked to draw a cladogram from sequence difference data, always group species with the fewest differences as sister taxa; this is the rule the AP exam expects.

## Common pitfalls

- **Wrong:** Claiming that analogous structures are evidence of close common ancestry
  - Why it fails: Students confuse homologies and analogies because both involve similar-looking structures, and forget that analogies arise from convergent evolution, not shared descent.
  - Correct: Always confirm if similarity comes from shared genetic/developmental origin (homology) or shared function (analogy) before using it to support common ancestry.
- **Wrong:** Stating that fossils in deeper rock layers are younger than fossils in upper layers
  - Why it fails: Students mix up stratification order after seeing textbook diagrams that rotate layers for readability.
  - Correct: Memorize the rule: undisturbed sedimentary rock forms from the bottom up, so deeper = older, shallower = younger.
- **Wrong:** Claiming vestigial structures must have no function at all to be classified as vestigial
  - Why it fails: Students misinterpret the definition of vestigial, thinking it means completely useless.
  - Correct: Define vestigial structures as structures that lost their original adaptive function from ancestors, even if they have a new minor function in the modern organism.
- **Wrong:** Assuming the molecular clock rate is constant for all genes and all species
  - Why it fails: Students generalize the molecular clock rule to all sequences, ignoring the effect of selection on mutation rate.
  - Correct: Only apply the molecular clock rule to neutral sequences (no selection), and note that differing mutation rates can alter divergence time estimates.
- **Wrong:** Claiming the fossil record must be complete to support evolution
  - Why it fails: Students think gaps in the fossil record disprove evolution.
  - Correct: Acknowledge that fossilization is extremely rare, so gaps are expected, and the existing fossil record fully supports descent with modification.

## Cheatsheet

| Category | Rule / Formula | Key Notes |
| --- | --- | --- |
| Radiometric Dating | $N(t) = N_0 \left( \frac{1}{2} \right)^{\frac{t}{t_{1/2}}}$ | Used for absolute dating; $\frac{N(t)}{N_0}$ = fraction of parent isotope remaining |
| Relative Stratigraphy | Deeper strata = older fossils | Only gives age order, not absolute age; applies to undisturbed sedimentary rock |
| Homologous Structures | Similar structure from common ancestry | May have different functions; direct evidence for common ancestry |
| Analogous Structures | Similar function from convergent evolution | Not evidence for close common ancestry; caused by shared selection pressure |
| Vestigial Structures | Lost original ancestral function, homologous to functional ancestor structures | Still classified as vestigial even if they have a new minor function |
| Molecular Relatedness | Fewer sequence differences = more recent common ancestor | Applies to neutral sequences under the molecular clock hypothesis |

## What's next

Now that you have mastered the evidence for evolution, you are ready to build on this foundation to learn more about how populations evolve and how new species form. Evidence of evolution is the core observational backbone for all subsequent topics in Unit 7, and understanding how to connect evidence to claims about common ancestry will help you tackle phylogeny, natural selection, and speciation questions on the AP exam. The skills you developed here, like analyzing sequence data and justifying evolutionary claims, are directly tested in both MCQ and FRQ sections, so practice applying these concepts to different scenarios before your exam.

- [Unit 7 Natural Selection Overview](https://www.owlsprep.com/study/ap-biology-u7-overview/)
- [Mechanisms of Natural Selection](https://www.owlsprep.com/study/ap-biology-u7-natural-selection/)
- [Common Ancestry and Speciation](https://www.owlsprep.com/study/ap-biology-u7-common-ancestry-and-speciation/)

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