AP Biology

## Learning Outcomes

- Explain why DNA and protein sequence similarities provide evidence for common ancestry.
- Interpret molecular data to decide which organisms share the most recent common ancestor.
- Connect mutations, codons, amino acid sequences, and inherited variation to evolutionary change.
- Distinguish molecular evidence from morphological evidence and recognize why both can support evolution.
- Explain why amino acid similarities may not reveal all DNA-level differences.
- Use molecular comparisons to support or reject claims about relatedness on AP Biology questions.

## AP Biology Syllabus

In AP Biology Unit 7, evidence of evolution is linked to the larger idea that evolution explains both the diversity and unity of life. Molecular evidence is especially testable because it connects Unit 7 with genetics, gene expression, biotechnology, and data analysis. Expect questions that ask you to compare DNA or protein sequences, interpret percent similarity, justify common ancestry, or evaluate a claim using sequence data.

- DNA sequences in shared genes can be compared among species to infer evolutionary relatedness.
- Organisms with a more recent common ancestor usually have more similar DNA sequences.
- Amino acid sequences in proteins can also be compared as molecular evidence.
- Differences in DNA sequences arise through mutations and are inherited when they occur in gametes or reproductive lineages.
- The genetic code is redundant, so different DNA sequences can sometimes produce the same amino acid sequence.
- Molecular data can be used to build or interpret phylogenetic trees.
- AP questions may combine molecular evidence with natural selection, mutations, or biotechnology methods such as PCR, gel electrophoresis, and sequence databases.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why does a high percentage similarity in a shared gene suggest a recent common ancestor?
2. Humans and chimpanzees have over $$99\%$$ similarity in the GAPDH gene, while humans and dogs have about $$91\%$$ similarity in that gene. What evolutionary claim does this support?
3. Why can two organisms have different DNA sequences but identical amino acid sequences for part of a protein?
4. How can molecular evidence help avoid mistakes caused by comparing only body shape or anatomy?
5. Why is it incorrect to say that an individual organism evolves a new DNA sequence because it “needs” one?

## Introduction

Evidence for evolution comes from several sources, including fossils, homologous structures, vestigial structures, biogeography, observations of living populations, and molecular data. Molecular evidence focuses on the molecules that store and express inherited information: DNA, RNA, and proteins. In AP Biology, the most common molecular comparisons involve DNA nucleotide sequences and amino acid sequences in proteins.

Molecular evidence is powerful because all living organisms use DNA or RNA as genetic material and use a shared genetic code to build proteins. When two species share highly similar DNA sequences in the same gene, the best biological explanation is usually that they inherited those sequences from a common ancestor. Over time, mutations accumulate in separate lineages, so more distant relatives generally show more sequence differences.

> **Key Term: molecular evidence**
> Evidence for evolutionary relationships based on comparisons of biological molecules, especially DNA sequences and amino acid sequences in proteins.
>
> **Key Term: common ancestry**
> The idea that two or more species descended from a shared ancestral population.

### DNA sequence similarity as evidence for relatedness

DNA is made of nucleotide sequences. A gene is a segment of DNA that can be transcribed and, in many cases, translated into a protein. If the same gene is found in different organisms, scientists can compare the order of nucleotides in that gene. The more similar the sequences are, the more recently the organisms usually shared a common ancestor.

A classic AP-style example is the comparison of the GAPDH gene. GAPDH stands for glyceraldehyde-3-phosphate dehydrogenase, an enzyme involved in cellular respiration. Because many organisms use this enzyme, the gene can be compared across species. The GAPDH gene sequence in humans and chimpanzees is over $$99\%$$ similar, while the GAPDH gene sequence in humans and dogs is about $$91\%$$ similar. This supports the claim that humans and chimpanzees share a more recent common ancestor with each other than humans and dogs do.

This reasoning depends on inherited changes. Mutations introduce differences into DNA sequences. If a mutation occurs in a reproductive cell or in a lineage that gives rise to offspring, that mutation can be passed to later generations. Over long periods, separate populations accumulate different mutations. Closely related species have had less time to accumulate differences since separating from their common ancestor, so their shared genes are expected to be more similar.

> **Key Term: homologous gene**
> A gene in different species that is similar because it was inherited from a common ancestor.
>
> **Key Term: mutation**
> A change in a DNA sequence, such as a nucleotide substitution, insertion, or deletion.

A useful way to think about sequence similarity is to imagine comparing the same sentence copied by two students from an older document. If the two copies have only one or two differences, they probably came from a recent shared copy. If they have many differences, they likely separated earlier and had more time for changes to accumulate. DNA sequence comparisons work in a similar way, except that the “letters” are nucleotides: A, T, C, and G.

A simple percent similarity calculation is:

$$
\text{Percent similarity}=\frac{\text{number of matching positions}}{\text{total positions compared}}\times 100
$$

For example, if two aligned DNA sequences match at 98 out of 100 nucleotide positions, their similarity is $$98\%$$. On the AP exam, you may not need to perform a long calculation, but you should be able to interpret what a higher or lower percent similarity means.

> **Test Tip:**
> When comparing molecular sequences, state the claim directly: “Species A and B share the most recent common ancestor because they have the highest DNA sequence similarity.” Then support it with the data.

Molecular evidence is considered especially strong because environmental conditions usually do not directly rewrite an organism’s inherited DNA sequence in a purposeful way. An organism may change its behavior, physiology, or gene expression in response to the environment, but those changes are not the same as inherited DNA sequence change. Natural selection acts on existing heritable variation in populations; it does not cause individuals to create the exact mutations they need.

### Protein comparisons and the genetic code

Proteins are made of amino acids. The amino acid sequence of a protein is determined by the DNA sequence of the gene that codes for it. Because DNA affects protein structure, protein sequences can also be used as evidence for evolution.

> **Key Term: amino acid sequence**
> The order of amino acids in a protein, determined by codons in mRNA and ultimately by the DNA sequence of a gene.

If two species have very similar amino acid sequences in the same protein, this suggests that their DNA sequences are also similar and that they may share common ancestry. For example, proteins such as cytochrome c, hemoglobin, and GAPDH are often compared among species because they occur in many organisms and perform essential cellular roles. Proteins that are required for basic cell processes often change slowly over time because many changes would reduce function and be selected against.

> **Key Term: conserved sequence**
> A DNA or protein sequence that has remained similar across different species over evolutionary time, often because it has an essential function.

However, AP Biology questions often test a key limitation: DNA sequences can differ even when amino acid sequences are the same. This happens because the genetic code is redundant. Most amino acids are coded for by more than one codon. For example, two different mRNA codons may specify the same amino acid. A nucleotide change that does not change the amino acid is called a silent mutation.

> **Key Term: silent mutation**
> A nucleotide change in a coding sequence that does not change the amino acid sequence of the protein.

This means amino acid comparisons may hide some DNA-level differences. Suppose two species have identical amino acid sequences for a short protein region. You cannot automatically conclude that their DNA sequences are identical. They may have different codons that code for the same amino acids. For this reason, DNA sequence data often provide more detailed information than protein sequence data.

> **Exam Warning:**
> Do not assume that identical proteins always mean identical DNA. Because the genetic code is redundant, different nucleotide sequences can produce the same amino acid sequence.

Protein comparisons are still useful. If a protein sequence is highly similar across many species, that can indicate shared ancestry and strong selection against major changes in that protein. If a protein sequence differs at many positions between two species, that may suggest a more distant relationship or a faster rate of change in that gene.

In exam responses, connect the levels of biological information clearly:

- DNA nucleotide sequence determines mRNA codons.
- Codons determine amino acid sequence.
- Amino acid sequence affects protein structure and function.
- Similar DNA or protein sequences suggest inheritance from a common ancestor.
- More sequence differences usually indicate more time since the lineages separated.

This chain of reasoning helps you earn points because it links molecular data to evolution rather than simply describing the molecules.

### Using molecular data to build evolutionary relationships

Molecular evidence can be used to construct phylogenetic trees, which are diagrams showing hypotheses about evolutionary relationships. A phylogenetic tree is not just a chart of similarity; it represents proposed patterns of common ancestry.

> **Key Term: molecular phylogeny**
> A hypothesis about evolutionary relationships based on comparisons of DNA, RNA, or protein sequences.

When scientists compare molecular sequences, they first align the sequences so that homologous positions are compared. Then they count similarities and differences. Species with fewer differences in shared genes are often placed closer together on a phylogenetic tree. A branch point represents a common ancestor.

For example, imagine three species are compared using the same gene:

- Species A and B differ at 3 nucleotide positions.
- Species A and C differ at 18 nucleotide positions.
- Species B and C differ at 17 nucleotide positions.

The best-supported claim is that A and B share a more recent common ancestor with each other than either does with C. The data do not prove that A evolved from B or that B evolved from A. Instead, A and B are sister lineages that descended from a shared ancestral population.

> **Test Tip:**
> In phylogeny questions, avoid saying “Species A evolved from Species B” unless the question explicitly describes an ancestor-descendant relationship. Usually, say “Species A and Species B share a more recent common ancestor.”

Molecular data are especially helpful when anatomy is misleading. Two organisms may have similar body shapes because they live in similar environments, not because they are close relatives. For instance, sharks and dolphins both have streamlined bodies, but sharks are cartilaginous fish and dolphins are mammals. Molecular evidence can help distinguish similarity due to shared ancestry from similarity due to similar selection pressures.

Molecular comparisons can also reveal relatedness that is not obvious from appearance. Some organisms look very different as adults but share molecular similarities that show common ancestry. This is why modern classification relies heavily on DNA and protein evidence, not only on visible traits.

Some questions may mention a molecular clock. This is an approach that estimates time since divergence by using the amount of sequence difference and an assumed rate of mutation or sequence change. Molecular clocks work best for genes or regions where changes accumulate at a fairly regular rate. They are estimates, not exact dates, and they must often be calibrated using fossils or other evidence.

> **Key Term: molecular clock**
> A method for estimating the time since two lineages diverged by comparing molecular differences and using an estimated rate of sequence change.

For AP Biology, you usually need the basic logic: more differences can mean more time since divergence, if the rate of change is similar. Be careful, though. Mutation rates can vary among genes and lineages. A gene under strong selection may change slowly, while a less constrained DNA region may change faster.

### Molecular evidence in AP Biology investigations

AP Biology often presents molecular evidence in experimental or data-based contexts. You may be asked to analyze a gene sequence alignment, a protein sequence comparison, a gel electrophoresis result, or a database search. The question may not require you to know the lab procedure in detail, but it will test whether you can interpret the data.

Several biotechnology tools can support molecular comparisons:

- PCR can amplify a DNA region so there is enough DNA to analyze.
- Gel electrophoresis can separate DNA fragments by size.
- DNA sequencing can determine the order of nucleotides.
- Sequence databases can compare an unknown DNA or protein sequence to known sequences from other organisms.

For example, a question might describe a researcher sequencing a gene from an unknown organism and comparing it with sequences from four known species. If the unknown organism’s gene is $$97\%$$ similar to species X and only $$84\%$$ similar to species Y, the stronger claim is that the unknown organism is more closely related to species X.

Some AP questions ask you to justify a prediction or conclusion using data. A strong answer does three things:

1. Identifies the pattern in the molecular data.
2. Connects that pattern to common ancestry.
3. Avoids unsupported claims about individuals changing because they need to adapt.

A weak answer might say, “The species became more similar because they lived near each other.” A better answer would say, “The species have highly similar DNA sequences in the same gene, which supports the claim that they inherited that gene from a recent common ancestor.”

Molecular evidence also connects to natural selection. Mutations create genetic variation. Natural selection acts on phenotypic variation that affects survival and reproduction. If a mutation changes a protein in a way that improves reproductive success in a given environment, that allele may increase in frequency in the population over generations. The key point is that populations evolve; individuals do not evolve.

> **Exam Warning:**
> Avoid Lamarckian wording such as “the organism changed its DNA so it could survive.” Instead, explain that heritable variation already existed or arose by mutation, and individuals with advantageous traits left more offspring.

Molecular evidence is not used in isolation. Scientists often combine DNA, protein, fossil, anatomical, and biogeographic data. If several independent lines of evidence support the same evolutionary relationship, the claim becomes stronger. For AP Biology, you should be able to explain how molecular data fit with other evidence, but you should not treat visible similarity as more reliable than DNA evidence.

A final exam skill is recognizing the scale of the claim. A short DNA sequence can support a hypothesis about relatedness, but a larger set of genes usually gives stronger evidence. One gene may have an unusual history due to mutation rate differences, selection, or horizontal gene transfer in microbes. Multiple genes or whole-genome comparisons provide a broader basis for evolutionary conclusions.

## Key Point Checklist

This article has covered the following key knowledge points:

- Molecular evidence compares DNA, RNA, or protein sequences to infer evolutionary relationships.
- More similar DNA sequences in shared genes usually indicate a more recent common ancestor.
- Humans and chimpanzees have over $$99\%$$ similarity in the GAPDH gene, while humans and dogs have about $$91\%$$ similarity, supporting closer human-chimpanzee relatedness.
- Mutations create DNA sequence differences that can be inherited across generations.
- Protein amino acid sequences provide evidence for evolution because they are determined by genetic information.
- The genetic code is redundant, so different DNA sequences can sometimes code for the same amino acid sequence.
- Silent mutations change DNA but do not change the protein’s amino acid sequence.
- Conserved sequences often remain similar because they perform essential functions.
- Molecular phylogenies use sequence comparisons to form hypotheses about common ancestry.
- A branch point on a phylogenetic tree represents a common ancestor, not one modern species turning into another.
- Molecular clocks estimate divergence time from sequence differences, but rates can vary.
- AP Biology answers should link molecular data to common ancestry and avoid saying that individuals evolve because they need to adapt.

## Key Terms and Concepts

- molecular evidence
- common ancestry
- homologous gene
- mutation
- amino acid sequence
- conserved sequence
- silent mutation
- molecular phylogeny
- molecular clock


Evidence of evolution - Molecular evidence (DNA, proteins)