Common Ancestry and Speciation

AP Biology· AP Biology CED — Natural Selection· 14 min read

1. Evidence for Common Ancestry★★☆☆☆⏱ 4 min

Multiple independent lines of evidence support the hypothesis of common descent, which states all living organisms share descent from a shared common ancestor, with more closely related species sharing more recent common ancestors. Three key lines are consistently tested on AP Biology:

Structural homology: Similarities in body structure derived from shared ancestry, even with different functions (e.g., human, bat, whale forelimbs). Contrast with analogous structures, similar functions from independent convergent evolution, not evidence for common ancestry.
Molecular homology: Similarities in DNA/amino acid sequences of conserved proteins, or the universal genetic code, shared across life. The molecular clock hypothesis states neutral sequence differences are proportional to time since divergence.
Fossil evidence: Chronological appearance of traits in the fossil record, including transitional fossils showing intermediate forms between ancestral and descendant groups.

t \propto d

📐 Worked Example

A researcher compares the amino acid sequence of the conserved respiratory protein cytochrome c across four species, with the following number of differences from the human cytochrome c sequence: Gorilla = 1, Horse = 7, Fruit fly = 29, E. coli = 43. Rank the species from most closely related to humans to least closely related, and identify which shares the most recent common ancestor with humans.

1
Recall from the molecular clock hypothesis that fewer neutral sequence differences mean less time has passed since divergence from a common ancestor.
2
Sort the species by number of differences from smallest to largest: Gorilla (1) < Horse (7) < Fruit fly (29) < E. coli (43).
3
The rank order from most to least closely related to humans matches this sorted order, because fewer differences indicate more recent shared ancestry.
4
Conclusion: Gorilla shares the most recent common ancestor with humans, followed by horse, fruit fly, and E. coli.

Exam tip:

Always double-check whether traits are homologous or analogous when asked for evidence of common ancestry. Analogous traits (e.g., dolphin flippers and shark fins) evolved independently, so they are never correct evidence for shared ancestry.

2. Biological Species Concept and Reproductive Barriers★★★☆☆⏱ 4 min

The most commonly tested definition of a species on AP Biology is the biological species concept (BSC), which defines a species as a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring, and are reproductively isolated from other such groups. Reproductive isolation stops gene flow between populations, allowing genetic divergence that leads to speciation.

📘 Definition

Reproductive Barriers

Factors that prevent interbreeding between different populations, divided into two categories based on when they act.

Example:

Temporal isolation, reduced hybrid fertility

Prezygotic barriers: Act before fertilization to block it. Examples: habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation.
Postzygotic barriers: Act after fertilization to reduce hybrid fitness. Examples: reduced hybrid viability, reduced hybrid fertility, hybrid breakdown.

📐 Worked Example

Two species of dragonfly live in the same wetland habitat. One species mates and lays eggs in early June, while the second mates and lays eggs in mid-July. No hybrid eggs or offspring have ever been observed between the two. What type of reproductive barrier is this, and is it prezygotic or postzygotic?

1
First, confirm whether fertilization ever occurs: the two species are reproductively active at different times, so mating and fertilization never happen between them.
2
Barriers that act before fertilization are prezygotic, so all postzygotic categories can be eliminated.
3
Barriers caused by mating/reproduction occurring at different times are classified as temporal isolation, a type of prezygotic barrier.
4
Conclusion: This is a prezygotic temporal reproductive barrier.

Exam tip:

If a question states no hybrid offspring are ever observed, always first check whether fertilization is prevented (prezygotic) rather than assuming postzygotic. Only label a barrier postzygotic if hybrids form but do not survive/reproduce.

3. Modes of Speciation★★★☆☆⏱ 3 min

Speciation, the process that forms new reproductively isolated species from an ancestral population, is classified by whether geographic separation blocks gene flow between diverging populations.

Allopatric speciation: Speciation occurs when a physical geographic barrier splits an ancestral population into two isolated subpopulations. With no gene flow, selection and drift cause divergence, leading to permanent reproductive isolation. This is the most common observed mode.
Sympatric speciation: Speciation occurs without geographic separation, within the range of the ancestral population. Common mechanisms include polyploidy (extra chromosome sets from cell division errors, common in plants), habitat differentiation, and sexual selection.

AP Biology also tests two models for the rate of speciation: gradualism (slow, steady change over millions of years) and punctuated equilibrium (long periods of stasis interrupted by rapid speciation after environmental change).

📐 Worked Example

A single species of wild sunflower grows in a large prairie. A subset of the population evolves to flower earlier in response to warmer temperatures, while the rest of the population continues to flower mid-season. Over time, the early-flowering group becomes reproductively isolated from the original population, forming a new species. Is this allopatric or sympatric speciation? Justify your answer.

1
Recall the core difference between the two modes: allopatric speciation requires a physical geographic barrier that blocks gene flow, while sympatric speciation occurs without geographic separation.
2
In this scenario, both populations live in the same prairie, with no physical barrier separating them. Reproductive isolation arises from a difference in flowering time, not geographic separation.
3
Divergence occurs within the range of the ancestral population without a barrier to gene flow, so it fits the definition of sympatric speciation.
4
Conclusion: This is sympatric speciation, justified by the absence of a geographic barrier between the diverging populations.

Exam tip:

On FRQ, always explicitly mention the presence or absence of a geographic barrier blocking gene flow when justifying your identification of speciation mode. This is the key point exam graders look for to award full credit.

4. AP Biology Style Concept Check★★★★☆⏱ 3 min

✓ Quick check

Test your understanding of core concepts with this AP-style multiple choice question:

Which of the following provides the strongest evidence that two living species share a very recent common ancestor?
- A. Both species have evolved webbed feet for swimming after colonizing the same lake environment independently
- B. Both species share 99.9% sequence identity in a conserved gene that codes for a ribosomal protein
- C. Both species are vertebrates and have a four-chambered heart
- D. Both species can mate to produce hybrid offspring that survive to adulthood but are completely sterile
Reveal answer
B —
Option A describes an analogous trait from convergent evolution, which does not indicate shared ancestry. Option C indicates shared ancestry but not recent ancestry. Option D confirms the two are separate species, not recent common ancestry. High sequence identity in a conserved gene means very few mutations have accumulated since divergence, indicating a recent common ancestor.

📐 Worked Example

Two closely related species of wild lettuce grow in southern California. Species A has a chromosome number of $2 n = 18$ , and Species B has a chromosome number of $2 n = 36$ that formed from an autopolyploidy event (doubling of chromosomes in a single Species A individual) 100 years ago. Can Species B interbreed with the original Species A to produce viable, fertile offspring? Justify your answer, and confirm if Species B meets the BSC definition of a separate species from Species A.

1
When meiosis occurs in a hybrid between Species A (gametes have $n = 9$ ) and Species B (gametes have $n = 18$ ), the hybrid will have $9 + 18 = 27$ chromosomes, an odd number.
2
An odd number of chromosomes cannot align and segregate properly during meiosis, so functional gametes do not form, making the hybrid sterile.
3
The new polyploid Species B is reproductively isolated from the original Species A, because any hybrid offspring produced are sterile.
4
Conclusion: No viable fertile interbreeding occurs between Species A and Species B, so Species B meets the BSC definition of a separate species. This confirms instant sympatric speciation via polyploidy can occur in plants in a single generation.

5. Common Pitfalls

Wrong move:

Claiming that analogous structures are evidence for common ancestry

Why:

Students confuse homologous and analogous structures; similar appearance leads to incorrect assumption of shared ancestry

Correct move:

Eliminate any argument that relies on analogous traits; only homologous structural or molecular traits count as evidence for common ancestry

Wrong move:

Labeling a sterile mule (horse + donkey hybrid) as an example of reduced hybrid viability

Why:

Students mix up the definitions of reduced hybrid viability and reduced hybrid fertility

Correct move:

Remember viability = ability to survive/develop, fertility = ability to produce offspring. A live but sterile hybrid is reduced hybrid fertility, not viability

Wrong move:

Classifying polyploid speciation in plants as allopatric speciation

Why:

Students assume all speciation requires geographic separation, forgetting polyploidy causes instant reproductive isolation without separation

Correct move:

Polyploid speciation is always sympatric, as the new species forms in the same geographic area as the parent population

Wrong move:

Using the biological species concept to classify fossil species or asexual prokaryotes

Why:

Students memorize BSC as the default species definition, but it only applies to sexually reproducing living organisms where interbreeding can be observed

Correct move:

Use the morphological species concept (shared physical traits) or phylogenetic species concept (genetic relatedness) for fossils or asexual species

Wrong move:

Interpreting more molecular sequence differences between two species as meaning more recent common ancestry

Why:

Students reverse the molecular clock logic, incorrectly assuming more differences equals closer relatedness

Correct move:

Remember: more mutations = more time since divergence = older common ancestor = less closely related

6. Quick Reference Cheatsheet

Category	Rule	Notes
Molecular Relatedness	Time since divergence $\propto$ number of neutral sequence differences	More differences = longer time since split from common ancestor
Evidence for Common Ancestry	Only homologous traits count as evidence	Analogous similarity from convergent evolution, not shared ancestry
Speciation Classification	Allopatric = geographic barrier blocks gene flow; Sympatric = no geographic barrier	Polyploidy in plants is always sympatric
Reproductive Barriers	Prezygotic = acts before fertilization; Postzygotic = acts after fertilization	No hybrids observed = usually prezygotic

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2023 · AP Biology
MCQ: Evidence for common ancestry
2022 · AP Biology
FRQ: Speciation mode justification
2021 · AP Biology
MCQ: Reproductive barrier classification

Going deeper

unit overviewAP Biology Unit 7 Overview: Natural Selection

What's Next

Common ancestry and speciation are core components of AP Biology's Unit 7 on Natural Selection, building on the mechanisms of microevolution you learned earlier and connecting to macroevolutionary patterns that shape the diversity of life on Earth. Mastery of these concepts is critical for both multiple-choice questions testing classification of traits and barriers, and free-response questions that require you to justify conclusions about speciation events based on data. After completing this guide, you can move on to review broader macroevolutionary patterns, practice interpreting phylogenetic trees, or review the full unit overview to consolidate your understanding of all topics in Unit 7.