Common Ancestry and Speciation — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Evidence for common ancestry including structural and molecular homology, fossil data; biological species concept, prezygotic and postzygotic reproductive barriers, and modes of allopatric and sympatric speciation.

You should already know: Natural selection as the mechanism of adaptive evolution, basic population genetics principles, how to read basic phylogenetic trees.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Common Ancestry and Speciation?

Common ancestry (also called common descent) is the unifying scientific hypothesis that all living organisms on Earth share descent from a single shared last universal common ancestor (LUCA), with closely related groups of species descending from more recent shared ancestors. Speciation is the evolutionary process that generates new, reproductively isolated species from an ancestral population, moving beyond microevolution (changes in allele frequency within populations) to macroevolution, the large-scale evolutionary change that produces new taxonomic groups.

This topic is part of Unit 7: Natural Selection, which accounts for 13–20% of the total AP Biology exam score, with this subtopic making up roughly 3–5% of that unit. It is regularly tested in both multiple-choice (MCQ) and free-response (FRQ) sections: MCQ typically present data sets to test identification of evidence for common ancestry or classification of reproductive barriers, while FRQ ask to connect speciation scenarios to natural selection or predict evolutionary outcomes.

2. Evidence for Common Ancestry

Multiple independent lines of evidence confirm the hypothesis of common ancestry, rather than independent origin of species. Three key lines of evidence are consistently tested on the AP Biology exam:

1. Structural homology: Similarities in body plan or structural traits derived from shared ancestry, even if the traits have different functions. For example, the forelimbs of humans, bats, whales, and cats all share the same bone structure, despite being used for grasping, flying, swimming, and walking respectively. This is contrasted with analogous structures, which have similar functions but evolved independently via convergent evolution, and do not indicate shared ancestry.
2. Molecular homology: Similarities in DNA nucleotide sequences, amino acid sequences of conserved proteins, or core cellular processes (like the near-universal genetic code) shared across all life. The molecular clock hypothesis states that the number of neutral sequence differences between two taxa is proportional to the time since they diverged from a common ancestor, expressed as: $$t\propto d$$ where $t$ = time since divergence and $d$ = number of sequence differences.
3. Fossil evidence: The chronological order of trait appearance in the fossil record matches the pattern of descent from common ancestors, with transitional fossils showing intermediate traits between ancestral and descendant groups.

Worked Example

Problem: A researcher compares the amino acid sequence of the conserved respiratory protein cytochrome c across five 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, as 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.

3. Biological Species Concept and Reproductive Barriers

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 is required for speciation, because it stops gene flow between populations, allowing genetic differences to accumulate via natural selection and genetic drift until populations become permanently separate.

Reproductive barriers are divided into two categories based on when they act:

• Prezygotic barriers: Act before fertilization to block it from occurring. Examples include habitat isolation (populations live in different areas and never meet), temporal isolation (mate/flower at different times), behavioral isolation (different mating signals), mechanical isolation (reproductive structures are incompatible), and gametic isolation (sperm cannot fertilize eggs).
• Postzygotic barriers: Act after fertilization to reduce the fitness of hybrid offspring. Examples include reduced hybrid viability (hybrid embryos do not develop or survive), reduced hybrid fertility (hybrids are viable but sterile, like mules), and hybrid breakdown (first-generation hybrids are fertile, but their offspring are weak or sterile).

Worked Example

Problem: 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 we can eliminate all postzygotic barrier categories.
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.

4. Modes of Speciation

Speciation is classified by whether geographic separation blocks gene flow between diverging populations:

• Allopatric speciation: Speciation occurs when a physical geographic barrier (e.g., a mountain range, river, ocean, canyon) splits a single ancestral population into two isolated subpopulations. With no gene flow, different selective pressures in the two environments and genetic drift cause the populations to diverge genetically, eventually leading to reproductive isolation even if the barrier is removed later. This is the most common mode of speciation observed.
• Sympatric speciation: Speciation occurs without geographic separation, within the range of the ancestral population. The most common mechanism in plants is polyploidy, where an error in cell division produces an individual with extra sets of chromosomes, which is instantly reproductively isolated from the parent population in one generation. Other mechanisms include habitat differentiation (when a subset of the population adapts to a new habitat/food source within the same area) and sexual selection (when divergent mating preferences lead to reproductive isolation without geographic separation).

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

Worked Example

Problem: 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 the original and early-flowering populations live in the same prairie, with no physical barrier separating them. Reproductive isolation arises due to a difference in flowering time, not geographic separation.
3. The divergence of the new species occurs within the range of the ancestral population without a physical 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.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Claims that analogous structures are evidence for common ancestry. Why: Students confuse homologous and analogous structures, because analogous traits look similar so they incorrectly assume shared ancestry. Correct move: On any question asking for evidence of common ancestry, automatically eliminate any option or argument that relies on analogous traits, only homologous structural or molecular traits count.
• Wrong move: Labels a sterile mule (hybrid of horse and donkey) 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". If the hybrid is alive but sterile, it is reduced hybrid fertility, not viability.
• Wrong move: Classifies 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, because the new polyploid species is reproductively isolated from the parent population in the same geographic area.
• Wrong move: Uses the biological species concept to classify fossil species or asexual prokaryotes. Why: Students memorize BSC as the definition of species, but forget it only applies to sexually reproducing living organisms where interbreeding can be observed. Correct move: For fossils or asexual species, use the morphological species concept (based on shared physical traits) or phylogenetic species concept (based on genetic relatedness) instead.
• Wrong move: Interprets more molecular sequence differences between two species as meaning more recent common ancestry. Why: Students reverse the molecular clock logic, assuming more differences equals closer relatedness. Correct move: Remember: More mutations = more time since divergence = older common ancestor = less closely related.

6. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

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

Worked Solution: First, eliminate option A: Webbed feet evolved independently in both species, making this an analogous trait that does not indicate recent shared ancestry. Eliminate option C: A four-chambered heart is shared by all mammals and birds, indicating shared ancestry but not recent common ancestry. Eliminate option D: Sterile hybrids are a postzygotic reproductive barrier that confirms the two are already separate species, not evidence for recent common ancestry. Option B is correct: High sequence identity in a conserved gene means very few mutations have accumulated since divergence, which indicates the two species split from a shared common ancestor very recently. Correct answer: B.

Question 2 (Free Response)

Two species of stickleback fish live in the same glacial lake in Alaska. One species lives in the open water and feeds on plankton, while the second lives near the bottom of the lake and feeds on invertebrates. Females of both species prefer to mate with males that have the body shape typical of their own species. No gene flow occurs between the two populations, and they are classified as separate species. (a) Why is this considered sympatric speciation rather than allopatric speciation? (2 points) (b) Identify the specific type of reproductive barrier that keeps the two stickleback species separate. (1 point) (c) Explain how natural selection led to the formation of these two separate species from a single ancestral population. (3 points)

Worked Solution: (a) Sympatric speciation occurs without a geographic barrier blocking gene flow between diverging populations. Both stickleback species live in the same lake, with no physical barrier separating the open water and benthic habitats that prevents movement between the areas. Therefore, speciation occurred without geographic isolation, making it sympatric. (b) This is prezygotic behavioral isolation, caused by female mating preference for species-specific body shape that prevents interbreeding. (c) The ancestral stickleback population had genetic variation in body shape and feeding preference. Individuals that specialized in open water plankton feeding had higher fitness in open water, while individuals that specialized in benthic invertebrate feeding had higher fitness near the bottom. Natural selection favored adaptations for each habitat, and also favored female preference for males of the same habitat, leading to gradual accumulation of genetic differences and reproductive isolation until two separate species formed.

Question 3 (Application / Real-World Style)

Two closely related species of wild lettuce grow in southern California. Species A has a chromosome number of $2n=18$, and Species B has a chromosome number of $2n=36$ 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.

Worked Solution:

1. When meiosis occurs in a hybrid between Species A ($n=9$) and Species B ($n=18$), the hybrid will have $n=9+18=27$ chromosomes, which is an odd number.
2. An odd number of chromosomes cannot align and segregate properly during meiosis, so gametes will not form correctly, 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 that instant sympatric speciation via polyploidy can occur in plants in a single generation.

7. Quick Reference Cheatsheet

8. What's Next

This chapter gives you the core framework for understanding how macroevolution generates biodiversity, building on the microevolution concepts you learned earlier in Unit 7. Immediately after this topic, you will apply your understanding of common ancestry to reconstruct evolutionary relationships in phylogenetics, where homologous traits are used to build and interpret phylogenetic trees. Without mastering the difference between homologous and analogous traits and how speciation produces branching relationships, correctly interpreting phylogenetic trees will be impossible. This topic also connects to Unit 8 Ecology, where you will study how adaptive radiation after speciation produces community-level biodiversity, and to applied evolution topics on emerging disease.

Phylogenetics and the Tree of Life Evidence for Evolution Origin of Life on Earth Community Ecology