Biology · Unit 7: Natural Selection · 14 min read · Updated 2026-05-10
Common Ancestry and Speciation — AP Biology
AP Biology · Unit 7: 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
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.
**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.
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).
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
Common Pitfalls
Why: Students confuse homologous and analogous structures; similar appearance leads to incorrect assumption of shared ancestry
Why: Students mix up the definitions of reduced hybrid viability and reduced hybrid fertility
Why: Students assume all speciation requires geographic separation, forgetting polyploidy causes instant reproductive isolation without separation
Why: Students memorize BSC as the default species definition, but it only applies to sexually reproducing living organisms where interbreeding can be observed
Why: Students reverse the molecular clock logic, incorrectly assuming more differences equals closer relatedness