AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/7/7-10)
Unit 7 · Topic 7.10
7.10 Speciation
A species, by the biological species concept, is a group whose members can interbreed and produce viable, fertile offspring. New species form when populations become reproductively isolated, with a geographic barrier (allopatric) or without one (sympatric), at rates ranging from slow gradualism to bursts of change (punctuated equilibrium) and rapid adaptive radiations.
Key terms
- biological species concept
- reproductive isolation
- allopatric speciation
- sympatric speciation
- prezygotic and postzygotic barriers
- punctuated equilibrium
What counts as a species
The biological species concept defines a species as a group of populations whose members can interbreed in nature and produce viable, fertile offspring, and that can't do so with other such groups. A horse and a donkey can mate, but their offspring, a mule, is sterile, so horses and donkeys are separate species. This definition works for sexually reproducing organisms; it can't be applied to organisms that reproduce only asexually or that are known only from fossils.
Speciation happens when two populations become reproductively isolated, so gene flow between them stops. After that, mutation, selection and drift act on each separately and they diverge.
Allopatric and sympatric speciation
Allopatric speciation happens when a physical barrier, such as a mountain range, a river, a new body of water or a stretch of ocean, separates populations geographically. When the Isthmus of Panama formed about 3 million years ago, it split marine populations into Atlantic and Pacific groups, and many have since become different species.
Sympatric speciation happens in populations that live in the same area. It's common in plants through polyploidy: an error in cell division produces offspring with extra chromosome sets, which can't produce fertile offspring with the parent species. It can also happen through shifts in habitat or behavior. Apple maggot flies (Rhagoletis) originally laid eggs on hawthorn fruit; in the 1800s some began using apples, which ripen earlier. Because the flies mate on their host fruit, apple flies and hawthorn flies now mate mostly with their own kind and are diverging.
Reproductive barriers
Prezygotic barriers act before a zygote (fertilized egg) forms; postzygotic barriers act after.
| Barrier | Type | Example of what blocks gene flow |
|---|---|---|
| Habitat isolation | Prezygotic | Two populations use different habitats in the same area and rarely meet |
| Temporal isolation | Prezygotic | Breeding at different times of day or year |
| Behavioral isolation | Prezygotic | Different courtship songs or displays aren't recognized |
| Mechanical isolation | Prezygotic | Reproductive structures or flower shapes don't fit |
| Gametic isolation | Prezygotic | Sperm and egg can't fuse, as in many sea animals that release gametes into water |
| Reduced hybrid viability | Postzygotic | Hybrid embryos die or are frail |
| Reduced hybrid fertility | Postzygotic | Hybrids survive but are sterile, like mules |
| Hybrid breakdown | Postzygotic | First-generation hybrids are fertile, but later generations are weak or sterile |
Tempo and patterns of evolution
Gradualism is evolution by slow, steady change over hundreds of thousands to millions of years. Punctuated equilibrium describes long periods of little change (stasis) broken by relatively rapid change, often linked to speciation. Fossil records show examples of both.
Divergent evolution happens when related populations adapt to different habitats and become more different in phenotype. When new habitats or resources open up, for example after a mass extinction or when a few colonists reach new islands, one lineage can rapidly split into many species adapted to different niches. This is adaptive radiation. Hawaiian Drosophila fruit flies (hundreds of species) and Caribbean Anolis lizards are examples.
Convergent evolution is the opposite pattern: unrelated species under similar selective pressures evolve similar traits, such as the streamlined bodies of sharks and dolphins. On different Caribbean islands, Anolis lizards independently evolved similar body types for similar perches, showing both radiation and convergence.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Instant speciation by polyploidy
A diploid plant species has 2n = 14. An error in cell division produces a tetraploid plant (4n). How many chromosomes do the tetraploid's gametes have? If it crosses with the original diploid species, how many chromosomes will the offspring have, and why are those offspring usually sterile?
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- Step 1: The diploid has n = 7, so a tetraploid has 4n = 28 chromosomes.
- Step 2: Tetraploid gametes carry half of 28: 14 chromosomes.
- Step 3: Diploid gametes carry 7. A cross gives 14 + 7 = 21 chromosomes (triploid, 3n).
- Step 4: In meiosis, a triploid has three copies of each chromosome, which can't be split evenly into two cells, so most gametes get unbalanced sets and don't work. The hybrid is sterile.
- Step 5: This is a postzygotic barrier (reduced hybrid fertility), so the tetraploid is reproductively isolated from its parent species right away, without any geographic barrier: sympatric speciation.
Answer: Tetraploid gametes have 14 chromosomes; hybrids have 21 (triploid) and are usually sterile because three sets can't pair and split evenly in meiosis, isolating the tetraploid as a new species.
- Example 2
Classifying barriers (common mix-up)
Classify each barrier as prezygotic or postzygotic and name its type: (a) Two frog species in the same pond breed in different months. (b) Two bird species have different mating songs, and females ignore the other species' song. (c) Two sunflower species produce hybrids that are healthy but produce very few seeds.
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- Step 1: (a) They never mate because their breeding seasons don't overlap. No zygote forms, so it's prezygotic: temporal isolation.
- Step 2: (b) Mating doesn't happen because of courtship differences: prezygotic, behavioral isolation.
- Step 3: (c) Hybrids form (a zygote did form) but have low fertility: postzygotic, reduced hybrid fertility.
- Step 4: Key test: did a zygote form? If not, it's prezygotic.
Answer: (a) Prezygotic, temporal. (b) Prezygotic, behavioral. (c) Postzygotic, reduced hybrid fertility.
Common mistakes
- Calling any physical separation 'sympatric'. Allopatric means geographically separated; sympatric means living in the same area.
- Classifying sterile hybrids as a prezygotic barrier. If a hybrid exists, a zygote formed, so the barrier is postzygotic.
- Thinking punctuated equilibrium means evolution happens instantly. 'Rapid' here usually means thousands of years, which is fast compared with millions.
- Confusing divergent and convergent evolution. Divergent: related groups become different. Convergent: unrelated groups become similar.
On the exam
- Questions often describe two populations and ask whether they're separate species or what kind of barrier keeps them apart. Name the barrier type and say whether it acts before or after the zygote forms.
- Be ready to predict what happens to two populations if gene flow stops (they diverge) or resumes (they become more similar).
Connected topics
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Check yourself
4 questions on 7.10 Speciation. Pick an answer to see if you got it, and why.
Sharks (fish) and dolphins (mammals) both have streamlined bodies and fins, but their most recent common ancestor did not. Which of the following best explains this similarity?
Two closely related frog species live in the same ponds. One breeds in early spring and the other in late spring, so they never interbreed. This is an example of
In a plant population, an error in cell division produces an individual with four sets of chromosomes (4n). It can self-fertilize but cannot produce fertile offspring with the normal diploid (2n) plants around it. This is an example of
In a series of rock layers, a snail species shows almost no change for millions of years, then is replaced in a relatively thin layer by several new species that also stay unchanged for millions of years. Which model of evolution does this pattern best fit?
0 of 4 answered