AP BiologyUnit 7: Natural Selection7.10
AP Biology · Unit 7 · Topic 7.10

Speciation

Play it as a 3-minute lesson → short cards, one idea at a time

The lesson3 min read

Find the zygote. Barriers that stop it forming are prezygotic; barriers acting on hybrids are postzygotic.

Speciation is one lineage splitting into two that no longer interbreed. Once gene flow stops, each changes on its own. On the exam, name the barrier and say whether it acts before or after a zygote forms.

Key terms

  • Biological species concept — a species is a group whose members can interbreed in nature and have fertile offspring, but not with outsiders.
  • Prezygotic barrier — blocks mating or fertilization, so no hybrid zygote ever forms.
  • Postzygotic barrier — acts after a hybrid zygote exists: the hybrid dies, is sterile, or its own offspring fail.
  • Allopatric speciation — a physical barrier, like a mountain range or open water, separates populations, which then diverge.
  • Sympatric speciation — a new species arises inside the parent species' range, with no geographic barrier at all.
  • Polyploidy — extra full sets of chromosomes, often from a cell-division error. In plants it can make a new species in one generation.

Barriers, first to last

  1. They rarely meet. They breed in different spots, even along one stream (habitat isolation), or at different times (temporal isolation).
  2. They meet but don't mate. The courtship song, dance, or scent doesn't match (behavioral isolation). Side-by-side flowers stay apart when each has its own pollinator.
  3. They try, but fertilization fails. Parts don't fit (mechanical isolation), or sperm and egg can't fuse (gametic isolation). Pollen that fails on a foreign stigma stops here too.
  4. A hybrid zygote forms, so barriers now are postzygotic. The hybrid dies before breeding (reduced hybrid viability) or matures sterile, like a mule (reduced hybrid fertility).
  5. Hybrid breakdown comes last: first-generation hybrids reproduce, but the next generation fails or is sterile.

Why it matters

Speciation is where the unit's forces pay off: selection, drift, and mutation pull separated gene pools apart. When two diverging populations meet again in a hybrid zone, hybrid fitness decides the ending. Unfit hybrids → selection favors mating within one's own population, so prezygotic barriers strengthen: reinforcement. Hybrids as fit as parents → the two fuse back into one. Hybrids fit only in the zone → the zone stays a stable, narrow band.

Exam traps

TrapActually
If hand crosses give fertile hybrids, the two are one species.Not if a prezygotic barrier stops them crossing in the wild. The species test is interbreeding in nature.
Courtship displays are mentioned, so it's behavioral isolation.Ask what actually stops mating. If they rarely meet, it's habitat or timing. Behavior only acts once they meet.
Polyploids must move away from their parents to become a new species.No move needed. A tetraploid crossed with a diploid gives triploids, which can't split chromosomes evenly in meiosis. Isolation is instant.
Speciation always takes millions of slow, steady years.Pace varies. In fossils, some lineages change gradually; others show long stasis broken by quick bursts, a pattern called punctuated equilibrium.
Worked example

Two frog species from the same region are paired in lab tanks, 200 cross-species pairs in all. 92% of pairs mate, 95% of those clutches are fertilized, and 90% of the hybrid tadpoles reach adulthood. When the adult hybrids are bred, 0% produce offspring. Which barrier do these data show, and is it prezygotic or postzygotic?

  1. Walk the stages in order and look for the crash.
  2. Mating (92%) and fertilization (95%) mostly work, so no strong prezygotic barrier shows up here.
  3. Hybrids survive to adulthood (90%), so hybrid viability is fine. The crash comes at hybrid reproduction: 0%.

Reduced hybrid fertility, a postzygotic barrier. Hybrids form and grow up, but they're sterile.

A hybrid zygote had to exist before the crash, so the barrier is postzygotic. The near-miss is hybrid breakdown. That needs first-generation hybrids that do reproduce, with the next generation failing. Here the first generation itself is sterile. And tanks force the frogs together, so any habitat or timing barrier in the wild can't show up in these data.

Now try it on real exam-style questions

Ten questions on Speciation, each with a ten-second explanation. Free, no account.

Three questions to check yourself

Question 1 · 7.10 Speciation

Which reproductive barrier acts before fertilization takes place?

  1. A Gametic isolation
  2. B Reduced hybrid viability
  3. C Hybrid breakdown
  4. D Reduced hybrid fertility
Show the answer

A — Gametic isolation is prezygotic: the egg and sperm of two species fail to fuse, so no zygote ever forms. Reduced hybrid fertility sounds like a fertilization problem, but it describes a hybrid that already exists and is sterile, which makes it postzygotic.

Question 2 · 7.10 Speciation

Under the biological species concept, what makes two organisms members of the same species?

  1. A They share most of their physical traits
  2. B They are descended from a common ancestor
  3. C They can interbreed and produce fertile offspring
  4. D They live in the same habitat during the same season
Show the answer

C — The biological species concept defines a species by reproduction: members can interbreed in nature and produce viable, fertile offspring. Looking alike is tempting, but very different-looking organisms can be one species and near-identical ones can be two.

Question 3 · 7.10 Speciation

In plants, which event can produce a new species in a single generation?

  1. A A point mutation in one flower-color gene
  2. B Polyploidy from an error in cell division
  3. C Geographic isolation by a newly formed river
  4. D Genetic drift in a small, isolated population
Show the answer

B — When an error in cell division doubles a plant's chromosome number, the polyploid offspring can breed with one another but form sterile or inviable offspring with the parent plants, so they are reproductively isolated at once. Geographic isolation also leads to new species, but only after many generations of divergence.