When one group becomes two

Science · Genetics & Evolution · ages 19-21

Name ______________________ Date ____________

Which observation is a postzygotic barrier?
- Parents breed in different seasons.
- Parents use different courtship signals.
- Hybrids reach adulthood but are sterile.
Mating has occurred. Which barrier still acts before a hybrid zygote forms?
- Egg and sperm fail to fuse after mating.
- Hybrid embryos die during development.
- Adult hybrids cannot make fertile gametes.
Two populations remain distinct while producing hybrids in a zone that stays stable over many generations. What does this observation show?
- Reproductive isolation is already complete.
- Hybridization continues between distinct populations.
- Fusion has erased the differences between populations.
Inherited mate preferences vary. Matings within either population produce more fertile offspring than hybrid matings. What can selection favour?
- Preferences that increase hybrid matings.
- Preferences that favour mates from the other population.
- Preferences that favour mates from the same population.
Why can the biological species concept not be tested directly for extinct organisms known only from fossils?
- Their reproductive compatibility cannot be observed.
- Their age alone establishes reproductive isolation.
- Their shape reveals whether their offspring were fertile.
A new river separates rodents that cannot cross it. Gene flow between the two sides stops. What has begun?
- Complete speciation caused by the river alone.
- Geographic isolation that may allow later speciation.
- Sympatric speciation within one undivided population.
An organism reproduces only asexually. Can a test of interbreeding directly decide its species boundary?
- Yes; similar appearance establishes reproductive compatibility.
- No; an interbreeding test cannot assess asexual reproduction.
- Yes; rapid growth demonstrates the ability to interbreed.
Two populations mate freely. Fertile hybrids breed with both parental populations, and no barrier maintains their differences. Which outcome becomes more likely?
- Fusion as gene flow gradually reduces population differences.
- Reinforcement as mate preferences restrict interbreeding.
- Stability as reproductive barriers keep the populations distinct.

Answer key

For grown-ups. Fold this page away before handing over the rest.

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Hybrids reach adulthood but are sterile. · A sterile adult hybrid has developed after fertilization. Its inability to reproduce is a postzygotic barrier.
Egg and sperm fail to fuse after mating. · A zygote forms at fertilization. Failure of egg and sperm to fuse is prezygotic, even when mating has occurred.
Hybridization continues between distinct populations. · Continuing hybridization can coexist with distinct populations. Stability alone does not establish how strong the reproductive barriers are.
Preferences that favour mates from the same population. · Inherited preferences that reduce costly hybrid matings can leave more descendants. Their spread can strengthen prezygotic isolation through reinforcement.
Their reproductive compatibility cannot be observed. · Fossils do not allow direct breeding observations. Preserved features and geological context provide other evidence, with limits.
Geographic isolation that may allow later speciation. · The river creates geographic isolation and an opportunity for allopatric speciation. Separation alone does not establish two species.
No; an interbreeding test cannot assess asexual reproduction. · An interbreeding test cannot be applied directly to organisms that reproduce only asexually. Other species criteria are needed.
Fusion as gene flow gradually reduces population differences. · Continuing gene flow without a barrier maintaining population differences makes fusion more likely. Fertile hybrids alone would not justify this prediction.