Speciation
Introduced| English |
|---|
| reproductive isolation/rɪprəˈdʌktɪv ˌaɪsəˈleɪʃn/ |
| species/ˈspiːsiːz/ |
| allopatric/ˌæləˈpætrɪk/ |
| prezygotic isolation/prɪzɪˈɡɒtɪk ˌaɪsəˈleɪʃn/ |
| postzygotic isolation/ˌpəʊstzɪˈɡɒtɪk ˌaɪsəˈleɪʃn/ |
| sympatric speciation/ˈsɪmpətrɪk ˌspiːʃɪˈeɪʃn/ |
| divergent evolution/daɪˈvɜːdʒənt ɪvəˈluːʃn/ |
| adaptive radiation/əˈdæptɪv ˌreɪdɪˈeɪʃn/ |
| convergent evolution/kənˈvɜːdʒənt ɪvəˈluːʃn/ |
A barrier can separate populations without a wall
- Two plant groups grow in the same field, yet chromosome differences prevent fertile crosses. Conversely, a river can separate populations that could still interbreed if reunited.
- Speciation depends on reproductive isolation 生殖隔离 and reduced gene flow. Identify the mechanism of isolation as well as the geographic setting.
What a species 物种 is
- A species is a group that can interbreed and produce fertile offspring.
- All members share one gene pool.
- Two organisms that cannot interbreed belong to different species.
- So the key to a new species is breaking that ability to interbreed.
Two organisms belong to the same species if they can…
A species is a group that can interbreed and produce fertile offspring together.
Reproductive isolation
- For one species to become two, the groups must stop interbreeding.
- This is called reproductive isolation.
- Once isolated, the two gene pools no longer mix.
- Each is then free to change in its own direction.
How a new species forms
Step through speciation - one population is split, the halves diverge, and eventually they can no longer interbreed.
What must happen for one species to split into two?
Speciation needs reproductive isolation — the groups can no longer interbreed, so their gene pools split for good.
Once separated, the two groups can accumulate different mutations and changes.
Separated gene pools diverge through different mutations, selection, and chance until they cannot interbreed.
Allopatric 异域 speciation
- A common route is allopatric speciation: a geographic barrier splits a population.
- A river, mountain range, or ocean divides the group in two.
- Over generations, different mutations and selection make the halves diverge.
- Eventually they differ so much they can no longer interbreed — two species.
Speciation caused by a geographic barrier splitting a population is called ____ speciation.
Allopatric speciation happens when a physical barrier — like a river — separates a population.
Select all true statements about speciation.
Looking different is not enough — the test is whether they can interbreed. The other three are correct.
Barriers can act before or after fertilization
- Prezygotic isolation 合子前隔离 prevents fertilization: populations may breed at different times, use different courtship signals or have incompatible gametes. Postzygotic isolation 合子后隔离 acts after fertilization: a hybrid may fail to develop or may be sterile.
- Geographic separation can reduce gene flow but does not alone prove that speciation has occurred. The biological species concept concerns viable, fertile offspring; it is hard to apply to fossils and organisms that reproduce only asexually.
Two populations mate but their hybrid offspring are sterile. Which barrier is described?
Fertilization occurs; the barrier acts in the hybrid offspring.
New species can arise in the same place
- Sympatric speciation 同域物种形成 occurs with geographic overlap. In plants, a change in chromosome-set number can create reproductive isolation; changes in host use and mating preference can also reduce gene flow.
- Worked comparison: a river separates two populations, an allopatric setting. Two plant groups share a field but different chromosome numbers prevent fertile crosses, a sympatric setting. Identify the barrier, not just the map.
Two plant groups occupy the same field but chromosome-number differences prevent fertile crosses. Which setting fits?
They overlap geographically while gene flow is blocked.
Compare evolutionary patterns
- Divergent evolution 趋异进化 produces different adaptations from a shared ancestry. In adaptive radiation 适应辐射, one lineage diversifies into several ecological roles. Convergent evolution 趋同进化 produces similar adaptations in independently evolving lineages under similar pressures.
- Gradualism describes accumulated change over long periods. Punctuated equilibrium describes long periods of relative stasis interrupted by comparatively rapid change. Neither requires an individual to transform into a new species overnight.
Similar adaptations in independently evolving lineages always show recent shared ancestry.
Similar selective pressures can produce convergent adaptations.
A lineage shows long intervals of little change separated by relatively rapid change. Which model fits?
The model combines stasis with comparatively rapid evolutionary change.
Looking different is not enough to make a new species. Two groups become separate species only when they are reproductively isolated — unable to interbreed and produce fertile offspring. Colour or size differences alone do not count.
A river makes two squirrels:
- Long ago, the Grand Canyon split one squirrel population in two.
- The two sides could no longer meet or interbreed.
- Over thousands of years each side changed separately — today they are two distinct species.
Speciation is one species splitting into two. It requires reproductive isolation — the groups can no longer interbreed, so their gene pools diverge. In allopatric speciation, a geographic barrier splits a population; the separated halves change until they become distinct species.