The big idea: Different species rarely interbreed. Barriers before and at fertilization stop hybrids forming, and hybrids that do form are usually sterile, so the alleles of two species do not mix.
The points to remember
- Courtship is complex and unique to each species: a partner giving the wrong signal is rejected.
- Timing: different breeding seasons or times of day stop mating.
- Place: species living in different areas never meet.
- Anatomy: reproductive organs may not fit together.
- Gametes: fertilization is species-specific; the sperm's acrosome must match the egg's outer coat (the vitelline layer).
Remember it as: Wrong song, wrong season, wrong sperm.
Real example: birds of paradise: Male birds of paradise in New Guinea perform elaborate displays of feathers and dance. A female watches and mates only with a male whose display matches her own species.
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The big idea: An interspecific hybrid is the offspring of two different species. Most are sterile: their chromosomes cannot pair in meiosis.
The points to remember
- A hybrid forms when members of two different species cross-breed.
- Different species often have different chromosome numbers or structures.
- In the hybrid's meiosis the chromosomes cannot pair: they are not homologous.
- Meiosis fails, so the hybrid makes no working gametes: it is sterile.
- So even when a hybrid forms, the alleles of the two species do not pass on.
Know the words exactly:
Sterile, not dead: A mule is healthy and strong. It is sterile: it cannot produce offspring. Do not write that hybrids always die.
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How it is tested: Paper 1A: how hybrids are formed, or why they are sterile. Paper 2: describe the barriers in four marks, sometimes with polyploidy (1.8.11).
IB-style questionDescribe[4 marks]
Two species of tree frog live in the same pond. Describe the barriers that prevent the two species from hybridizing.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.
Say why the hybrid is sterile: 'The hybrid is sterile' earns one point; 'because its chromosomes cannot pair in meiosis' earns another.