The big idea: Speciation happens gradually, not in one step.
The points to remember
- Speciation splits one species into two or more.
- It starts when populations stop interbreeding, for example when a barrier divides them.
- Each population then changes in its traits, a little every generation.
- Over time they become more and more different, until they can no longer interbreed.
- There is no single moment when one species becomes two.
Remember it as: Split, then drift apart, slowly.
Real example: snapping shrimps and the Isthmus of Panama: About 3 million years ago the land bridge between the Americas closed and split the snapping shrimps of the Caribbean from those of the Pacific. Pairs that were once one population now look alike but rarely mate.
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The big idea: Because the change is gradual, calling two populations one species or two can be an arbitrary decision.
The points to remember
- A population is a group of one species living in one area and interbreeding.
- Two populations part-way through speciation may still sometimes interbreed.
- Different biologists can draw the line in different places.
- So the decision whether they are one species or two can be arbitrary.
Signs of one species
- They meet and mate freely
- Their offspring are fertile
- Their traits overlap
Signs of two species
- They never mate, even when together
- Hybrids are rare or sterile
- Their traits no longer overlap
Real example: one giraffe species, or four?: Giraffes were long treated as one species. A 2016 study of their DNA suggested four, and biologists still disagree about where to draw the lines.
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How it is tested: Paper 1B shows two forms or populations and asks which idea is better supported. Either answer can score, if the reason is about the evidence shown.
IB-style questionExplain[1 mark]
Grove snails from two valleys separated by a river were collected. Snails from the left bank are yellow; snails from the right bank are brown with dark bands.
Explain, with a reason, whether the snails are more likely to be one interbreeding population or two separate populations. [1]
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Different looks do not prove two species: Colour can vary within one population. Say what the evidence suggests, not that it proves it.