Speciation at Higher Level: The same statement as SL, with different real cases: Galapagos tortoises, the Isthmus of Panama and New Zealand. At HL, the next statements go further: the kinds of reproductive isolation, and why hotspots hold so many species.
Practise this as you read
- Explain both sides of plate movement: land pulled apart and land brought together.
- Explain why an island has many endemic species, not just that it is isolated.
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Split, change, cannot mix: Speciation happens when part of a species is cut off and changes in its own way, until the two groups can no longer breed together.
The points to remember
- A species is a group that can interbreed and produce fertile offspring.
- Speciation starts when a population is isolated: no more interbreeding, no gene flow.
- Each part meets different conditions and selection pressures, and gets different mutations.
- Each adapts in its own way, so the parts diverge over many generations.
- In time they cannot interbreed to produce fertile offspring: a new species.
Remember it as: Isolation + different conditions + time = new species.
Real example: giant tortoises reached the Galapagos from South America. On each island they met different food: on wet islands, low plants, and domed shells evolved; on dry islands, tall cactus, and saddle-shaped shells that let the neck stretch up evolved. There are now over ten species.
Use the full test: Say 'cannot interbreed to produce fertile offspring'. A horse and a donkey can mate, but their young, the mule, is infertile, so they are separate species.
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The slow movement of Earth's tectonic plates is one of the main ways populations become isolated.
The points to remember
- Plates split continents, open oceans, raise mountains and build volcanic islands.
- These barriers divide one population into isolated groups: no gene flow.
- Drifting plates carry land into new climates, so each group meets different selection pressures.
- Each group adapts differently and gets different mutations: divergent evolution, then speciation.
- New mountains and islands give new habitats at different heights: more niches to fill.
- But a population carried into harsher conditions may go extinct, lowering biodiversity.
Join
- About 3 million years ago plate movement and volcanoes joined North and South America.
Meet
- Land animals crossed: armadillos and opossums went north; cats, deer and bears went south.
Compete
- Newcomers competed with native species; many South American mammals went extinct.
Split
- The same land cut the sea in two, isolating Pacific and Caribbean marine life.
Species
- Sea animals on each side diverged into new species.
Plates pulling land apart
- Populations isolated
- Divergent evolution
- Speciation: biodiversity rises
Plates bringing land together
- A land bridge: populations meet
- Competition: some species go extinct
- Hybrids can merge species, or form new ones
Keep the plates in every point: In a tectonics question, link each step to the moving plates. A general account of natural selection with no link to plate movement does not count.
The fossil record shows the link between moving continents and the number of species.
The points to remember
- Give a time with units: 'about 250 million years ago'; a bare number does not count.
- Positive correlation: the more continents, the greater the species diversity.
- Reason: separating continents isolates populations, which leads to speciation.
- Reason: continents move into different climates, so different species evolve.
- Reason: plate movement makes new habitats, such as mountain ranges and islands.
Real example: the Atlantic Ocean opened as Pangaea split, and it still widens by about 2.5 cm a year. The dip at X was a mass extinction, when an asteroid hit Earth 66 million years ago.
Units, or no mark: When you read a time off the graph, always write 'million years ago'. '250' on its own is marked wrong.
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Isolated islands are full of endemic species. Speciation explains why.
The points to remember
- An endemic species is found nowhere else.
- A remote island is too far for gene exchange with the mainland: few new arrivals.
- Isolated for a long time, species evolve independently: speciation.
- Varied altitude, climate and habitats give many niches, so many species.
- Adaptive radiation fills empty niches, e.g. with no ground predators.
- Limited resources push species to specialise; a stable climate gives them time.
Real example: New Zealand broke away from the southern supercontinent about 80 million years ago. With no native land mammals except bats, birds filled their niches: the kiwi and the kakapo, a flightless parrot, evolved to live on the ground.
Explain the isolation: 'The island is isolated' alone is not enough: say what isolation does (no gene exchange, so species evolve independently). 'Adapted to no predators' alone, or one habitat only such as rainforest, is not enough either.
How this comes up: Paper 2, Section B (b): explain how plate movement has influenced biodiversity and evolution [7].
About 3 million years ago the Isthmus of Panama joined North and South America.
Explain how the movement of tectonic plates has influenced biodiversity and evolution.
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