Allopatric and sympatric speciation sound the same in my notes. What is the real difference?

Both end in the same place: two populations that can no longer interbreed to produce fertile offspring. The difference is what stopped them interbreeding in the first place, and it comes down to geography.

The words tell you, if you take them apart. Allo means other, patris means homeland: other homeland, so the populations are in different places. Sym means together: same homeland, same place. Learn the roots and you will never mix them up again under pressure.

Allopatric: separated by geography

A physical barrier splits one population. A river changes course, sea level rises and isolates an island, a mountain range forms, a glacier advances.

Gene flow between the two groups stops. Each is now under its own selection pressures, with its own mutations arising and its own genetic drift, and the allele frequencies diverge. Given enough time the two gene pools differ so much that even if the barrier disappears, members of the two groups can no longer produce fertile offspring. They are separate species.

This is the common route and the one most examples illustrate.

Sympatric: separated without moving

Harder, rarer, and more interesting, because the populations are living in the same place the whole time. Something other than geography has to stop them breeding with each other:

• Polyploidy, especially in plants. A chromosome-number error produces individuals that can breed with each other but not with the parent population. This creates a new species in a single generation, which is why it is the example most worth knowing.
• Behavioural isolation. Courtship rituals or mating calls diverge, so individuals stop recognising each other as potential mates.
• Temporal isolation. The two groups breed at different times of year, so they never meet in a breeding state.
• Ecological isolation. They specialise on different habitats or food sources within the same area and simply stop encountering one another.

Where the marks go

The question nearly always asks you to explain how, not to define. A definition scores one mark at best. The chain is what pays:

Barrier or isolating mechanism → gene flow between the populations stops → different selection pressures, mutation and drift act on each → allele frequencies diverge → eventually reproductive isolation → separate species.

That middle step is the one candidates skip. "They were separated so they became different species" misses every mark in between. Say the words "gene flow" explicitly — it is usually on the mark scheme, and it is the mechanism that makes the whole explanation work rather than a sequence of events.

And if a question gives you an unfamiliar organism, do not look for a remembered example. Identify whether the populations are physically separated or not. That single observation tells you which type it is.