Allopatric Speciation – Geographic Isolation and Reproductive Isolation
BiologyEvolutionAges 17–18
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Sign in to playA population of mice is split in two by a geographic barrier (a river, a mountain range or the sea). The two sides have different soil colours, so natural selection pushes fur colour in opposite directions, alongside mutation and genetic drift. Track mean fur colour, genetic distance and the share of pairs still able to interbreed over the generations, then remove the barrier to test whether the populations still interbreed or have become two species; a control line shows what happens with no barrier.
Lesson: Allopatric speciation: geographic isolation, natural selection, genetic drift and reproductive isolation
What it shows
New species often form when a population is split by a geographic barrier. Gene flow stops, so each part evolves on its own: natural selection adapts it to local conditions, while mutation and genetic drift add random differences. In this model mice on dark soil evolve dark fur and mice on pale sand evolve pale fur. Genetic distance grows, and once it passes a threshold the two groups can no longer produce offspring, even if they meet again. That is reproductive isolation, the point at which biologists count them as separate species. A control population without a barrier shows how gene flow holds a species together.
How to use
Choose River from the barrier menu and press Run. Watch the red genetic distance line climb towards the dashed threshold while the green line of pairs able to interbreed falls. When the status says two species, press Remove barrier and count the hybrids. Then press Reset, raise Environment difference or Selection, or choose No barrier, and compare how fast speciation happens. Tap a mouse to see its genes.
Parameters you can change
- Geographic barrier No barrier, River, Mountain range, Sea (rising water forms an island)
- Individuals per population 30–150
- Difference between the two environments 0–100 %
- Selection strength 0–10
- Mutation rate per locus per generation 1–10 ×10⁻⁴
- Genetic difference threshold for reproductive isolation 10–40 %
Questions to explore
- Why does the population not split into two species when there is no barrier, even though the environments differ?
- Does a larger difference between the environments make speciation faster or slower, and why?
- What happens to the two populations if the barrier is removed too early?
