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Speciation example (2 of 5)

Image caption:
The scene: a population of wild fruit flies is minding its own business on several bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit.

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Speciation example (1 of 5)
The branching points on this partial Drosophila phylogeny represent speciation events that happened in the past.

Speciation example (3 of 5)
Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea. The banana bunch washes up on an island off the coast of the mainland. The fruit flies mature and emerge onto the lonely island. The two portions of the population, mainland and island, are now too far apart for gene flow to unite them. At this point, speciation has not occurred — mainland and island fruit flies can mate and produce healthy offspring.

Speciation example (4 of 5)
The populations diverge: Ecological conditions are slightly different on the island, and the island population evolves under different selective pressures and experiences different random events than the mainland population does. Morphology, food preferences, and courtship displays change over the course of many generations of natural selection.

Speciation example (5 of 5)
So we meet again: When another storm reintroduces the island flies to the mainland, they will not readily mate with the mainland flies since they've evolved different courtship behaviors. The few that do mate with the mainland flies, produce inviable eggs because of other genetic differences between the two populations. The lineage has split now that genes cannot flow between the populations.