Speciation is a lineage-splitting event that produces two or more separate species. Imagine that you are
looking at a tip of the tree of life that constitutes a species of fruit fly. Move down the phylogeny to
where your fruit fly twig is connected to the rest of the tree. That branching point, and every other
branching point on the tree, is a speciation event. At that point genetic changes resulted in two separate
fruit fly lineages, where previously there had just been one lineage. But why and how did it happen?
The branching points on this partial Drosophila phylogeny represent long past speciation events. Here is
one scenario that exemplifies how speciation can happen:
- The scene: a population of wild fruit flies minding its own business on several bunches of rotting
bananas, cheerfully laying their eggs in the mushy fruit...
- Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to sea.
The banana bunch eventually washes up on an island off the coast of the mainland. The fruit flies mature
and emerge from their slimy nursery 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 any
fruit flies that got back to the mainland could mate and produce healthy offspring with the mainland flies.
- 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.
- 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.
This is a simplified model of speciation by geographic isolation, but it gives an idea of some of the processes
that might be at work in speciation. In most real-life cases, we can only put together part of the story from the
available evidence. However, the evidence that this sort of process does happen is strong.