Understanding Evolution: your one-stop source for information on evolution
Resource library Teaching materials Evolution 101 Support this project







The big issues

en Espanol en Español     print print

Defining speciation

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.


    Download the graphics on this page from the Image library.

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.

page 3 of 7
previous | next  >

More details
Read about speciation in plants and how speciation factored into the history of evolutionary thought. Or explore different modes of speciation, including:

Evo examples
Learn more about speciation:

Teach this
Teach your students about speciation:

Find additional lessons, activities, videos, and articles that focus on speciation.