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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:
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.
Causes of Speciation
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Spanish translation of Understanding Evolution For Teachers from the Spanish Society of Evolutionary Biology.