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:
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The scene: a population of wild fruit flies minding its own business on several bunches of rotting
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Disaster strikes: A hurricane washes the bananas and the immature fruit flies they contain out to
bananas, cheerfully laying their eggs in the mushy fruit...
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.
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The populations diverge: Ecological conditions are slightly different on the island, and the island
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So we meet again: When another storm reintroduces the island flies to the mainland, they will not
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.
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.
http://evolution.berkeley.edu/evosite/evo101/VBDefiningSpeciation.shtml
1. Choose a population of organisms.
2. Choose one trait that the organism has. (body color, eye color, mating dance,)
3. What are variations for that trait? (for example: thick stripes vs. thin stripes)
4. Draw below how the population could get separated. Put different variations of the trait
you selected for on each side of the geographic event. What is different about the two
environments?
5. Explain how natural selection is pressuring the variations on the separated
populations. Be specific about how the it will favor (or not favor) the some of the traits
you chose. How will the traits be beneficial in the new environment in which the
organisms find themselves?
6. Draw what the future organisms of the separated populations would look like in the
future if the environmental pressures continued.
7. How would you know that these individuals were now separate species? Hint: think
about the definition of species. What has happened to make them separate species?
8. This example shows how one species changed over time into two separate species.
This is called divergent evolution; two species share a common ancestor, but have
become more different over time. Can you give another example in nature that
shows how 2 species have diverged from a common ancestor. Explain what
happened using evidence such as homologous bone structures.
9. Adaptive radiation is a type of divergent evolution where one species is changed
into many species in a quick period of time. This usually happens on islands where
there are many different environments and with little competition. The Galapagos
Finches illustrate this. Can you explain how this might have happened?
11. Convergent evolution is when unrelated organisms evolve to have similar phenotypes due
to living in the same environment or having a similar niche. They have analogous body
structures. Examples include all flying organisms, such as insects and birds; both use wings to
catch food and escape predators, but these wings are not built the same way. Can you
describe another example of convergent evolution, and explain why it occurred?