Download Defining Speciation

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
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Disaster strikes: A hurricane washes the bananas and the immature fruit flies they
bunches of rotting bananas, cheerfully laying their eggs in the mushy fruit...
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.
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The populations diverge: Ecological conditions are slightly different on the island,
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So we meet again: When another storm reintroduces the island flies to the
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.
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.
http://evolution.berkeley.edu/evosite/evo101/VBDefiningSpeciation.shtml
1. What is the organism?
2. What are three traits that the organism has? (wings, body color, eye color,
mating dance,)
3. What are the variations for those three traits? (for example: thick stripes vs.
thin stripes)
4. What was the geographical isolating event for the fruit flies?
5. Draw and explain how this isolating mechanism will separate the
populations, and how the 2 environments are different from each other.
What can they not do anymore?
6. 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?
7. Draw what the future organisms of the separated populations would look
like in the future if the environmental pressures continued.
8. 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?
9. 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.
10. 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 lots of empty
niches to be filled. 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?