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Polymorphism Polymorphism
wrote...
13 years ago
Why are balanced polymorphisms typically older than neutral ones?

How is horizontal gene transfer dealt with in reconstructing phylogenies?

Why do some recognized boundaries in geological column coincide with a mass extinction event?

Why is it difficult to determine if speciation has occurred parapatrically or sympatrically?


Thanks so much for any help you can offer.
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wrote...
Valued Member
Educator
13 years ago
Why are balanced polymorphisms typically older than neutral ones?

Balanced occurs when different forms coexist within a given population in equilibrium, that is, at a constant proportion from generation to generation. From the point of view of evolutionary biology, a polymorphism that persists over many generations is usually maintained because no one form has an overall advantage or disadvantage over the others in terms of natural selection; in other words, all the forms possess, for one reason or another, survival value for the species.

How is horizontal gene transfer dealt with in reconstructing phylogenies?

This will help:

http://lacim.uqam.ca/~chauve/Enseignement/BIF7001/H05/PHYLOGENIE/BIF7001-Phylo-COMicrobio6.pdf

Why do some recognized boundaries in geological column coincide with a mass extinction event?

These will help:

http://mypage.direct.ca/w/writer/extinct.html
http://www.earth4567.com/talks/time/timeex2.pdf

Polymorphism Author
wrote...
13 years ago
Thanks! The link helped alot.  Do you have any idea about the last question?
wrote...
Valued Member
Educator
13 years ago
I found this online for you Polymorphism, perhaps it could help.

Because it's difficult to prove that reproductive isolation of the sort necessary to lead to speciation is possible in the absence of geographic isolation. The late Dr. Thomas K. Wood researched the Membracid species complex of Enchenopa binotata for years as a model of the Island Theory of Bio-geography method of sympatric speciation without a definitive answer.

The idea is that an organism tied to one host (plant) is forced off that host by a catastrophic event. They become linked to different novel host plants and through association with the life cycle of the novel hosts, their life (reproductive) cycles are separated enough that speciation is possible. Though the creatures on the new host plants share the same geographic area, they are separated by the host plant they mate on, and possible separated developmentally if their new host plants affect the organism's development such that they do not mature at the same rate.

Dr. Wood and his colleagues found 9 different cryptic species of Enchenopa, differentiated by pronotal shape of the adults and by nymphal morphology. Each of the nine species specialized in a different host plant. Each host plant budded at different times in the spring and the insect's egg-hatch was linked to budding. Statistical differences were proven in some, but not all, of the species gonadal development. IE, it was proven that there is some separation of mating season based on host plants, but not proven that the difference was enough to provide reproductive isolation in itself between all nine species.

Other research showed that Enchenopa experimentally forced onto novel host plants will suffer high initial mortality, but survivors will show fidelity to the novel hosts within 2-3 generations and refuse to mate/lay eggs on the original host plant. Additionally, there were no differences in chromosome number or C-banding patterns between the species. Enough data to show that the possibility of sympatric speciation in Enchenopa could neither be conclusively proven or discounted.

It's even harder to show evidence of sympatric speciation under other circumstances.
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