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Diversifying selection

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Diversifying selection

Postby Juanita1 » Mon Jul 06, 2009 9:50 pm

Hi there

I am slightly confused about diversifying selection and its effect on variation. I would have thought that diversifying selection since it leads to diversity would increase variation. However, It also leads to the elimination of intermediate traits thereby reducing variation.

So I am unsure what the overal effect of diversifying selection is-increase in variation or decrease? For example, in the galapagos finches, the diversifying selection within a single population-would it lead to an increase or decrease in variation?
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Postby kolean » Tue Jul 07, 2009 12:38 am

http://en.wikipedia.org/wiki/Disruptive_selection

The explanation in this example of white and black rabbits seems to support variation only in the two extremes, and not in the middle (though blacker gray rabbits might still be able to stay alive long enough to procreate considerably - perhaps even faster ;-) - and light gray white rabbits the same).

Overall it does create at least two variations (extreme ends) and not just one (all black in the example). The possibility for variation is still there for a black rabbit to mate with a white rabbit, especially if acid rain makes all the stones turn into gray stones.

With the finches, it was small beak vs. big beak depending on the type of seed that grew on which of the islands. Even though the middle variation dies out, the possibility is always there for variations if you have the two extremes for the genetic pool. So that if a new plant with middle size seeds is introduced to both locations, then the possibility of variation from interbreeding could still support variation.
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Re: Diversifying selection

Postby Juanita1 » Tue Jul 07, 2009 11:42 am

Thabnks for the reply.

So are u saying that there is more variation when a population is not diversifying into two sub populations and less when they are on the verge of diversification?

I know you said the possibility is still there for variation to occur if their is interbreeding between the two types eg big beak and small beak but that would mean there is more variation when the intermediate forms are around than when diversification is occuring and the intermediates dieing out?

I look forward to your reply.
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Re: Diversifying selection

Postby kolean » Tue Jul 07, 2009 9:46 pm

There is genetic potential for variation when you have the two extreme genetic pools available. If you do not have the two extremes, then the spectrum of variation may die out.

Say that all the big beaks live, and all the small beaks died out. You could only have variation now from the big beak birds which will evolve in a way that may bring variation, but the spectrum will now be limited.
Variation now only has one source, or one genetic pool to evolve from, and only if there is environmental pressure for mutation.

So presently, as the intermediates are dying out, their variation is also dying out. But as long as the two extreme gene pools are kept viable, variation has a promise of returning when conditions are conducive to the intermediates.
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Re: Diversifying selection

Postby Juanita1 » Tue Jul 07, 2009 11:01 pm

OK so if there is a population on one side of the island with diversifying selection acting with just big beaks and small beaks, intermediates dieing out while on the other side of the island, there is a population with big beaks, small beaks and intermediates and no diversifiing selection acting. Which population has the greater variation?
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Re: Diversifying selection

Postby kolean » Wed Jul 08, 2009 3:23 am

The side of the island that already has the intermediates and the two extremes.

In the Galapagos, there is a big beak finch on one of the islands eating the big nuts that only grow on that island. And on another island, there is only small beak finches because there is only small seeds on this island for them to eat. Thus together, there is only 2 variations. There can be no intermediates and no variation between the two finches because of their location and their available food.

So any spot that has intermediates would have more variation than just those two. As long as there are already an established intermediates.
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Re: Diversifying selection

Postby Juanita1 » Wed Jul 08, 2009 10:00 pm

Ok I see what u r saying. But I thought diversity increased variation, is this not true since u say that greatest variation would be where the two extreme phenotypes and the intermediates live but there is no diversification occuring in that population.
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Re: Diversifying selection

Postby AFJ » Fri Jul 10, 2009 2:15 am

Say that all the big beaks live, and all the small beaks died out. You could only have variation now from the big beak birds which will evolve in a way that may bring variation, but the spectrum will now be limited. Variation now only has one source, or one genetic pool to evolve from, and only if there is environmental pressure for mutation.

Kolean, you sound very studied, but there are two points here.

1)You forgot about recessive traits in the big beak birds--just because the small die out does not guarantee the trait will never reappear. Theoretically, it would have to be a time of unchanging direction in environment and resources before the recessive gene would be deleted, and I could not tell you how long.

2)There is extended research (over 30 years or so) that has observed this phenomenon. The environment was the trigger. This is something I have read and would have to search in order to site. I will leave the details because I don't have the material with me--but basically in a less harsh environment ("normal mild") both large and small beaks were alive. During drought the small disappeared and later reappeared after normal conditions returned.

3) This and other studies (bacterial ecological specialization) could suggest that selection is directionless--what is good in one environment is not beneficial, sometimes even detrimental in another.
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Postby kolean » Fri Jul 10, 2009 4:34 am

Are you sure you are not confusing diversfying selection with diversity? The term diversfying does not mean diversity in the context of this terminology.
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Re: Diversifying selection

Postby AFJ » Sat Jul 11, 2009 5:43 am

It's my understanding that diversifying selection causes speciation within a non-isolated varied population. In the example of black and white rabbits more than likely you will not have totally gray, but rabbits that are individually black and white in the intermediates who will blend among black and white rocks.

Speciation and it's theories sound nice, but I am from a farming community and think some of these scientists need to go to the country. I have seen domesticated and wild rabbits, and their instinct is to hide and sit in brush or tall grass, not rocks. Most wild rabbits are gray. They can traverse green grass, a golden harvested corn field, black dirt, or white snow. Their defense is their speed, agility and instinct to hide in vegetation , not their color.

[Diversifying] Selection is believed to be the driving force behind sympatric speciation.Wikipedia


NS is a fact, but it is not so much a driving force or mechanism as it a natural result. It does not cause the environment and resources but conversely it is caused by them, and it's direction can be changed by them. NS only acts on what is already there--it can "purify" a trait in a population, causing it to be predominant, or delete traits (not always permanently). A gene must be there first before it can be selected for.
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Re: Diversifying selection

Postby AstusAleator » Sat Jul 11, 2009 6:41 pm

it's "divergent selection"
What did the parasitic Candiru fish say when it finally found a host? - - "Urethra!!"
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Re: Diversifying selection

Postby AFJ » Sat Jul 11, 2009 11:03 pm

Mr. Astus, I haven't studied this alot, but I was just reading on it. I may be wrong, but if you look up "diversifying" or "disruptive selection" and "divergent selection" or "divergent evolution" it seems very close, but perhaps they are a bit different. Perhaps you can clear this up. One thing is sure--they are both non geographical speciation--that is they occur within a population that is together--sympatric speciation.

From what I studied divergent selection is sympatric, but it has to do with sexual selection by sexual preference of certain traits. In one study more female fruit flies preferred certain cuticular hydrocarboned males, causing more predominance of this trait in the novel populations (from a common ancestor).

Diversifying selection is sympatric also, but seems to be caused more by the environment. The environment tends to make selection for polarized extreme traits and deletes the intermediates. Example of black and white rabbits living among black and white rocks--intermediates are rooted out by selection because of a survival disadvantage.

Disclaimer: In my reading of the fruit fly study there was a self admitted lack of actual research in this area, and that most of the theories on speciation are theories. Most modern creationists do not have a problem with small speciation--which would be what Blythe called variations. But evidence for new orders or families of organisms is lacking. Assumed time is always the evolutionist defense though.
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