Bottleneck Effect - Randomness or Predisposition?

[Oblivion]

I have always questioned the randomness of the bottleneck effect. Most textbooks state that the remaining alleles following a population reduction will generally be non-representative of the original population, which I agree with. However, they then go on to underline the fact that the alleles of the surviving population are a completely random selection.

There is one concept that makes me question this:

Consider a volcano eruption that wipes out the majority of a population of rabbits, but leaves a few. The few that survived lived why? Is it possible that some members of the population had an inherited genotype from their surviving predecessors that said hey man, build your home on the top of this hill so the lava won't hit you when that baby erupts? Could some members of the population have a genetic predisposition to surviving the disaster? And if so, wouldn't that mean that the bottleneck effect is only partially random?

Give me something to chew on...

 

[Mentat]

Originally posted by Oblivion
I have always questioned the randomness of the bottleneck effect. Most textbooks state that the remaining alleles following a population reduction will generally be non-representative of the original population, which I agree with. However, they then go on to underline the fact that the alleles of the surviving population are a completely random selection.

There is one concept that makes me question this:

Consider a volcano eruption that wipes out the majority of a population of rabbits, but leaves a few. The few that survived lived why? Is it possible that some members of the population had an inherited genotype from their surviving predecessors that said hey man, build your home on the top of this hill so the lava won't hit you when that baby erupts? Could some members of the population have a genetic predisposition to surviving the disaster? And if so, wouldn't that mean that the bottleneck effect is only partially random?

Give me something to chew on...

Well, if the rabbits had indeed evolved some genetic predisposition toward whatever quality eventually saved them, wouldn't this evolution (of the aforementioned predisposition) have come about randomly? In this case, it can still be said that the survival of those rabbits was "pure luck" since they didn't have to evolve that tendency.

 

[iansmith]

Originally posted by Oblivion
However, they then go on to underline the fact that the alleles of the surviving population are a completely random selection.

This is true because your example is natural selection rather than the bottle neck effect. Text book are not that great for excellent explanation of theory. It should also stated that bottle neck do not select survivor, they survive by chance. For example, a deer pack of 10000 individuals is crossing a river, suddently the river over flow and kills 9902 individuals. 108 survive by chance. In ooposite, if a virus is affecting the same pack and 99% of the individuals dies. Then 100 individuals survive and these were selected for their resistance to the virus.

 

[Oblivion]

deer pack of 10000 individuals is crossing a river, suddently the river over flow and kills 27 individuals. 8 survive by chance.

Alright, so 8 deer survived by chance. But the fact that they survived could be due to some unknown factor, correct? Is there such thing as pure chance? And is the fact that those 8 survived totally random?

 

[iansmith]

For the scenario there migth be many scenario:

The 8 were either the leaders of the pack (A), the rear of the pack (B), or were inbetween the leader and the slowest (C). This offers many combination events. You could have only A, only B, only C, A and B, A and C, B and C or ABC. The leading group migth be homogenous such as only strong individuals but could also be heterologous such as a mix of strong and weaker individuals. and so on. In bottle neck effect there is a large combination events possible whereas in natural selection there is only 2 opinion, you have it or you don't.

 

[Monique]

It totally depends what the bottleneck was (forgive me, I haven't read the previous posts yet so I might repeat something already said).

For instance: the plague. In England 1/3 fo the population was killed in a half year in 1349, it took 300 years to regrow the loss.

Now, I am working on a disease in which we can see a very prevelant mutation that is present only in caucasians. Strange, since if it is very prevelant it should be old, if it is old, other populations should be affected. The hypothesis could be that the mutation lead to a increased survival of the plague and thus can be seen to have been enriched.

But in general I would say bottle necks are random.