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Natural Selection

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RTFishall | 16:38 Tue 17th Mar 2009 | Science
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I can understand how natural selection and a large gene pool leads to an "improved" species. But why is it that a fixed gene pool leads to "degeneration"?
Why doesn't a fixed pool just remain constant?


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Because there is no such thing as a "fixed" gene pool.

Mutations occur randomly.

If the mutation leads to a "fitter" individual then they have a better chance of survival and of passing on the mutated gene to their own offspring.

If the mutation leads to a "less fit" individual then that individual may well not survive.
Question Author
Thanks Rollo.
What I meant by "fixed" gene pool was say brother/sister or father/daughter etc offspring . Why does this seem to lead to a higher incidence of "bad" mutations rather than just the norm?
Because, if there is a "bad " mutant within the family already, it is more likely to be passed on if both parents have it - especially if that gene is dominant rather than recessive.
Question Author
Does that mean then that if both parents have a "good" mutant gene then the result would be (by definition) better?
I suppose what I'm asking is, would the result be as likely good as bad?
If so, would highly intelligent parents (with the "right" dominant gene) be more likely to breed more intelligent children and visa versa?
O level biology was a long time ago, but I seem to remember that a faulty gene that doesn't code for anything sensible is inherently recessive. Lets say
G = good, average gene
B = bad, inherently recessive gene
In a mixed gene pool you may have:
GG + GB --> GG + GB + GG + GB
so 50% of offspring carry the bad gene harmlessly
In the limited gene pool you would see much more occurences of:
GB + GB --> GG + GB + GB + BB
condemning 25% of offspring to have a harmful, quite possibly fatal defect.

Now what happens if you have a super gene. In the mixed gene pool:
SG + GG --> SG + SG + GG + GG
but the S could be dominant or recessive, so only 25% of offspring exhibit the super trait. In the limited gene pool you will get many more:
SG + SG --> SS + SG + SG + GG
so 50% of offspring exhibit the super trait.

Unfortunately there are many more ways to get a B than an S. So, in the limited gene pool you are going to get a lot more offspring with problems than you will get super traits. Hence the reason most of us are programmed (by our genes) to not fancy our own parents, siblings or children.
Indeed if both parents have a particular beneficial mutation, then it is guaranteed to be passed on to their offspring. However, the overwhelming majority of mutations are weakly or strongly deleterious, so the consequence of increased rates of inbreeding is to increase the amount of lethal recessive alleles in a population.
Question Author
Thank you all for your replies. By pure coincidence there was a television program about Darwin on the BBC last night that dealt with the subject we have been discussing!

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