Natural Limitation to Evolutionary Processes (2/14/05)

The rates of mutation don't need to be high in order to maintain variation because much of variation is protected from selection. For traits that are affected by genes at many different loci, low mutation rates can maintain a populations variability. Many different genotypes generate intermediate phenotypes that are favored by stabilizing selection. If the individuals within a population are all just as likely to survive and reproduce, then a lot of variation is protected from selection. When a large number of loci affect a single trait, only a small fraction of the genetic variability present in the population is expessed phenotypically. Selection removes variation from the population very slowly, if this happens. Recombination and reshuffling will expose these hidden variation to natural selection in later generations. Because of this protection from selection, a low mutation rate can maintain variation despite natural selections tendency to decrease variation.Hidden variation is always present in continuously varying traits.This is the reason that we can get different forms of species via artificial selection. A chihuahua(sp?), which is smaller than the smallest wolf, has been and could be selected for by humans over a very short period of time because within the genes of wolves, there must have been these hidden variations!

(My bold)But the fact is that novel mutations DO take place and breeders do USE them in breeding. There are different breeds of domestic cats which came about due to some novel mutation.For example:

quote:

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On a sunny day in June 1981 in Lakewood, California, a longhair silky black female kitten with unusual ears wandered up to the doorstep of Joe and Grace Ruga. Joe scrutinized the situation and determined that the most effective solution to this stray kitten problem was to ask Grace not to feed the kitten. Grace, not abiding by her husband's wishes but listening to her heart instead, left a bowl of food on the porch. The affectionate black kitten quickly worked her way into the Ruga's hearts (especially Joe's) and they named her Shulamith, which means "black but comely". Such are the beginnings of the American Curl as it is known today. True American Curls must trace their pedigree back to Shulamith, the foundation female.

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In December 1981, Shulamith delivered her first litter of kittens. Out of four kittens, two had the same curly ears as Shulamith. A geneticist was contacted to study this phenomenon and he confirmed that this unusual ear was a genetic trait and was inherited in every case, causing it to be labeled a dominant gene, with no deformities attached to it. Referred to as a spontaneous mutation, the gene that causes the ear to curl appeared to be following a single dominant pattern.

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From: Cat Fanciers Website

The process of natural selection works on the phenotypes. This can cause a reduction in variation but you mustn't forget that there is still hidden variation within the genotype which can continue on in later generations and not be expressed.

How do you think that that adaptive trait came about in the first place? This, I think, is a fundemental problem that you are falling under. Mutation had to have happened at one time or another. This mutation may already have been present in the population but not expressed, until some selective pressure made it beneficial to HAVE that mutation. How do you think you get variations in allele's??

Isn't the Hardy-Weinberg Equilibrium equation often used as an indicator that evolution is happening? You can compare allelic frequencies from the equation to actual allelic frequencies in the population to show that/if evolution is happening.

Codons can often have a change in the last base without affecting the amino acid it codes for. There are 20 amino acids in biological systems and DNA has 4 base pairs arranged in complementary pairs. That gives us, what, 64 different combinations for the codons. As you can see, those 64 different combinations code for 20 amino acids. As an example: GCT, GCC, GCA all code for alanine. A point mutation can affect the last base of the codon and this would essentially be a neutral mutation. It wouldn't change the code for the amino acid.Now, if you look at which parts of the DNA strand actually used to code for the proteins produced you would see that often there is much of the strand that is no longer used or codes for nothing. Any change to this portion of the code, via point mutation, would again be neutral. In fact, often mRNA is "snipped" of portions of its chain before it is sent out of the nucleus to be read by the tRNA. (Obviously it is more complicated than this but I hope you get the picture.) That snipped portion might have had a mutation but is ignored because it doesn't pass through.So you can see that DNA has such redundancies to it, that mutations (specifically point mutations), may not even affect the survival of the organism.

Which explains why this mutation is NOT selected against even though having homozygous allele's for sicke-cell is VERY deleterious. I don't think they even live to reproductive age, correct? Having this allele in populations exposed to malaria carrying mosquitos is a positive thing. If you're heterogenous for this trait, you have some resistance to the disease and are able to pass YOUR gene to your offspring. You have a reproductive advantage over the homozygous normal carrying individual in THAT environment.(I hope you don't mind me adding my comments here. Just hoping he reads through the posts and isn't selective I guess.)Which reminds me. The change in the gene is a point mutation. IT can be deleterious when you are homozygous for the trait, but heterozygous individuals have a reproductive advantage.

and Natural selection does NOT ALWAYS cause a reduction in variation because hidden variation is ALWAYS present in continuously varrying traits. Natural selection can INCREASE variation. How about an example of how this can come about, eh?Suppose you have an environment that favors larger beaks in birds. We'll let + equal the gene for large beaks and - as the gene for small beaks. Because deeper beaks is determined by genes at many loci, a population of even large beaked birds + have some small beak genes -. Now when the birds with the smallest beaks die, alleles for the small beaks are removed from the breeding popuation. This may increase the frequency of the + gene at every loci, but because even the larger beaked birds have some - alleles, variation still remains. Reproduction shuffles these genes within the population and because + genes are more common, individuals in future generations will have more + alleles. Because the more + genes you have the deeper your beak, the population will show a shift to larger beaks, and the new generations biggest beaks will be bigger than the previous generation. If this continues the same thing happens again. The individuals with the biggest beaks will have beaks even larger than the previous generation. As you can see you have MORE variation. This process can even be reversed.An example I have seen used is an experiment on oil content within seeds. The experiment conducted on corn showed that oil content could be increased, after 80 generations, beyond the initial oil content of 4-6%. The researchers were also able to reverse the process and select for low oil content instead. This showed that selection could INCREASE the initial range of variation.
(From Boyd and Silk, ibid, p. 74 with the citing found in the url below) Evolution: Online courseAnd finally, where do you think our domestic dogs have come from? If they come from wolves and we SELECTED traits then of course you should be able to see that we do have MORE dog breeds and MORE variation in this species. It is EXPRESSED variation that is selected for. Hidden variation is later expressed through reproduction/recombination.

I apologize if I misunderstood what you were saying. It seemed to me that you were talking about variation. Specifically a reduction in variation due to genetic drift/ NS.I do think I percieve what you are saying and perhaps a misunderstanding about alleles that you might have. In your first post you talked about a reduction in alleles: This is infact not the case. There is no genetic reduction, or reduced allotment of alleles, as if they are somehow reduced. Instead there is a shift in the frequency of alleles by selection working on the expressed alleles. This may or may not cause a reduction in the "FREQUENCY" of an allele, but that is because the frequency of a selected allele is increased. So if we see in our finches with small beaks that they are selected against, we still have a population of larger beaked birds that may have the small beak alleles. In the case of a drastic reduction, such as a bottleneck the allele frequency may shift so much that the allele for small beaks doesn't exist in the population, but the number of alleles for that trait are still there, they may all be large beak alleles.To express this visually, suppose you have a population of birds that vary in beak size according to expressed alleles, as shown below (with - allele for small beaks and + allel for larger beaks). Now remember we are talking about continuouly varrying traits or multiple alleles affect a trait (such as height in humans.)(numbers in parenthesis show hypothetical population)(5) (11) (20) (30) (21) (10) (3)
-- -- -- -+ -+ -+ ++
-- -- -+ -+ -+ ++ ++
-- -+ -+ -+ ++ ++ ++Now if some selection pressure caused the smallest beaked birds to die off you still have - genes in all but the largest beaked birds. There was no "reduction" in the alleles that affect beak size but a reduction in the FREQUENCY of the - allele.This I think is the misperception you have about some reduction.As far as there being a reduction in variability. Variability is the "ability" to vary. What is stopping a mutation to act on or change one of the alleles, even if they were all + alleles? If a mutation acts or changes one of these alleles it will remain hidden and reshuffled among the population if it isn't detrimental to the reproduction of the species UNTIL selective forces act in such a way that this NOVEL ALLELE becomes expressed. This novel allele may or may not help the species to survive.This message has been edited by DBlevins, 02-17-2005 19:09 AM