Evolution: why are mutations retained by descendants?

Factual Questions
1

For some reason I had this thought while half asleep, and it occupied my entire brain throughout the night, giving me weird dreams.

I would’ve thought a mutation was recessive, and a creature’s descendants would dilute it and breed it out, rather than keep it and make it permanent.

Surely in order to keep it, both male and female would have to have the same mutation, it would have to remain in the gene pool for an extended period of time.

Please explain the process of evolving, especially regarding retaining mutations.

2

Mutations don’t need to be recessive. But even if it is, the reason it isn’t bred out is that it gives a reproductive advantage to individuals that have it. (Those that don’t are bred out fast.) Genes that only give a slight increase in reproductive success can proliferate through the population very quickly.

Also, keep in mind that much of evolution is about the mutation of regulator genes. Genes that tell other genes what to do or when to do it or how much of “it” to do.

3

If the gene is to create something (i.e. more melanin-darker skin) then that gene is dominant - it only takes one to get the result. If the gene is a copy error (more likely) that does NOT make a particular enzyme/protein/whatever (or causes a failure in one step in its creation) then it is recessive; usually the “good” gene from the other parent will fill in the gap and sufficient of that chemical will be made to keep the body normal.

If the mutation is developmental (i.e. the person has the right genes, but something interfered with their expression, so, for example he’s born with no arms) then that is environmental not hereditary.

Usually mutations that stay with us because they have an advantage. The classic example is that one gene for sickle cell anemia helps create a greater resistance for malaria, a significant advantage in populations where the gene is usually found.

Also, some hereditary diseases spontaneously appear through new mutations. Queen Victoria’s descendants and hemophilia is a good example. The thought is that she developed the bad gene spontaneously. Apparently this happens frequently. Thus, although it is a distinct disadvantage in reproductive survival, enough new mutations happen to keep the disease popping up in the population.

4

To answer your last question, the process of a species evolving works something like this;

[ol]
[li]A species lives in environment 1 (E1). Some are green, some are blue. Neither of which give them any sort of disadvantage in environment 1.[/li][li]Some of the species move to environment 2 (E2). E2 contains a predator which is very good at spotting blue creatures. This means that fewer blue creatures survive, leaving more green ones in E2 compared to E1.[/li][li]Some of the creatures from both E1 and E2 move to E3, which has the same predators as E2.[/li][li]A genetic mutation in the population of E3 (which is cut off from the other two environments) results in creatures which are orange. As it turns out the colour orange gives these predators severe headaches. As a result the genes for being orange spread rapidly through the population of E3.[/li][/ol]

End result;

[ul]
[li]E1 has an equal mix of green and blue creatures because neither colour offers an advantage, hence both phenotypes are passed on.[/li][li]E2 has fewer blue creatures because more of them are eaten than the green ones, meaning the alleles for being green are more likely to be passed on.[/li][li]E3 has fewer blue creatures for the same reason and a large population of orange creatures because the advantage being orange gives them over predators meant that the mutation which caused it was spread (conversely, any predators which didn’t get headaches from the colour orange would be more likely to eat enough to survive…). Note that even a recessive gene (mutations aren’t always recessive as a previous poster noted, of course) would still exist if it wasn’t expressed so it’s less likely to disappear completely than you think.[/li][li] Were the mutation introduced to E1 or E2 it would spread through E2 as it did E3 (because there is the same predator) but would be less common in E1 (particularly if it was recessive) because there’s no advantage.[/li][/ul]

5

So, the mutated allele that causes sickle disease, which is recessive, does not “create” anything? Hint: It does.

from wikipedia: Sickle-cell anaemia is caused by a point mutation in the β-globin chain of haemoglobin, causing the hydrophilic amino acid glutamic acid to be replaced with the hydrophobic amino acid valine at the sixth position.

That doesn’t even make any sense. All mutations are heritable. There are non-genetic causes for many outcomes, but the mutations I was talking about are ones that affect the expression of other genes, but do not create a new feature. For instance, a mutation might cause the delay of sexual maturity or it might cause an existing appendage to grow longer or shorter, or grow more than the standard copies. There are also some genes that are not expressed except under certain environmental factors. I’m really not sure what you’re talking about when you refer to mutations that are not hereditary.

6

If someone gets a mutation on one of his genes (one of two paired genes) then he has a 50% chance of passing it to each child. Counting him and his mate as one generation, half of them have the gene, and half of the next generation has the gene, and, if the gene has nothing to do with survival, then half of each succeeding generation will have the gene. If you mix this family with others who do not have the gene, say in a 10% ratio, then the frequency will drop with the mixing to one in 20 but will stay constant after that (on the average). If the gene does cause increased or decreased survival or fertility, then its frequency in the population will change with time.

7

No. Only germline mutations are.

8

Yes, mutations in somatic cells are not heritable across generations. I don’t think the post I was responding to was referring to mutations in somatic cells.

9

I think they might be referring to teratogenic influences (think thalidomide). Calling that a “developmental mutation” is a bit of a stretch, though.

10

I don’t know of any reason that a mutation would be more likely to be recessive than not. Also, recessive genes don’t get diluted and bred out, they just get expressed in the phenotype less frequently.

11

Genes, whether dominant or recessive, do not dilute.

Actually, let’s start with the distinction between dominant and recessive genes. The distinction has nothing at all to do with how they’re passed on, only with how they’re expressed. At each gene locus, you have two genes. One of them came from your father, and one of them came from your mother. When you have kids, each kid will get one of your two genes from that locus, and one of the other parent’s two genes. Then, each of their kids will get one of those and one from your kid-in-law, and so on. At every stage, the person involved either has that gene, or they don’t. Nobody ever “sort of” has a gene, or half-has it, or the like.

Now, if the gene is dominant, then everyone who has the gene shows some trait associated with it. If it’s recessive, then you don’t show the trait unless you have two copies of the same gene. But even if it’s recessive and hiding behind a dominant gene, it’s still there, and will still be passed down (or not) just as easily as a dominant gene.

Now, then, suppose you have a mutation, that gives you a new gene, never before seen. Any given kid you have will be 50% likely to have it. If it’s a dominant gene, those kids will then show some trait, but even if it’s recessive, they’ll still have it; it’s just not doing anything yet. And maybe, just maybe, the gene will end up being more prevalent in the gene pool. This could be because it’s a dominant gene that gives some benefit, or it might just be due to luck (maybe the coin comes up heads for a larger-than-normal portion of your descendants, or maybe (either due to other good genes or due to more luck) you just happen to have more descendants than average). If this happens, then eventually, a couple of your descendants who both have your new gene will mate and have a child, and maybe that child will happen to get your mutant gene on both sides. If it was a recessive gene, this will be the first opportunity anyone has to see what it does, but either way, it gets passed down the same.

12

This is a lot more complicated than I expected, I may not truly understand. But Chronos’s simplification is familiar from High School science class, so I think I’m getting the gist.

13

Also recall that the dominance/recessive relationship taught in high school is not the only possible relationship between alleles. Many traits show co-dominance, in that individuals that are heterozygous show intermediate traits or combined traits compared to homozygotes. One example is human skin color, in which the offspring of people with dark and light skin will often be intermediate. Another is human blood types, in which O (neither A nor B), A, B, and AB all occur.

14

Human skin color is even more complicated than that, since it’s determined by several different gene loci (I think I heard somewhere that it’s mostly from nine different loci, but don’t quote me on that). If it were only one locus, you couldn’t get nearly as much variation as we see.

15

In other words, you expect 1/16 of one’s g-g-grandchildren to inherit the full gene, rather than each g-g-grandchildren to inherit 1/16 of the gene.

It should be noted that Charles Darwin did not understand this, and the point would not be clear until Mendel’s Laws were appreciated. If genes instead were diluted by inheritance, as Fleeming Jenkins discussed in 1867, evolution would proceed much more slowly than required by Darwin. It was Jenkins’ paper that led Darwin to equivocate in later editions of Origin of Species, even embracing Lamarckism as part of the answer.

Ironically, Mendel had published the paper which showed the way out of the dilemma two years before Jenkins’ paper, but Mendel’s work went unnoticed.

16

Aha! That makes a whole lot of sense!

17

OK, for the sake of discussing the OP, let’s say we are only refering to mutations of the genetic material that make it to the next generaton. Therefore, the mutation has to occur somehwere in the genetic material of cells that end up in the reproductive organs, creating the next generation of sperm or eggs… or while creating the egg/sperm for the next generation.

Cancer, for example, is a mutation that occurs in the lung/bone marrow/pancrease/skin/whatever, but of course that does not affect the genetic material passed on to offspring.

The problem is - if a genetic problem can cause death (or reduce the probability of reproduction) and its a simple single gene, then simple Mendel laws can apply. You get one gene from each parent. A pair of parents with one good and one bad gene - AB; the odds are that a child will have AA, AB, BA, or BB; A is bad, the AA child does not reproduce. Thus from a generation where everyone has one bad gene, 50-50 we have a situation where the next generation of parents, only 66% have the gene. And so on.

A basically “bad” gene can eventually breed itself to almost extinction. Those BA offspring will either find another BA or a BB; Again, any AA will not reproduce. However, when A becomes so rare that the odds of a BA-BA pairing are pretty low, the odds of AA children become very very low.

Then you have to account for social trends. When parents had 5 to 10 kids, in the middle ages, and the healthiest survived, the odds of producing some completely good BB along with the mixed AB were pretty good. Today, lets say they have tests and an AA might be aborted. Plus, parents only have one or two children, so odds are they will have AB children and stop there. (Of course the odds that any one child’s mix will be AB, BA, or BB stays the same).

Over the extremely long run, bad recessive genes will reduce to a marginal (endemic?) level. You cannot guarantee they will become “bred out” completely.

18

And just to be especially clear, we’re talking about dominant alleles, not dominant genes. Each gene is composed of two alleles, like the stuff we all learned in junior high:

AA Aa aa. A= dominant, a=recessive

AA and aa considered the same gene, just different expressions of it.

19

Yes, people frequently talk about genes when they mean alleles. A gene is a specific string of DNA on the chromosome that controls some trait. Alleles are the different variants of DNA that may cause the gene to have different effects. (Note that some alleles/mutations are “silent” in that they do not produce any change in the action of the gene.)

20

Strictly, it is A and a that are considered different expressions of the same gene, though. AA and aa are couplings of genes.