I’ve been browsing through some threads on Evolution in the Great Debates bit. I’m in no way qualified to tell anyone anything about Evolution, but I’ve always had an interest in it, and as a result read anything concerning it that I stumble upon. Reading the posts here reinforced the ideas I’d already gathered, gave me new ones to think about and loads of interesting links to follow. While I was following some of these links, I realised that I’d never really heard the Creationist argument from the horses mouth, I’d only really based my opinion of them on that Bill Hicks rant. So I googled creationism and came up with www.creationism.org. To be perfectly honest, i have no idea if this site is serious. In the ‘Fossil’ section, it claims that “humans could live for over 900 years” before Noah’s flood. Surely this is someone having a laugh, like the sites that claim to take Time Cube seriously. Anyway, theres one bit that I wasn’t sure on, in the ‘Mutation’ section:
Mutations are almost always harmful or (if not fatal) get reabsorbed into successive generations in any event.
I’ve never heard anyone say this before, really doesn’t sound right (‘almost always’ bit seems to be missing the crux of evolution). Can someone shed some shiny rational light on it please?
I think they’re saying that, since a mutation is a change in the genes of an individual, the mutation would quickly be “swallowed up” by interbreeding with the overwhelming majority of the unmutated population. Not that it bears any resemblance to reality, but that’s my understanding of their point.
And no, they’re not having a laugh about such ridiculisms as the 900 years thing. Most of the “creation science” groups really believe such things as that there was a real man named Methuselah, and that this Methuselah really lived 969 years. Believe me, if Jack Chick isn’t a deliberate parody, the “creation science” sites aren’t…
Most mutations are of the sort that confer no selective advantage (positive or negative) to an organism. For example, a large fraction of the human genome is noncoding DNA. While some of it does have another function, large portions of it appear to do exactly nothing
Even if you look at DNA sequences that code for protein, there are many places where you can substitute a leucine for an isoleucine codon and have absolutely no effect whatsoever on the form, function or lifetime of the coded protein.
Mutations of this sort are subject only to random propagation. They are as likely to disappear from the gene pool as to continue on into succeeding generations.
I don’t know what they mean by “reabsorbed”. Do they mean such mutations disappear?
Mutations that are not fatal tend to accumulate through the generations. In fact, such neutral mutations are darn useful for tracking down the ancestry of related groups (like Jewish males). Some alleles (the forms of a gene) disappear in a population, just due to chance. For instance, if you have a child with purple eyes and that child dies without living offspring, that mutation will disappear. But other mutations will remain in the gene pool indefinitely.
We all possess a number of mutations that could be traced to ancestors who lived multiple generations ago. In fact, we all possess the mutations that belonged to our ancestral mother. They are embedded in our mitochondrial DNA.
Now, there is a slim chance that a mutation will revert to it’s “wild type” state. Let’s say your purple-eyed offspring have a single dominant mutated gene which is responsible for this trait. There’s the possibility that your grandchildren will be born with brown eyes (like yours) if their mutant gene mutates AGAIN…either disenabling it or turning it into the “brown eye” gene. But this is even rarer than getting struck by lightening twice.
Some accumulate and some don’t. Neutral and even beneficial mutations can and do disappear from a population if the individual who first got the mutation has no offspring or if all of his direct decendants fail to reproduce. This is caused simply by the random effect of genetic drift. The most fit don’t always survive to repoduce, sometimes a rock falls on them out of nowhere before they get a chance.
Would I be correct in thinking that human evolution is pretty much a moot process in the modern western world? Given that people can now choose whether or not to procreate, and that food is easily available, (in the western world at least), I cannot imagine a mutation that would give you an advantage over the rest of the breeding population.
However, say a child was born in, lets say Ethiopia, that had a gene that meant said child only needed 1/2 the calorie intake of a ‘normal’ person, then that child would have a distinct advantage, and if the trait passed on, his descendants would have that advantage, until, eventually it would become the norm in that regions population.
I know that evolution is a very commonly misunderstood process. Am I beginning to get to the crux of it or am i spouting rubbish?
Well, as I understand it, evolution is a pretty moot process in historical time… and yes, modern society and technology may be making it ‘moot-er’
It takes hundreds of thousands of years for evolutionary processes to make themselves felt… so they’re of interest for figuring out how we got to where we are now, not so much how we’re changing. That’s as far as I’m aware, at least.
To the OP:
In terms of the molecular mechanism of mutation, a mutation on one strand of DNA first has to be incorrectly repaired. Then only 50% of the progeny will ever inherit this mutation (if it is autosomal) as each progeny only gets one chromatid. So it is easy to imagine that if a man undergoes a mutation and passes it on to one of two children and that child fails to pass it to his one child, that mutation is lost. We can assume that a significant amount of mutations end up like this. Although it is quite true that neutral and slightly positive and slightly negative alleles accumulate through random genetic drift. This increases the genetic diversity of a population, which is the starting material by which evolutionary selective processes work. Evolution, again, is just when this mishmash of alleles at a locus gets winnowed by a selective pressure.
Also gene reversion may be a bigger process than monstro has put it out to be. Any second mutations close to the first that cause apurination or double-stranded breaks would be repaired by homologous recombination, as would many other standard polymerase misincorporation or demethylation changes. This can easily erase a single-base pair mutation. In this process, the cell recognizes a damaged allele (the second mutation) and repairs it by copying the DNA from the other chromosome. This leads to a process called gene conversion and is far, far more common that acquiring another cis suppressive or reversion by direct mutagenesis. If this makes any sense…
One and Only
Evolution works on very small margins. It has been calculated that a difference in fitness of less than 1% (measured in amount of progeny) is enough to be a strong genetic selection. The definition of evolution as a shift in allele frequency means that we focus (usually) on subtle, small changes. So when we talk about populations evolving, at least in molecular genetics, we rarely talk about large phenotypic changes. Although it is very exciting when we can link the two.
We in the Western world are constantly under evolutionary processes and it is too simplistic to think of evolution as only at disease and evergy usage loci. For example, the mean temperature of the Earth is changing, as it has always, and we are changing along with it. We don’t notice these changes, because we have only been looking at population frequency of alleles for about 10 years or so, but one has to assume that they are occuring. The existence of air conditioning and other factors may blunt some of these changes, but they are still happening. There are many other loci at which we can assume there is reasonable levels of genetic selection in the Western world – I would think HIV nonresponding genes, genes which induce a higher metabolic rate (less propensity for obesity), and skin pigment genes (Northern Europeans in Florida, California, Texas, Australia have a rough time of it).
Another thing is that evolution has almost never selected for energy usage. There are always malnourished populations and these generally don’t produce as much offspring as other well-fed populations. These well-fed ones end up outcompeting the malnourished ones. Our body has too many levels of redundancy and complexity which waste energy to, in one step, become significantly more energy efficient.
Thanks for all the replies, that ones been cleared up. Edwino - you seem to know your stuff The first two paragraphs in that post went way above my head, but the rest sunk in, although I had no idea what progeny was. The definition i found was ‘a result of creative effort’, which didn’t make too much sense - couldn’t get my head round fitness being measured in… ahh. Think i just got it. A 1% increase in the success of your efforts (efforts concerning fitness)?
Yeah, sorry. On rereading that I have no idea what I was thinking.
Easily put – Most mutations do little or nothing to fitness. These tend to accumulate in populations. But this doesn’t mean that all mutations are bound to accumulate. It is a toss-up whether a single event (mutation) in a single germ cell passed will be a) passed on to children and b) passed on enough to spread throughout the population over time. It is quite easy to envision a scenario by which a single event is bred out after one or two generations if it is even passed on to begin with.
Next, there are many repair mechanisms. Obviously with the 3+ billion base pairs of DNA in each cell, accumulating a mutation (let’s say from an A to a G) at a single residue of DNA and then acquiring the exact same reversion mutation of that allele (let’s say from that G back to the A) is impossibly rare (though we do see this type of thing, especially in smaller organisms like yeast). This is called a cis change. But, the cell has a suite of repair mechanisms and some of those will unwittingly (or even sometimes wittingly) overwrite the new mutated strand of DNA. It an individual carrying one copy of the A->G mutation allele across from the original A allele, if there is some DNA damage anywhere in the area of the A->G gene, the cell will copy the damaged part completely from the other copy (the original A allele) – repair in trans, or gene conversion.
A good example of this is loss-of-heterozygosity (LOH) in cancer prone conditions. The original description of LOH comes from individuals carrying one mutation of gene which suppresses tumor formation called Rb (for retinoblastoma). The individual has one copy of normal Rb and one mutated copy, thus is a heterozygote. In these individuals, one sees early pediatric retinoblastoma in both eyes – a condition resulting from the loss of the normal copy of the other retinoblastoma gene. This is not from acquired mutations in the normal copy of the gene. It is from gene conversion. The normal strand acquires some new repairable mutation (it doesn’t have to be even in the Rb gene) and the cell copies it from the mutated strand (the cell has no way of telling which is mutated), thus converting that cell from Rb +/- to Rb -/- – loss of heterozygosity. This only has to happen in one cell in a retina – that cell now becomes a cancer as the Rb brake to the cell cycle is removed and that cell begins dividing uncontrollably. In practice, you see events of this nature in both eyes by two or three years old.
The first two paragraphs in that post went way above my head, but the rest sunk in, although I had no idea what progeny was. The definition i found was ‘a result of creative effort’, which didn’t make too much sense - couldn’t get my head round fitness being measured in… ahh. Think i just got it. A 1% increase in the success of your efforts (efforts concerning fitness)?