Jeff Heusby says, "can environment change gene expression, SNPs, et cetera, within one
lifetime? In other words, if you didn't have a SNP in a certain gene early in life, could you
develop one later on say as a result of extreme chronic stress? Correct answer
probably is that it takes hundreds of years, but I thought maybe we had more science on it now."
Well, generally speaking, environment mediates, I mean, it depends what you mean by environment
mediating, right? So environment mediates changes in gene expression
through epigenetics, but gene expression is not SNPs. SNPs
are produced through mutations in the heritable genome, and then by environment changing,
do you mean the production of the mutation that is the SNP for the first time? Or do you mean
the change in the proportion of the population? So environment will change your gene expression,
which means the epigenetics in your lifetime in you, and there may be some
limited heritability of that that might last up to generations if environment doesn't reverse it.
But environment's going to be a lot more likely to change lasting things during development,
or during pregnancy and nursing in the mother. But, changes in gene expression
happen over the course of minutes sometimes and generally hours and days all the time.
So environment's constantly changing your gene expression on a micro scale of hours to days,
and maybe weeks. But that doesn't last, that's the type of gene expression change
that can happen over hours, to days, to weeks, or especially days to weeks to months is the type
that can be reversed in the same timeframe by the environment. So that's generally going to be more
responsive to cycles in the environment, fasting and feeding cycle is an example. Seasonal changes
in macro nutrient availability is another example. Seasonal changes in sunlight is another example.
But if you're going to produce lasting epigenetic changes that are semi-heritable,
then that's probably going to be driven by things that were sufficiently developmental to make your
body perceive that it's going to last generations. And a famine is a great example of that.
Now, the environment of course is going to dictate gene mutations
because it's going to dictate the type of stress,
such as oxidative stress and things like that, that could damage DNA and make it mutate.
And evolutionary theory talks about gene mutations being random,
but they aren't random and evolutionary theory never meant that they were random.
It meant that they were random with respect to the environment's demand for,
the environment's imposed fitness level of that genetic mutation.
In other words, Darwinism being correct, as opposed to Lamarckism being wrong,
as taught in high school science classes, means that the reason genes mutate is not directly a
result of the fact that they would be more fit to be in that environment. But there's got to be
at least some exceptions to that, at a minimum being that when you express a gene more often
that the chromatin structure will be more open, because you have more things coming in to read
the gene and make proteins out of it. Whereas when you express the gene less, the chromatin structure
will be more tightly wound because you don't need to express the gene. And I can't fathom that that
would not affect more highly expressed genes being mutated more often than less expressed genes. So
while it's probably not true that the giraffe stretching its neck is what makes
the neck longer through successive generations, it probably is true that genes that are in high
demand to be expressed often are the ones that are more likely to accumulate more mutations, and thus
accumulate a higher amount of genetic variability that accumulates more opportunity to be acted on
by natural selection. And then the process of natural selection takes generations.
So if this is in bacteria, then it's not going to be hundreds of years, but in humans,
it's going to be hundreds of years, because that's what takes multiple generations to act out.
But that has nothing to do with epigenetics and nothing to do with gene expression. That's
about natural selection acting on the existing amount of the mutations, the existing variability
to change the proportions over time in the population. But that has nothing to do with you.
That's about the proportions in the population. So these are just different fundamentally things.
Let me summarize that. So gene expression is about epigenetics. You have cyclical things
that act on the timeframe of the cyclicality that are reversed as easily as they're imposed.
Those can be short scale like the fasting feeding cycle, or they can be longer scale
like seasonal changes in environment. And then you have things that would be expected to
impact in general at least a lifecycle, if not generations, that will act primarily through
the state of the mother's body and the child's body during pregnancy, nursing, and development.
And those are large-scale things like long famines or changes in food availability. And then you have
environment will immediately and always be affecting the rate of mutation. And then on
the variability caused by that mutation, natural selection will act over generations,
but will not act on you. Natural selection acts on you by killing you.
So natural selection acts on a population by killing some people more than others,
which is complimented by sexual selection, which acts to make some people have more
children than others. And that has no effect on gene expression in an individual,
but has strong effects of the proportions of certain SNPs in a population over time
as played out over generations. Okay, I hope that helps.