No. Just no. The more genetic diversity a species has, the better positioned it is for survival, long term. The persistence of “bad eye” genes does not make “good eye” genes disappear. You’re making the same mistake folks make when they claim that more brown eyed people in the US will make the blue eyed phenotype disappear.
I agree with some of what you said, but don’t get too carried away…
Sure, but you’re kind of missing the point here. Evolution doesn’t have an “aim”, but we do. We would prefer to have genetically perfect eyes that work well without glasses or surgery. The straighforward point is that the more that we ameliorate genetic defects with technology (and with a society that involves mutual support), the more we shield those genetic defects from natural selection, and we increase the frequency of bad eye genes in the next generation. Sure, it’s technically not correct to say that natural selection has “stopped”, but that’s only because natural selection has a tautologous definition; the meaning is obviously that the kind of natural selection that would operate without the shielding of technology has stopped.
But you seem to be conflating genetic diversity, the abundance of genetic variants with roughly equal fitness in the current environment, with a failure to purge deleterious mutations by hiding them from selection. Granted, to some extent, even “deleterious” is environment-dependent (sickle cell granting malaria resistance is the well known example), but in general, it’s not going to improve the chances of survival of a population if straightforward loss-of-function mutations are hidden from selection and not purged.
Exactly, thank you.
Every person born with deleterious genes that manages to survive through technology, is one less person who might have died off, and if early enough, been replaced by their parents until they popped out a combination of genes that survived. First world people seem to limit their total reproduction once rich enough. If detrimental genes accumulate through mutation and are not winnowed, then yes, they do displace more fit ones over time. Given long enough the problem could result, if the environment changes, in a serious population bottleneck.
not that I’m advocating population cull, speaking as someone who wears glasses. I’m just pointing out an accelerating flaw in our civilization; and the obvious non-genocidal future fix - genetic manipulation - holds its own risks. Like global warming, it’s a serious flaw in our technology.
In practice, I don’t really think it’s a concern, even if you have an incredibly pessimistic view that society will implode into some low-tech dystopian future. The pace of technological advance in our civilization is so incredibly fast compared to the glacial pace of natural selection. Within a couple of generations we will be actively intervening to modify defective genes in the germline (check out CRISPR technology), at which point evolution by natural selection is completely irrelevant; and if we crash and burn back to hunter-gatherer societies without opticians before then, far too little time will have elapsed for the gene pool to be seriously compromised by molly-coddling. A few myopic nerds like you and I will be taken out quickly by the zombies, but humanity will survive.
I don’t think so. It’s rather near sighted (hah!) to think that someone with poor vision has only “the poor vision gene” to add to genetic diversity. Every person who lives has the potential to add some diversity to the gene pool. And they are not crowding out the “good vision genes” unless for some reason people with poor vision are seen as more suitable mates than people with good vision, thus reducing the chances of the “good vision genes” from being passed on to future generations. We are now 7B individuals as opposed to 1B only about 100 years ago. Our genetic diversity is greater with those 7B than it was with the 1B, and our chances for survival are better as a result.
At the beginning of the agricultural revolution we were somewhere between 1M and 10M individuals. We’re are much, much better set as a species with 7B than with 10M. If all those extra people we’re clones of one individual, that might be of concern. But they aren’t. Civilization also has led to one of the largest interbreeding experiments within our species since we got started as a species.
Of course, I’m not advocating eugenics as an ideology - we don’t want anyone to die because they have one bad gene or even a thousand bad genes.
But in natural population genetics, the way you envision things is not really how things work. Evolution is the change in allele frequency in a population. Suppose a locus that codes for an important eye protein has two variant alleles, one coding for a functional protein and one for a defective protein. If the vision phenotype is exposed to purifying natural selection (the “no opticians” environment), then all the people carrying the defective allele are less likely to produce offspring, so the proportion of defective alleles in the next generation is smaller, and the proportion of functional alleles is correspondingly higher - so it is zero sum between “good” and “bad” alleles at a given locus.
The action of natural selection does not usually entail loss of diversity at other loci in the genome, because sexual recombination and assortment shuffles all the other genes around randomly*. One less individual with the defective allele is offset by one more individual with the good allele, and on average they contribute similar diversity at other loci in the genome. Under extremely strong selection there may be loss of diversity at tightly linked loci on the same chromosome, when there are not enough generations for sexual recombination to shuffle things around. But, typically, diversity is related most closely to population size, and loss of diversity occurs only with a population crash. In a steady population, purifying selection does not usually entail loss of diversity at other loci.
*This is one reason why sex is a good ideas. There may be others.
ETA:
To expand on this a bit: in a species with a single chromosome and no sexual recombination to shuffle things around (bacteria, say), the loss of overall diversity in the genome that you envision under strong natural selection at one locus is precisely what happens.
Consider beneficial mutations, because it’s more interesting:
In bacteria, evolution proceeds in a more “dramatic” fashion from the point of view of the organism. If a beneficial mutation arises in one individual in a bacterial population, the only way for the population to evolve to fix that new mutation is for every other bacterium in the population to die off without descendants, and for the entire future population to be descendants of the one individual in which the mutation occurred. They are all clones with zero diversity until new mutations have time to occur to regenerate diversity. Effectively, bacteria must evolve pretty much one mutation at a time. This works ok for bacteria, because their generation time is measured in minutes.
In sexual species such as humans, many different mutations at different loci can arise in multiple individuals simultaneously and all undergo simultaneous natural selection, because sexual recombination and assortment of chromosomes allows different combinations at different loci from many different individuals to come together.
IOW, it’s a spread-spectrum jam-resistant redundant-path communications system. Very clever of these DNAs.
To speak to the population size question:
In an equilibrium population (and with a few other assumptions), genetic diversity is proportional to population size. That’s quite intuitive, because the source of diversity is mutation, and the more individuals there are, the more copies of each gene that can mutate.
However, mutation is slow, and populations can change in size very rapidly. Our current population is nowhere near equilibrium. The genetic diversity of our modern population of 7 billion is no higher than the diversity of the population of 1 billion 100 years ago (it may even be lower, given the number of Africans killed by colonialism). At best, we just have 7X more copies of all the same alleles, because 100 years is such a short time. There has not been nearly enough time for new mutations to arise in the larger population increase the diversity significantly.
When you consider the relationship between population size and genetic diversity in a population that fluctuates in size over time, the relevant statistic is the harmonic time-weighted mean of population size. In other words, periods with small populations (crashes, population “bottlenecks”) cause an immediate loss of diversity; but periods with large populations take a long time to add to diversity, because the mutation rate is slow.
So, in the case under consideration: even if the population growth rate were constrained somewhat by purifying selection on (say) poor eyesight (i.e. we just couldn’t breed fast enough), removing that purifying selection by providing people with glasses does not add significantly to diversity, because it makes virtually no difference to diversity if the population takes 100 years or 300 years to increase from (say) 1B to 7B. The diversity of an equilibrium population of 7B will only be achieved over many thousands of years of sustaining the larger population size, giving time for new mutations to occur.
I hope you also compliment that falling apple for knowing all about the law of gravity.
Genetic diversity isn’t only about adding new alleles. It’s also about novel combinations of existing alleles. There has been more interbreeding among populations in the last 100 years than probably any other time in history, much of that happening in the Americas, where Asian, European, African and Native American* populations are all colliding with each other more and more every year. Where I live, in CA, bi-racial people are a dime a dozen. Tri-racial is the new bi-racial, in terms of the stand-outs.
*Let’s not forget how much Native American genetic legacy resides in so-called Hispanic populations.
As Riemann points out, the bottleneck 75,000 years ago had much more effect on net human diversity than the large population today. And even ignoring this, there’s a strong diminishing-returns effect operating on genetic diversity. I won’t try to quantify this, but the chance for evolutionary progress would be about the same with 1B humans as with 7B.
And the final clause quoted assumes that genetic diversity is the main influence on “our chances for survival.” I doubt that and would guess that a 1B population would improve humanity’s long-term chances compared with today’s excess numbers.
But the catch is, each major recovery is permitting more diversity to happen faster than previous recoveries. Mutations that could be a serious flaw in hunter-gatherer societies (colour blindness, serious myopia, weaker muscles, flat feet, etc.) are less of an impediment with a more advanced civilization. The last hundred years have accelerated the level of diversity.
Take as an example Queen Victoria and her hemophilia. Presumably only the most privileged could expect to survive a decent lifespan, and even for the heir to the Russian throne it was touch-and-go. Today, the only thing that stops it from spreading is deliberate self-restraint by carriers and victims. Plus, apparently, it’s a relatively easy mutation to spontaneously occur. So I would guess regardless of population size, the proportion of population living with the disease is larger. Ditto for many other detrimental traits that are hereditary, like type I Diabetes.
Look at it this way - the mutations that allowed the population of poodles to explode and thrive in our society are probably precisely those would make it impossible for them to survive a major disaster fending for themselves. Are we becoming a nation of Foo-Foos?
But to put this in context:
(1) The great majority of diversity found in non-African populations is still a subset of African diversity. To a good approximation, there would be no loss of diversity if everyone outside of Africa died tomorrow.
(2) Human genetic diversity is lower than chimpanzee genetic diversity.
This shows how little the recent massive population growth has added to diversity - and by recent, I mean in evolutionary terms, i.e. several tens of thousands of years, not just the last 100 years.
No, you seem to be just re-stating what JM said originally about diversity from a different angle. As I’ve tried to explain above, population genetics doesn’t work that way - adding or removing purifying selection at a few loci in a sexually reproducing species has little effect on the overall genetic diversity in a population. A few previously deleterious mutations are more prevalent because they are now neutral, but they represent a tiny fraction of overall diversity.
That’s the $64,000 question isn’t it? How fast has the gene pool diversified in a bad way?
How many people do you know with Type I diabetes? There was one person out of 30 in my high school class. There’s one out of 12 where I work now. (Actually 2 out of 12, because his twin brother is also diabetic) One out of 10 of my nieces and nephews is diabetic. in 1900, that ratio was probably close zero, especially insofar as surviving to reproduce. I don’t see that as “tiny fraction” when it’s gone from zero to fairly significant in 4 generations. We saw what less than a decade of ignorance (in the sense of “not knowing enough”) did to the hemophiliac population when many were infected with aids, demonstrating the fragility of technological reliance.
So the rate of Type 1 will triple in 34 years - but the population sure won’t. By 2050 well over 1% of the US population will have this one potentially fatal condition dependent on a functioning civilization.
Well, genetic diversity means something quite precise. It can be measured several ways, but it is the average across the genome of nucleotide diversity or locus diversity or heterozygosity. Most of this diversity is neutral, it has no fitness phenotype at all in the current environment. So, whatever the origin of the higher incidence of diabetes, it really has no relationship to the concept of genetic diversity.
We were discussing situations such as poor eyesight where a previously deleterious trait is no longer subject to purifying selection, it proliferates because it is not a handicap in the modern environment. I don’t see how diabetes fits that paradigm at all. What is your thesis here? Are you suggesting that the increased prevalence of diabetes is due to a genetic change? That seems implausible to me. Isn’t it attributable to modern behavioral changes such as sedentary lifestyle, poor diet? These behavioral changes surely have a cultural origin, not genetic.
ETA:
Reading up on Type I which I knew less about, it’s autoimmune & associated with certain HLA haplotypes. But, there’s no evidence that the increase in incidence derives from increased prevalence of the susceptible haplotypes. Those alleles were always present in the population; but speculation seems to be toward some recently prevalent viral or other environmental trigger for the dysfunctional immune response. So, as with Type II, there’s no evidence that I can find that the increased incidence of the disease is attributable to any population genetic change.
And here’s the other thing that is frequently misunderstood - the difference between genetic defects and environmentally caused defects.
There’s a lot of evidence that myopia is caused NOT by “bad genes” but by how the eyes are used early in life. Close up work like reading is what tends to cause nearsightedness, NOT genes - groups of people who are preliterate have very little myopia, it’s a rare condition, but as soon as their kids are entered into conventional schooling and start spending a lot of time at that close up work like reading they start all needing glasses. The stereotype of the kid needing glasses being a “brain” and oriented towards academics has some basis in reality.
Myopia isn’t caused by a sudden mutation in the genes as much as by a sudden “mutation” in the environment.
Except… bacteria actually DO have sex.
At least a bunch of them do. They don’t have sex to reproduce, but they do exchange genetic materials via things like plasmids, which enables the same mixing and reshuffling of mutations that occurs in multi-cellular species like us. It’s just that we do it every time we reproduce whereas bacteria don’t require it to reproduce and indeed having sex and reproducing are two completely different things for them.
Now, our mitochondria are like what you describe - lots of cloning and since they’re inside our cells they don’t have a change to get jiggy with any mitochondria outside their clone. Yet someone we continue to muddle along, probably a half a billion or more years after mitochondria first moved into the cell(s) of our long-distant ancestor that gave rise to the eukaryotes. How do they prevent fatal degradation of their genome? Damnifino - I don’t think anyone else knows either but there seems to be some sort of mechanism for keeping them viable since we’re all still here. There are some mitochondrial disorders that can lead to early death so the error-correction isn’t perfect, either.