Myopia genetics: I’m not an expert, but it took me about 30 seconds to find this:
Note the last sentence, my bold.
What do you think is the probability that I don’t know this? Does it change the important point that I was making about the significance of sexual vs asexual reproduction?
Except, again, you are conflating genetic flaws with environmentally induced flaws.
Serious myopia, as you put it, is more an artifact of our schooling methods than the our genes. Color blindness is not necessarially a problem - many primates are “color blind” by human standards but have no problems surviving in the wild, and people who are colorblind, under certain conditions, actually see better than color normals. Granted those are a limited subset, but it’s not as crippling as people assume it to be. Weaker muscles? The history of the human race has been one long saga of us getting weaker - maybe we sacrificed brute strength for dexterity, or maybe intelligence and technology more than offset that disadvantage. Fact is, humans are pretty damn weak compared to our primate cousins yet they’re the ones on the endangered list and we’re over-running the planet. Flat feet may well be more a problem of our shoes than our genes - people who live their lives barefoot tend to have much stronger food muscles and tendons and fewer of the problems that keep podiatrists in business (of course, they are risk types of injury to their feet that shoe-wearing people don’t).
Prior to the late 19th Century it would have been rare for any hemophiliac to reach physical adulthood and pass on the gene.
Which can be surprisingly effective, actually - many people with a serious genetic defeat simply choose not to pass it on. They may remain childless, or they might adopt, or they might focus more on helping their nieces and nephews, but they elect to NOT pass on the gene of their own free will. There’s an organization that exists to help Jews at high risk of being Tay-Sachs carriers avoid marrying other Jews who are carriers. People who know they are a carrier for a defect sometimes choose preimplantation diagnosis as another way to avoid passing on the gene - access is limited by cost, unfortunately. And you might not know you and your mate are carriers until you have an affected child. But the point is, voluntary avoidance of passing the a bad trait on to the next generation is a real phenomena.
Type 1 used to be called “juvenile diabetes” for how commonly it struck children - who wouldn’t live to pass on the genes that affect it. That’s another one that’s, apparently, a fairly common spontaneous mutation.
The poodle is actually a hunting breed developed for retrieving waterfowl - that wooly coat actually provides the dog protection in cold, wet conditions. So, yeah, they need human tending, like most other specialized dog breeds, but they aren’t the wimps popular opinion makes them out to be.
And how does that contradict the studies that when a primarially pre-literate society converts to conventional schooling the next generation gets myopia?
Of course identical twins will both have the same tendencies for (or against) myopia, they share they same genes, but it’s the environment that is triggering the myopia, not the genes. Given a different environment - one that you can’t provide in western societies because it would be deemed child abuse - they wouldn’t become myopic. Put two twins boys in a pre-literate society where they spend all day at target practice and looking into the distance they might have perfect vision. Make them spend 6-8 hours in a small room, much of it reading books or screens, and the environment triggers the problem.
It’s somewhat analogous to how populations with a lot of intestinal parasites have very, very few food allergies but as soone as you eliminate those parasites the next generation starts getting allergic - the change in the environment is the main trigger for the problem. Of course, there’s a lot of negatives associated with intestinal parasites, they can be deadly just like food allergies can.
You didn’t mention it, so I assumed you didn’t know about it. Yes, it does change your point because it doesn’t require every other clone lineage of a bacteria to die off and just one take over for a mutation to spread through a population of bacteria, it just requires the bacteria with the mutation to have sex with other bacteria. It can then spread without reproduction, although of course in real life reproductive rate is also a factor.
Unlike multicellular critters which only exchange genetic material during reproduction and not at other times.
nm
I was drawing the distinction between sexual and asexual reproduction, and how natural selection under each system affects diversity in the rest of the genome. My original point was that recombination in sexual organisms means that overall diversity is not lost when one locus is subject to selection; I brought up asexual reproduction to highlight the difference.
Do you disagree that bacteria are the best common example of asexual evolution that we have? Conjugation is important, but most bacterial mutation, reproduction and evolution takes place asexually. Most bacterial mutations that arise do not find themselves fortuitously located on a plasmid or within a transposon that gets passed around by conjugation.
Mitochondria are interesting, but highly unusual.
Myopia is definitely a combination of genetic and environmental influences. It seems to be strongly influenced by the amount of natural light illumination during childhood. Singapore, having one of the highest myopia rates worldwide has done a lot of research on the issue: From BMJ
http://bjobeta.bmj.com/content/85/5/521.full
No one really proposes such a speedy genetical change; however the above paper also shows a significant genetic component. The interaction is still not understood but it may be similar to many afflictons of modern humans.
But what you are saying here is logically equivalent to: if we all lived in caves in pitch darkness, visual acuity would have no genetic basis.
In order for myopia to arise, certain environmental conditions must occur, and the requisite environmental conditions are presumably much more common in recent modern society. I don’t dispute that. We could reduce myopia by modifying the environment.
But that does not mean that myopia does not have a genetic basis. In order to determine whether it has a genetic basis, we ask: under current environmental conditions (in which myopia is prevalent), how much of the phenotypic variance is explained by genetic variance? That’s the final sentence that I highlighted in the extract from the paper - it is somewhere between 50% and 96% heritable. Thus we could (if we believed in eugenics!) reduce myopia in the modern environment rather easily by selective breeding. If we were talking about breeding dogs rather than humans, we really could do this of course.
ETA:
Broomstick, we’re focusing on a few relatively technical points of contention, but I should say, more importantly: in your response to md2000 above, I completely agree with your central point - the notion that humans are in any significant sense “genetically weakening” is misguided.
Looks like the effective population size for humanity as a whole is around 10,000.
Not only conjugation (bacterial sex) but also transformation (bacterium dies and breaks apart or living bacterium ejects genes, other bacteria suck up the genes) and transduction (virus picks up the genes out of one bacterium, inject them in another.)
(Link) ('Nother link.)
Not really. We certainly prefer perfect eyes, but I don’t think it actually matters much how we get them. Genetically-perfect eyes give the same results as eyes that are perfect as a result of technological intervention. The only reason why genetically-perfect eyes might be preferred is that it saves the cost of glasses, but glasses are incredibly cheap in the grand scheme of things, and there are many, many other genetic traits that make much more of a difference in cost-benefit analyses.
Now, it’s true that this depends on our current environment. And maybe our environment will change in such a way that it changes the relevant pressures. But that works both ways: Sure, maybe our tech level will plummet for some reason, such that we can no longer make glasses and those who need them remain blind. But on the other hand, maybe VR computer interfaces will take off, and those people used to having a piece of hardware mounted on their head will have an easier time working with them, and so wearing glasses will actually become a benefit over naturally-perfect eyes.
How much did vision count before we started reading and writing? Far sightedness may have been an advantage for hunters. Artisans were distinguishing writing, maybe they could hold an object close enough or far enough away from the their eyes or just squint to do what was needed. Assuming there is a genetic factor it doesn’t mean much for humans as long as there’s a mix of sight abilities, humans live in groups, we divide labor based on ability, we balance each person’s abilities and deficiencies, and we carry along our brethren past their point of utility. We as humans survive not only because of our individual genes but because as a species our genes are superior to the other species we compete against.
Just a few comments -
One, the early comment by Chronos
is most pertinent.
Human brains individually are smart but it was the creation of language and then written language allowed for the meta-organism of society to develop and to facilitate, even to catalyze, the accomplishments of human intellect, preserving and building on what was learned and invented before, including written language as an accomplishment itself. Thing is that a developing system of writing with many symbols requires a society of a certain level of complexity and developing a society of much more complexity requires a written language.
How much has the genetic underpinnings of brain function changed as an adaptation to life within societies since the development of agriculture and since the written word? Not sure there are definitive answers but certainly humans have changed genetically otherwise quite a bit.
Were early scribes more likely to reproduce more? Were those with a natural facility for reading and writing able to achieve greater reproductive success? Or were they sent to religious orders with traditions of celibacy? Knowing the answers to those sorts of questions within different early societies could at least help formulate some informed hypotheses to the op I think.
Also the idea of farming was apparently well practiced in early rain forest societies. Certain trees were cultivated and slash and burn techniques much more widely applied than is generally appreciated.
Agriculture as it developed within the Fertile Crescent and spread from there seems to have been a mix of the idea spreading and the people who did it migrating.
Finally, what is this “genetically-perfect” being referenced with eyes as an example? There is no such thing.
Eyes are part of a package with the organism functioning within its environment inclusive of the society (and the technologies of the society) in which the individual organism exists. Eyes well adapted for hunting and discriminating between which plants to gather were a vital feature selected upon for many thousands of years but do not hold to any Platonic ideal of “perfect” … The only ideal is defined by the environmental demands of the time. Within the current environment, one that is more defined by technologies and the use of them, that feature is no longer very important of an adaptive/fitness trait is all.
Current humans possibly being genetically less well adapted to survive if suddenly placed within an early hunter-gatherer community is not humans becoming genetically weakened or less perfect, any more than the ability to digest lactose and starch better is us becoming genetically stronger.
Yes, I agree. I rambled a bit across multiple posts above - my view is that there probably are real population genetic effects at a few loci like vision (although in retrospect that may not be a good example), but that it’s ultimately unlikely to be significant:
That’s well understood. But, like Broomstick, perhaps you take this “correction of a common misconception” a little too far to the opposite extreme. While the notion of absolute genetic perfection is mistaken, so is too extreme relativism: there are loss-of-function mutations with phenotypes with lower fitness in almost any conceivable environment; and the less harsh modern environment means that there may be some such loss-of-function mutations that undergo weaker purifying selection.
I think it’s a quantitative question addressed by population genetics - and we are certainly in agreement that any effect is not significant, and any purported overall “weakening of the gene pool” is a mistaken notion.
Pretty confident one cannot even say that they have a “lower fitness in almost any conceivable environment.”
Any possible genetic predisposition to myopia, as our specific example, is neutral on fitness in our “less harsh modern environment.” It becomes a tautology really. If any current environment creates a circumstance in which a trait/ability/predisposition is immaterial to the success of the genes associated with that trait being passed on to more of the next generations then neither presence or absence of that trait/ability/predisposition is more fit. A catfish that is adapted for no light cave living is not “less fit” for having loss of visual function. It is instead well adapted for its environment.
If a loss of function mutation has no significant impact on reproductive fitness in our current environment, one that obviously can be easily conceived of since it exists, then clearly it cannot be true that it has lower fitness in almost any conceivable environment.
There is no such thing as “purifying selection.”
ETA: The issue gets confusing to some I think because they forget that “fitness” in this context has a very specific meaning. If it is selected for it is more fitness; if no impact then it is neutral on fitness. No more complex than that.
The deeper point folks are wandering around but not quite saying is that high tech society is an artificial construct. One that might vanish in an evolutionary eyeblink. And they are uncomfortable with a species well-adapted to current high-tech life that would be ill-adapted to low-tech life.
About 24 hours ago I wrote a post for this thread that got lost before posting. Dumb guy me. In it I talked about the dichotomy between slow-changing environments, where selection favors highly adapted specialists who evolve slowly. Versus fast changing environments which favor A) generalists, and B) fast adaptors to environmental change. And how the optimal rigging for any given species would depend on which meta-environment they were placed in. The two meta-environments are close to polar opposites in terms of which evolutionary style they promote.
The integrating point of these two ideas is that technical society is slow-changing and lacks many of the traditional selecting pressures. (Albeit substituting new selecting pressures all its own. e.g. post 2000 nerds get laid *en masse * and presumably reproduce for the first time in millennia.)
Which means today is a stasis-type environment favoring specialists. The unstated, but deeply held, fear of technological collapse is all about worrying that we need generalists well adapted to primitive living to cope with the inevitable impending (on evolutionary timescales) collapse of tech & society.
This yin/yang is what’s driving all the handwringing.
But by the same token that technology could collapse quickly, it’s also been built up quickly. We don’t need to worry about our low-tech survival traits having been washed out by our high-tech environment, because we haven’t had our high-tech environment for long enough.