A queston on Human evolution

[Broomstick]

@guestchaz

bolding mine.
is there a variety of rhubarb I’m not familiar with? as a child, I would often pluck a stalk out of Gma’s rhubarb patch, break the leaf off, wash and then eat the stalk raw. It was the leaves that I was always told were inedible and poisonous.

Among other factors, domesticated rhubarb is less toxic than wild, and yes, the toxins are concentrated in the leaves. There are still residual amounts in the stalks which are (mostly) deactivated by cooking.

Bottom line: eating one raw stalk is unlikely to be a problem. Eating a lot of them could be, but virtually no one eats that much raw rhubarb.

@md2000

I specifically excluded Huntingdon’s and Type II Diabetes, as post-reproductive problems. High blood pressure may be an issue, but usually again has its most serious consequences after the children have left. (My maternal grandmother, I think, died from high blood pressure in the early 1950’s at about 55.)

Except Huntington’s does affect one’s reproductive success - those with the gene tend to have more children than those without. Type II diabetes doesn’t help you reproduce but if the traits that lead to that disease in our current environment can help you survive famine then, under those conditions, they might help you raise more children by surviving food shortages.

Another one is hemochromatosis, or genetic iron overload. Excessive iron levels damage vital organs like the liver and can lead to relatively early death (late 30’s or the 40’s). BUT - women with the gene almost never suffer negative effects until after menopause due to normal iron losses during menstruation and pregnancy. The trait might help women during their reproductive years, especially in areas where the diet is low in iron. And if they men die at 39? Well, that’s plenty of time to sire offspring, even if he’s not around to see them all the way to adulthood. By helping women survive their reproductive years the trait is not as negatively selected as you might assume.

A trait that impairs health but by one mechanism or another assists in reproductive success tends to persist.

A quick check, the busiest clinics in the USA did almost 100,000 IVF cases last year. This is hardly trivial.

At best IVF has a success rate of 30%. That’s not 30% per attempt, that’s 30% overall. Most couples need multiple attempts to get to the goal of a live birth. So fewer than 30,000 of those 100,000 attempts at IVF were successful. That’s a drop in the bucket compared to a world population of 7 billion. It’s not significant.

Now, if IVF became more successful/reliable/common that might become a problem but we’re a long, long, long way from that.

Given that at least some of those IVF attempts are done not to overcome infertility but to screen for genetic disorders and prevent the birth of people with them it’s arguably either neutral or a net positive (people of normal fertility who opt for IVF to avoid genetic problems tend to be more successful with fewer attempts, which probably shouldn’t be surprising as their issues aren’t fertility.)

There are some fascinating long-term risks to our present homo technophilia environment that could cause problems where gene drift would add more risk. There’s the whole health care industry looking after people who are unhealthy, for example. Will that become even more of a drag on the economy?

Again - there has not been enough time with high technology civilization to make a difference in the human gene pool. It requires many many generations in an environment to cause the sort of problem you bring up.

And I’m not going to argue that we should abandon the sick - which is what you’re suggesting with “There’s the whole health care industry looking after people who are unhealthy, for example. Will that become even more of a drag on the economy?”. First of all, “chronic illness” is not incompatible with “employed”. A LOT of people with birth defects, amputations, permanent injuries, chronic disorders, and genetic flaws work for a living and are a net gain for society. It’s not a binary situation of either perfect health or helpless invalid. Second, all those people employed by the healthcare industry contribute to the economy, and if you eliminated healthcare they’d all have to do something else or else they’d be an unemployed drag on the economy.

[LSLGuy]

@Riemann

It is Ronald Fisher. There’s a Wiki article on sex ratio:
Fisher's principle - Wikipedia

Thank you.

I recall now that we’ve discussed this before in the context of a post-apocalypse a la Dr. Strangelove where rebuilding the population would benefit from a skewed sex ratio. Which could be artificially introduced in the first couple of generations but would quickly revert to 50/50. Which scenarios always seem strangely more popular with the males than the females. One wonders why.

I hadn’t known the name or the details of the idea though.

[Riemann]

@LSLGuy

The mere fact hermaphrodism might (?) be more energy efficient globally doesn’t *require *that we passed through that region of genetic space some time in the past. Those species that did, are. Those that didn’t aren’t. The cause need be no more complex than that roll of the historical dice.

This is a good point, evolution can only explore what is possible. And even if something is possible, it may not be able to get there from here. Natural selection is myopic, it can only make incremental steps where every step is a point of higher fitness on the evolutionary landscape. Natural selection cannot drive evolution to a distant peak of higher fitness if that means crossing an intervening valley of lower fitness.

However, that limitation is not likely to be the reason for dioecy (sexual specialization, males and females). Hermaphroditism is common in plants and sessile animals. And in plants we see frequent evolution from hermaphroditism to dioecy and the reverse. So it seems that hermaphroditism probably was an evolutionary option, likely an intermediate stage in an ancestor of we dioecious animals, and that dioecy arose through evolutionary choice rather than necessity.

I agree with doubleminus that it’s a challenging problem. There’s an obvious drawback to self-fertilization (inbreeding), but that does not explain why we are not all outbreeding hermaphrodites. Imagine a hermaphrodite mutant in a population of specialist males and specialist females. The hermaphrodite could accept sperm from males to mother babies and also impregnate females with its own sperm. Bear in mind that the human case of high male parental investment in childcare is exceptional, in most animals the male is little more than a sperm donor. So the hermaphrodite would appear to have an immense advantage, producing twice as many offspring.

As I recall there’s a clear non-technical account of the most compelling proposed solution in The Red Queen by Matt Ridley. I’ll try to summarize the key ideas tomorrow, but I need fresh brain.

[md2000]

As I said - that’s the $64,000 question - how quickly will detrimental genes diffuse through the general population to the point where a significant proportion of the population is affected? The answer , it seems, is - it’s a matter of debate and we don’t know, so let’s leave it at that.

BTW, those 30,000 successes at IVF are not over 7 billion; that’s in one year, for a USA population of 340M; so about 0.01% of the population each year; assuming the trend continues, assuming say 50 or 60 year replacement cycle, say about half a percent of the population conceived by IVF. That’s with a procedure so expensive that it’s probably out of reach of a significant portion of the population. What happens when it isn’t?

Yes, there are traits that may have been beneficial in early times when chronic scarcity was the bigger problem, that are now detrimental. According to Jared Diamond, high blood pressure from salt is one such problem. I read a suggestion once that “berserker” personality was a benefit for warrior HG societies, but in more settled societies it was a negative trait as disrupting law and order, and so on.

Death in your 40’s may be irrelevant to breeding success nowadays, but keep in mind for HG and early agricultural societies, children whose parents died before they matured may not have fared a well.

Further on the cost-benefit issues - keep in mind that generally, any beneficial (genetic) attribute has a cost as well. Big muscles need more food; big brains need more food; as race horses show, a physique designed for speed means that bones are as light and thin as the organism can get away with, meaning more risk of fractures; we see the same effect in very tall people and athletes. Along these lines, hermaphrodite is a significant cost. Given the amount of food needed to maintain two sets of organs, the double risk of complications associated, etc. - where’s the benefit? Very rarely does a society find itself with a shortage of one or the other (China and India nowadays may be exceptions), so specialization allows for finer tuning for either role. HG women were not expected to go into combat or strenuous hunting while pregnant or nursing, while men evolvd the physique to meet these challenges to the benefit of both. The ones who need to be physically strong for the greater good do not shed iron every month. And so on… I’m suspecting this is why, with a very few mutational exceptions, vertebrates specialize in sex roles, especially mammals where the cost of the role is highest.

[doubleminus]

@Riemann

Imagine a hermaphrodite mutant in a population of specialist males and specialist females. The hermaphrodite could accept sperm from males to mother babies and also impregnate females with its own sperm. Bear in mind that the human case of high male parental investment in childcare is exceptional, in most animals the male is little more than a sperm donor. So the hermaphrodite would appear to have an immense advantage, producing twice as many offspring.

It’s even worse than that. You have the bigger individuals (most mammal species have bigger males than females) consuming most of the food, killing or maiming each other, killing the offspring of other males and really doing nothing “productive”, besides perhaps lining up to be chosen by the picky female. Also, in most species you have a huge variance of reproductive results of different males: you may have a few males monopolizing harems, and other that hardly reproduce at all.

IMHO, this has a lot to do with sexual selection, which is a much faster way of moving through evolutionary space than purely survival statistics. Basically, the intelligence of the female is used to assess the fitness of the male and implicitly predict the fitness of the offsprings.

This has also to do with The Handicap Principle that tries to explain apparent misadaptations such as the peacock’s tail. (“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick,” Darwin wrote)

[md2000]

But even without hermaphrodites double-teaming the reproductive cycle, most species produce more than enough offspring to keep Malthus happy. Specialization must have its benefits.

[doubleminus]

@Riemann

This is a good point, evolution can only explore what is possible. And even if something is possible, it may not be able to get there from here. Natural selection is myopic, it can only make incremental steps where every step is a point of higher fitness on the evolutionary landscape. Natural selection cannot drive evolution to a distant peak of higher fitness if that means crossing an intervening valley of lower fitness.

Yes, I understand this is the received wisdom, and is also a good argument for the IDs and the creationists, but I doubt it is entirely true.

If you look at optimization algorithms, you see that, in addition to following a continuous path to local maxima, you will have randomizations throwing you away into distant territory. Sometimes you just have to cross the valley. I believe evolution learned these tricks.

You have for instance the mutation rate. It is the organism that “chooses” what it will be and how powerful the DNA repair mechanisms will be. If you have a great enough population you (well the pseudo you in evolution) may risk a high mutation rate even if most results are destructive. If you look at microbial adaptation to, say, antibiotics or cancer cell adaptation to all kind of wrenches thrown into their wheels, the resistance to those agents cannot be gradual, or at most may contain a very small number of steps.

And there is perhaps the biggest question of it all: how did the DNA/RNA evolve gradually? Many tons of ink have been written on different hypotheses, but even if you start with simpler replicators, the information content necessary seems to be so great that it is hard to see how it can be due to chance only; on the other hand there is a need for a minimal complexity. This is one take on the “Fermi Paradox”. Maybe we are alone due to the extreme improbability of randomly creating sophisticated enough replicators.

[doubleminus]

@md2000

But even without hermaphrodites double-teaming the reproductive cycle, most species produce more than enough offspring to keep Malthus happy. Specialization must have its benefits.

When you see exquisite adaptations increasing the survival rate by fractions of a percent versus the extreme wastefulness of male/female specialization (especially since if there are 2 sexes there must be an equal number of individuals of each), you start thinking what is so useful about males to compensate for their abysmal behaviour. :rolleyes:

[Riemann]

@doubleminus

…is also a good argument for the IDs and the creationists, but I doubt it is entirely true…

You’re thinking of the “what good is half an eye” ID argument? Yes, we need to demonstrate that there are a series of intermediates, each with increased fitness, or that could plausibly be reached from a prior step (as you suggest) without natural selection by chance mutation alone.

But the “myopia of natural selection” is one of the strongest arguments against ID. I’m sure you’re familiar with outrageously circuitous path of the recurrent laryngeal nerve in the giraffe, the antithesis of “intelligent” design:

Richard Dawkins demonstrates laryngeal nerve of the giraffe

[doubleminus]

[QUOTE=Riemann;19814909
But the “myopia of natural selection” is one of the strongest arguments against ID. I’m sure you’re familiar with outrageously circuitous path of the recurrent laryngeal nerve in the giraffe, the antithesis of “intelligent” design:

Richard Dawkins demonstrates laryngeal nerve of the giraffe

Sorry, this was my bad English. When I said “for the ID, etc” I meant “to be served to…”, so, yeah, of course.

And the Giraffe thing is amazing, I was not aware of it. My field is really algorithms and cryptography, I dabbled in evolution when dealing with genetic algorithms. That spurred my interest long time ago. I read the popular books, they are so well written - but my knowledge in biology/zoology is amateurish at best.

[DSeid]

Re crossing a negative fitness divide - correct me if I am wrong but while distant peaks may be improbable crossing small valleys from one local fitness peak to another do happen. Not every step of the path needs to be to higher fitness.

“Exaptations” or “spandrels” may be pertinent. A feature may exist not because it has positive fitness impact selected for in and of itself but as a consequence of a trait that does. It may spread even as it would itself, if it was by itself, be neutral or even weakly negative on fitness, until eventually another mutation occur that takes advantage of that trait in a novel way, making it something to select for.

And of course as has been discussed populations populations will also often carry sizable amounts of genotypes and phenotypes that would be by themselves subject to negative selection pressure in the environment of the time but are not eliminated, may even become fixed … due to weak negative selection and/or just random events or due to a founder effect and so on. It is possible to then become beneficial upon further modification of that by itself of negative impact gene, or by modification of another gene that has differential impact with that mutation present than when there was the previous form, a modification that would have been deleterious in and of itself in isolation, but that is beneficial in the presence of the other weakly deleterious but tolerated mutation - “sign epistasis”.

One explanation of such escape paths from one peak to another through a valley:

Fitness interactions between loci in the genome, or epistasis, can result in mutations that are individually deleterious but jointly beneficial. Such epistasis gives rise to multiple peaks on the genotypic fitness landscape. The problem of evolutionary escape from such local peaks has been a central problem of evolutionary genetics for at least 75 years. … valley genotypes can also be subject to mutation while transiently segregating, giving rise to copies of the high fitness escape genotype carrying the jointly beneficial mutations. In the absence of genetic recombination, these mutations may then fix simultaneously. The time for this process declines sharply with increasing population size, and it eventually comes to dominate evolutionary behavior.

Not trying to raise special relativity here; it is just pertinent the question. Reimann I am sure can explain in more detail.

In case anyone is interested in a subject close that which the op was originally after, this 2010 NYT article may be of interest: human culture as both the result of evolution and as a driver of it.

As with any other species, human populations are shaped by the usual forces of natural selection, like famine, disease or climate. A new force is now coming into focus. It is one with a surprising implication — that for the last 20,000 years or so, people have inadvertently been shaping their own evolution.

The force is human culture, broadly defined as any learned behavior, including technology. The evidence of its activity is the more surprising because culture has long seemed to play just the opposite role. Biologists have seen it as a shield that protects people from the full force of other selective pressures, since clothes and shelter dull the bite of cold and farming helps build surpluses to ride out famine.

Because of this buffering action, culture was thought to have blunted the rate of human evolution, or even brought it to a halt, in the distant past. Many biologists are now seeing the role of culture in a quite different light.

Although it does shield people from other forces, culture itself seems to be a powerful force of natural selection. People adapt genetically to sustained cultural changes, like new diets. And this interaction works more quickly than other selective forces, “leading some practitioners to argue that gene-culture co-evolution could be the dominant mode of human evolution,” Kevin N. Laland and colleagues wrote in the February issue of Nature Reviews Genetics. …

… This instance of gene-culture interaction turns out to be far from unique. In the last few years, biologists have been able to scan the whole human genome for the signatures of genes undergoing selection. Such a signature is formed when one version of a gene becomes more common than other versions because its owners are leaving more surviving offspring. From the evidence of the scans, up to 10 percent of the genome — some 2,000 genes — shows signs of being under selective pressure.

These pressures are all recent, in evolutionary terms — most probably dating from around 10,000 to 20,000 years ago, in the view of Mark Stoneking, a geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Biologists can infer the reason for these selective forces from the kinds of genes that are tagged by the genome scans. The roles of most of the 20,000 or so genes in the human genome are still poorly understood …

… Many genes for taste and smell show signs of selective pressure, perhaps reflecting the change in foodstuffs as people moved from nomadic to sedentary existence. Another group under pressure is that of genes that affect the growth of bone. These could reflect the declining weight of the human skeleton that seems to have accompanied the switch to settled life, which started some 15,000 years ago.

A third group of selected genes affects brain function. The role of these genes is unknown …

… Mathematical models of gene-culture interaction suggest that this form of natural selection can be particularly rapid. Culture has become a force of natural selection, and if it should prove to be a major one, then human evolution may be accelerating as people adapt to pressures of their own creation.

[md2000]

Yes, that cultural selection kind of goes along with what I said about a “berserker” gene - useful in more anarchic situations like warrior-centric HG societies; but a detriment to peace, order, and good government in more settled societies. Plus, the more physically aggressive and uncontrolled types are more likely to either end up executed, imprisoned, or in armies where they could die early.

[John_Mace]

How does a H/G tribe benefit from having a berserker as part of its group? If anything, I’d think it would worse in a small society of non-specialists who rarely encounter hostile groups but who have to live intimately together and depend on each and every member of the group to help the group survive.

[DSeid]

Also not so sure that a berserker gene would be useful even in a hypothetical warrior tribe. But it does raise a closely related subject - punishers.

Cooperation between individuals that are not closely genetically related happens in even very simple societies. Some research has been focused on how altruism may be selected for but some has also been focused on how cooperation is contingent upon having some number of members of the group being willing to be the punishers of cheaters and freeloaders, even though punishing is a costly behavior that benefits all. If the willingness to engage in punisher behavior has a genetic basis how is it selected for when it is on its face behavior that is likely to reduce fitness by virtue of its cost (including risk of death or injury during the act of attempting to punish)?

It’s not immediately straightforward. The answer seems to require either selection at the level of the group (cultural selection) or if directly then by including punishers making probablistic decisions on when to punish based on how many others have also signalled a willingness to punish, the impact of punishment being not only to decrease the fitness of cheats and freeloaders but more to rehabilitate them for future cooperation, and the ability to partipate some across groups.

I also wonder if a mechanism might work similar to that proposed by some for costly displays by males competing for females … the display advertises that one has enough health and resources to be able to afford the costly display … and thus even though costing health and resource reserve attracts mates.

[doubleminus]

Cooperation vs cheating in groups has been modelled by the *iterated prisoner dilemma * game. Tournaments have been played with different strategies. To the first approximation, the best strategy is tit for tat. Which means: start cooperating and then copy the move of your opponent. You may be especially forgiving and after a loop of both sides defecting, you may try to cooperate again.

[DSeid]

With some trepidation I would like to again try to have a discussion about what I find objectionable about the words and phrases used to describe some very basic concepts in population genetics. I am hoping that I can articulate my thoughts clearly enough that they are not misunderstood as attacking the validity of a few basic concepts. I recognize that the words are indeed the words used by the academics in the field and that the concepts themselves are valid. My comments are that the words used to represent the concepts misrepresent to what the concepts actually are when heard by many of us outside that academic discipline.

Let’s start with just one: “purifying selection.” All it means is the process of genes being selected out based on factors other than random factors and chance: negative selection pressure.

I took population genetics in college and was exposed to it some in med school (that would be late 70s) and have no recollection of the term. Looking it up on Google ngram it is easy to understand why: it was rarely used at that time and only relatively took off in the late 90s and into the 2000s. As that nGram illustrates “negative selection pressure” was the preferred term in my ancient era. Simply directional, pressure to decrease the number as opposed as pressure to increase the number. Not better (beneficial) or worse (deleterious) and certainly not “pure” (which can only be in comparison to “impure”)

Now to my ear the phrase “purifying selection” has several problems. First (to my ear, and assume that all that follows is to my ear) it inadvertently implies an intent or goal of evolution to create some pure version of a species. Of course evolution has no intent and has no goal, and of course there is no pure version of a species. Second and perhaps more viscerally it is redolent of eugenic implications. At the risk of Godwinizing it sounds Aryanish. If it was a historical artifact of the pre-WW2 era I could understand, but it took off only in the last two decades. To me it at best reflects that many population genetics academicians function in a space of their own.

This post of course depends on the definition of purifying selection as synonymous with negative selection, as multiple sources (see here* for example) state, being accurate. But of even that I am not sure when I see this article.

The study focuses exclusively on the type of evolution known as purifying selection, which favors mutations that have no or only a small effect in a fixed environment. This is in contrast to adaptation, in which mutations are selected if they increase an organism’s fitness in a new environment. Purifying selection is by far the more common type of selection.

“It’s the simplest, most boring type of evolution you can imagine,” Plotkin said. “Purifying selection is just asking an organism to do what it’s doing and keep doing it well.” …

… This result — that later mutations were dependent on the earlier ones — demonstrates a feature known as contingency. In other words, mutations that are accepted by evolution are contingent upon previous mutations to ameliorate their effects.

The researchers then asked a distinct, converse question: whether it is possible to revert an earlier mutation and still maintain the protein’s stability. They found that the answer was no. Mutations became “entrenched” and increasingly difficult to revert as time went on without having a destabilizing effect on the protein.

“At each point in time, if you make a substitution, you wouldn’t see a large change in stabilization,” Shah said. “But, after a certain number of changes to the protein, if you go back and try to revert the earlier change, the protein structure begins to collapse.”

The concepts of contingency and entrenchment were well known to be present in adaptive evolution, but it came as a surprise to the researchers to find them under purifying selection.

“We thought we would just try this with purifying selection and see what happened and were surprised to see how much contingency and entrenchment occurs,” Plotkin said. “What this tells us is that, in a deep sense, evolution is unpredictable and in some sense irreversible because of interactions between mutations.”

Such interactions, when the effect of a mutation is dependent on another, are known as epistasis. The researchers’ investigation found that, unexpectedly, purifying selection enriches for epistatic mutations as opposed to mutations that are simply additive. Plotkin explained that this is because purifying selection favors mutations that have a small effect.

So not sure if the phrase always means the same thing.

*“Because more DNA changes are harmful than are beneficial, negative selection plays an important role in maintaining the long-term stability of biological structures by removing deleterious mutations. Thus, negative selection is sometimes also called purifying selection”

[DSeid]

@doubleminus

Cooperation vs cheating in groups has been modelled by the *iterated prisoner dilemma * game. Tournaments have been played with different strategies. To the first approximation, the best strategy is tit for tat. Which means: start cooperating and then copy the move of your opponent. You may be especially forgiving and after a loop of both sides defecting, you may try to cooperate again.

Again though, specifically talking about the evolution not of cooperation so much as of those traits that create those willing to punish at potential cost to themselves and at some benefit to the entire group, some of whom are unrelated.

[LSLGuy]

@DSeid

Again though, specifically talking about the evolution not of cooperation so much as of those traits that create those willing to punish at potential cost to themselves and at some benefit to the entire group, some of whom are unrelated.

Totally NOT my field of expertise. IMO …

But once you have a small-group or tribal society, then the unit of evolution becomes your band, not yourself. Those bands that had some punishers who (for whatever random reason they arose) were willing to enact tit-for-tat did better than those bands who did not. Bands that culled their punishers (or never had any) lost out over the long term.

It’s accepted that organisms exist as a means for genes to compete and propagate themselves forwards through time. Or at a minimum it’s accepted that it’s a reasonable lens through which to view the situation. It provides some useful insight, if not the total picture.

Given that, IMO in a real sense human society exists as a means for organisms to propagate themselves forward through time.

Said another way, if it’s sensible to talk of an organism as a bag of disparate genes with differing goals and “behaviors” which end up unwittingly cooperating to succeed as a group (of genes), you’d expect the same dynamic to obtain on the next level up. I.e. it would be sensible to talk of a social unit as a bag of disparate people with differing goals and behaviors which end up unwittingly cooperating to succeed as a group (of people).

[John_Mace]

@doubleminus

Cooperation vs cheating in groups has been modelled by the *iterated prisoner dilemma * game. Tournaments have been played with different strategies. To the first approximation, the best strategy is tit for tat. Which means: start cooperating and then copy the move of your opponent. You may be especially forgiving and after a loop of both sides defecting, you may try to cooperate again.

“First approximation” is not really the right term to use here as that generally refers to the first mathematical approximation (often a linear equation). While some strategies, like tit for tat, do well in repeated competition, the strength of a given strategy is also highly dependent on what other strategies are out there. In short, the iterative PD is interesting, but we don’t know how well it models the real world. You can shift things dramatically in the game by changing just a few parameters.

[doubleminus]

Insofar as I remember Axelrod’s paper, TfT won the tournament against all other much more sophisticated algorithms, and it continued to do so even at a repeat tournament. Only significantly later it had to be slightly tweaked. I may be wrong though, didn’t check.