A Question: Evolution and Over-Population

[Sweetie]

I was just thinking about human behavior, particularily the question of murder and how that would apply to the case of over-population. It led me to the thought though of evolution, nature taking care of itself. It seems that where there is too little of something nature needs, something else rises up to balance as much as nature is capable which would not be the case in such instances as large amounts of pollution which nature does not have time to digest, man against nature, for example. Where there is too much, something suffers or becomes less so nature can become balanced again.

So, an imbalance comes to mind, over-population. Now, for years we have been hearing news reports of lower sperm counts in men and things of that nature, though I think it's mostly attributed to pollution, if I can recall correctly, pollution and things of the nature of pesticides in foods and other chemicals.

My question is really, is there anything we would actually physically need to do or should do to stop overpopulation, or if left alone to the winds of evolution, would nature in this instance, take care of itself? Low-sperm counts maybe, or perhaps the nature of the beast (the human being) where there are too many people, go a little bit mad, start wars or something else along those lines or a dehumanizing or insensitive to humanity at large because there are just too many humans to deal with. I wouldn't assume even for the sake of arguement that the Earth itself could perceive over-population and thence cause natural disasters on a large scale to balance it out, protect itself. It has not perceived it as regards to pollution so I wouldn't think so.

But it just seems to me that evolution and nature should be able to take care of itself in this regard without direct human influence, in the sense of Asian countries perhaps, that they may only have two children and forced sterilization and things of that nature. It could be that our evolutionary instinct to survive and not die out could trump any other evolutionary consideration as far as blind but balanced nature is concerned.

I don't know, just a thought. I've never read anybody say things of this nature before, so if anybody knows of any essays or anything, I would like to read them just out of interest.

Any thoughts?

[Socratoad]

If humans where not destroying nature at quite so fast a rate then its quite feasible that by evolutionary process we would evolve into a mammal that would indeed be able to compensate for scarcity or other such pressures by radically lower sperm counts, actual longer gestation periods and a whole host of other such things. Other creatures have developed just such biological survival mechanisms so of course its possible. But man is when all is said and done is the one animal that craps in its own nest, so to speak. our rapid development of technology, our inability to understand or respect nature do to our removal from same does not give me a hell of a lot of hope. Increasingly due to our technological oriented lives I cannot help but despair that evolution has time to save us from ourselves.

Just a couple of thoughts off the top of my head. ... not deep ....not scientific ...... perhaps connected to moral philosophy.

[wei yau]

I have to admit to a very rudimentary understanding of evolution. That being said, I don't think evolution will work in the way described in the OP. As I understand it, evolution is the process in which certain traits which best enable an individual to survive are passed down from one generation to another. The overall affect on the species is that those individual without these traits are most likely to die off without offspring and those that do have these traits will most likely live on to procreate.

As to how this relates to human overpopulation, I'm not sure. The traits needed to survive have to be able to handle the consequences of overpopulation. However, as Socratoad has pointed out, man has the ability to adapt the environment to his needs, as opposed to having no choice but to adapt to the environment.

I suppose that some of the consequences of overpopulation such as famine or disease can be a force to shape the evolution of man. Those with stronger immune systems or more efficient metabolisms might better survive the dire consequences of extreme overpopulation.

In the most extreme cases, overpopulation is a problem that can right itself. Ultimately, the population will be reduced through famine, disease and the inevitable wars fought over scarce resources. But, I don't see this as evolution...as I understand it.

But, I'm largely ignorant of evolution and am willing to be corrected.

[wade-w]

Eldar is on the right track here. Evolution is usually defined as "A change in allele frequencies in a population over time."

Evolution is driven by random mutation and natural selection. The primary force reason this happens is the famous "survival of the fittest." I never cared for that phrase; it's misleading. A better way to say what is meant by that is "differential reproduction." The long term survival of any particular organism in the population doesn't matter in the long run; what counts is how many offspring the organism has. IOW, those organisms who have the most offspring have the most influence on the gene pool of the next generation. It's hard to see how any mutation that would lead to lower populations could be successful in light of this.

[The Lone Ranger]

Back in the 60s, researchers such as J. B. Calhoun studied the effects of overpopulation on mice and rats. What they discovered was that rats in overcrowded environments developed an awful lot of “abnormal” behaviors.

They tended to become much more aggressive, for instance. Many of them, both male and female, lost interest in mating, while others, paradoxically, became hypersexual. Mature rats often attacked and killed younger ones. In very crowded conditions, females would often abandon and sometimes even kill their own offspring. Young males often organized themselves into “gangs” and would chase down and “rape” females. Even when food and water were present in sufficient supply, individuals would fight viciously – sometimes even to the death – for access to them.

In short, their behaviors became quite pathological, and one effect was that birth rates (or at least, survival rates of the young) plummeted, until the population density became fairly low – at which point they reverted to their “normal” behavior.


It’s awfully tempting to draw 2 lessons from these studies (hint: one of them is dubious at best, and the other is flat-out wrong). First: these studies demonstrate that overcrowding leads to pathological behaviors, and explains the antisocial behaviors that many of us associate with the stress of living in crowded urban areas. Rape, murder, child abandonment, etc. – all of these are pathological behaviors triggered by the stress of overcrowding. Second: these studies demonstrate that species have specific, evolved mechanisms to prevent overcrowding, so that they don’t wind up extinguishing themselves through using up all their resources.

The first lesson is not entirely unworthy of consideration. It does seem clear, after all, that the stress of overcrowding can lead to all sorts of bizarre and even pathological behaviors, after all. If for no other reason then, the studies have provided us some useful information. It’s dangerous to extend these lessons gained from laboratory rats to human populations, however.

The second lesson sounds quite plausible to most people. So, what we need to do is spend some time discussing why it is not – indeed, cannot – be true. Species do not have built-in mechanisms that have evolved for the purpose of preventing overpopulation.


What drives adaptive evolution? It’s generally agreed that the main mechanism is natural selection – those individuals who’re best-adapted to their environment are the ones most-likely to pass on their genes. The problem here is that natural selection would never favor the evolution of behavior that involved the individual sacrificing his or her genetic future for the benefit of the group. This would be a form of “group selection,” and group selection (in this form – there are forms that can work) simply cannot work.

Okay, so we’ve got a very dense population. If the population gets much denser, individuals will find themselves unable to get enough food (or living space, or water, or shelter, or whatever else it is that’s in short supply). At this point then, it’s to the advantage of the group as a whole for individuals to lower their reproductive output, so that the population’s density goes no higher. The problem, though, is that it is most-definitely not in the best genetic interest of any given individual to lower his or her reproductive output. After all, if “everyone else” is lowering their reproductive output, but you maintain or even increase your own, your genes will tend to dominate the gene pool. And that’s what natural selection’s all about. So, in short, natural selection would never favor such a strategy, even though it’s in the best interest of the group, because it’s not in the best interest of the individual. It’s the “tragedy of the commons” as applied to reproductive behavior. Natural selection would actually “punish” individuals who behaved “prudently” in this case, because they’d fail to contribute their genes to future generations – so, any genetic propensity toward “selfless” behavior would quickly disappear from the population, because “selfish” individuals will certainly out-reproduce “selfless” individuals.

[N.B.: I'm not saying that natural selection never favors the evolution of "altruistic" behavior – there are situations in which is most-certainly does – but only when it's to the benefit of the individual to behave "altruistically."]



Yes, birth and survival rates tend to plummet when a population becomes too crowded, but this is not because of any evolved mechanisms to prevent overpopulation. It is because of pathological behavior that is symptomatic of organisms’ inability to deal with the stresses caused by overcrowding.



If humans where not destroying nature at quite so fast a rate then its quite feasible that by evolutionary process we would evolve into a mammal that would indeed be able to compensate for scarcity or other such pressures by radically lower sperm counts, actual longer gestation periods and a whole host of other such things. Other creatures have developed just such biological survival mechanisms so of course its possible.
Sadly, that does not, and cannot happen.

Animals do evolve longer gestation periods, lower birth rates, and so forth under the proper circumstances – but not as a means of preventing overpopulation.

What matters from an evolutionary perspective is the number of surviving offspring you produce. Organisms will adopt whatever “strategy” works under their given circumstances. In some circumstances, the “best strategy” is to produce lots of offspring – they’ll not be as healthy on average, so their life-expectancies will be low, but if you produce enough of them, some will survive. This is the strategy adopted by salmon and most insects. The opposite extreme is to produce a very small number of offspring over a prolonged period of time, but to ensure that those offspring are provided with every chance of survival – such offspring will generally be healthier and much more likely to survive to adulthood. This is the strategy of elephants and humans. Despite the fact that an individual salmon might produce thousands of offspring in her lifetime, and an elephant fewer than a dozen, the average elephant probably has more surviving offspring than does the average salmon, because the elephant mother invests so much more in ensuring that her offspring survives.


But man is when all is said and done is the one animal that craps in its own nest, so to speak.
Not at all. Time after time, it has been shown that when organisms are introduced into environments where they are not native – where they are not limited by the predators, competitors, and parasites with which they evolved – that their populations explode until massive starvation and disease outbreaks either force the populations back down to manageable sizes or they all die.

The reason we don’t normally see populations eating themselves into extinction or killing themselves off from their own accumulated wastes is because predators, parasites, and competitors prevent this from happening. Nevertheless, it happens quite often, as all population biologists are well-aware. Evolution simply has not provided species with any mechanisms to prevent this, and so if predators, starvation and disease don’t suffice to keep their densities sufficiently low, the populations eventually render themselves extinct as a result of their own “success.” It’s as simple as that.




In the most extreme cases, overpopulation is a problem that can right itself. Ultimately, the population will be reduced through famine, disease and the inevitable wars fought over scarce resources. But, I don't see this as evolution...as I understand it.
You’re correct – this is not evolution.

Ultimately, you’re right inasmuch as overpopulation is a “self-correcting” problem. The world’s resources are necessarily limited. This places a ceiling on how large our population can grow. Eventually, famine, disease, and scarce resources will take their toll. What often happens is that a fast-growing population “overshoots” the environment’s “carrying capacity,” and then starvation and disease set in on a huge scale, leading to a population “crash” in which huge numbers die in a short time and the population is reduced to a much lower number – sometimes zero.

Let us hope that, as a species, we are wise enough to see the danger in time and take action to prevent this.

Cheers,

Michael

[livius drusus]

Fascinating post, Michael. Thank you. :thankee:

[viscousmemories]

The long term survival of any particular organism in the population doesn't matter in the long run; what counts is how many offspring the organism has.
Which amuses me to no end when I consider that my siblings - some of whom may not even believe in evolution - are far more 'fit' than I in those terms.

[Socratoad]

Michael, methinks you misunderstand the short blurb I posted. Seeing that evolution takes place over such a vast expanse of time and that all of life started out as single celled entities or even less developed then certainly what I indicated is possible.

Every living thing on the planet is an adaption. I realize that the polar bear and the scorpion are related is a concept for most of us cannot get our heads around. Either one believes in evolution or one does not, methinks.

By saying that man is the only creature that craps in its own next I was merely pointing out in the very very long run, in that our use/misuse of resources may very well not even begin to give us, and by extension the other lifeforms on the planet time to adapt/evolve ....... perhaps into creatures totally unimaginable to our present understanding.

In short, we may very well be the last example of evolution. Methinks man's arrogance in thinking that we humans can control the future with our knowledge of science and by manipulation of genes, chemistry, etc probably will bring it all to a crashing halt as we know it at least.

Such concepts as I have in mind certainly are incapable of being proved in one way or the other in labs. Gestation periods changing ? why not? I simply don't think we have the time.
If the feline species and many others have evolved into creatures that can and do reabsorb fetuses during times of stress, or avian species only breed every second or third year, if amphibian species are capable of burrowing into the sand and lying dormant for several years, then what I'm suggesting cannot be dismissed as being a false understanding of evolution. :yup:

I submit that evolution is almost beyond the human powers of conception, sort of like the national dept

PS: Of course after we fuck it all up, the long slow process of evolution will start all over again. See ya again in a mere few hundred million years. :wave:

[viscousmemories]

Michael, methinks you misunderstand the short blurb I posted. Seeing that evolution takes place over such a vast expanse of time and that all of life started out as single celled entities or even less developed then certainly what I indicated is possible.

Every living thing on the planet is an adaption. I realize that the polar bear and the scorpion are related is a concept for most of us cannot get our heads around. Either one believes in evolution or one does not, methinks.
If I understood Michael's point correctly it wasn't that it was impossible for such changes to occur, but rather because population growth is an evolutionary imperative it's impossible for an organism to evolve to compensate for overpopulation. I'm pretty sure it's the same basic point wade made above: Evolution is all about producing as many offspring as possible, diaper shortages be damned. So organisms simply can't evolve to under-produce offspring. At least that's what I think they're saying.

[wade-w]

That's the gist of it, vm. In biology, "fitness" means producing viable offspring who in turn produce more offspring, and so on. Evolution can only work on what's already there, so a species that evolves in an environment where resources are already scarce will be more likely to have adaptations like the one Socratoad cites. It all depends on what evolutionary pressures are present and what material there is to work on.

And Socratoad, I think you'll find that Michael understands quite well that a scorpion and a polar bear are related.

[Socratoad]

That's the gist of it, vm. In biology, "fitness" means producing viable offspring who in turn produce more offspring, and so on. Evolution can only work on what's already there, so a species that evolves in an environment where resources are already scarce will be more likely to have adaptations like the one Socratoad cites. It all depends on what evolutionary pressures are present and what material there is to work on.

And Socratoad, I think you'll find that Michael understands quite well that a scorpion and a polar bear are related.

Although I addressed my PM to Michael, perhaps that was a mistake, as I meant our species in general generally being incapable of grasping the vastness of evolutionary concepts.

Yes I'm well aware that Michael is aware that scorpions and polar bears are related. His mother certainly did not raise a fool.

Methinks I better steer clear of threads such as this, as I understand fully well what I'm saying ...... problem is nobody else does

[The Lone Ranger]

If I understood Michael's point correctly it wasn't that it was impossible for such changes to occur, but rather because population growth is an evolutionary imperative it's impossible for an organism to evolve to compensate for overpopulation. I'm pretty sure it's the same basic point wade made above: Evolution is all about producing as many offspring as possible, diaper shortages be damned. So organisms simply can't evolve to under-produce offspring. At least that's what I think they're saying.

Precisely.

It's not that it wouldn't be wonderful if it happened -- it's simply that natural selection functions in such a way that organisms can't evolve to "under-produce" offspring. Lowered birth-rates due to overcrowding are pathologies, not adaptations.

None of this means that there aren't circumstances in which having fewer (but better-protected) offspring is advantageous to the individual, but there are very few circumstances under which natural selection would ever favor an adaptation that causes an individual to have fewer offspring than its competitors.



Yes I'm well aware that Michael is aware that scorpions and polar bears are related. His mother certainly did not raise a fool.
More to the point, I'm an evolutionary biologist; I like to think I have some grasp of the finer points of evolutionary theory and the logic of natural selection.


In short, we may very well be the last example of evolution. Methinks man's arrogance in thinking that we humans can control the future with our knowledge of science and by manipulation of genes, chemistry, etc probably will bring it all to a crashing halt as we know it at least.
I'm the last person to argue that our ignorance and arrogance won't prove disastrous, but we couldn't sterilize the planet even if we wanted to. We'd exterminate ourselves in the process long before we could wipe out all the insects and such -- much less bacteria. Life will survive anything we can dish out, even if we do wipe out 90% of species (including Homo sapiens) in our foolishness.


If the feline species and many others have evolved into creatures that can and do reabsorb fetuses during times of stress, or avian species only breed every second or third year, if amphibian species are capable of burrowing into the sand and lying dormant for several years, then what I'm suggesting cannot be dismissed as being a false understanding of evolution.
The point I was making is that this happens only when it is in the best genetic interests of the individual. Whether or not it's in the best interest of the population as a whole is irrelevant to natural selection. If it's in the individual's best interest to produce fewer offspring (as is the case when resources are limited and pregnancy would endanger the mother's chances of survival, making it in her best interest to resorb the fetus and wait 'til better times to get pregnant), it will happen. Unfortunately, since natural selection will "punish" individuals who behave "prudently" when their competitors do not, such behaviors cannot evolve as a means of population control.

Cheers,

Michael

[JoeP]

Michael, I’m going to challenge you fearlessly despite you being an evolutionary biologist. :cool:

Species do not have built-in mechanisms that have evolved for the purpose of preventing overpopulation.

I offer a counterexample. Take it as a black box; don't tell me "selection only works at the individual level". What happens if population A has a mechanism that limits population growth when there's overcrowding and population B doesn't ... Suppose that they're otherwise identical and live in initially identical habitats, but are geographically isolated. Suppose further that the single genetic difference between them only has this effect and isn't tied to any other positive or negative effects. It might be necessary to suppose some variability in food sources or in predators, but I don't think so.

Both populations grow to a state of overcrowding. Population A then limits its growth rate. It needn't be a perfect steady-state rate, it could involve a crash, just as long as it crashes to a larger population than Pop B, which in the same period got even more overcrowded, wiped out most of its food plants and crashed to a very small population. Both populations then continue to grow, but Pop A starts from a higher base and maintains a higher average population. Sure, they both achieve similar maximum populations at various times, but when a bigger food supply or a wider territory becomes available, Pop A is going to be larger.

I admit this may never have happened, but you have to agree it's possible. Note that I'm not talking about individuals in the same population, but two individuals or families split apart at the time of the mutation. In the same population, the "polite" mutation will indeed be wiped out.

The problem, though, is that it is most-definitely not in the best genetic interest of any given individual to lower his or her reproductive output.But it can be...
What matters from an evolutionary perspective is the number of surviving offspring you produce. Not purely...

If I am born with a mutation that allows me to have 10 litters of 10 pups, compared to my sister who can only manage 10 litters of 5 pups, who's more reproductively successful? Me! But suppose this mutation has an unfortunate side-effect - as many do - that all my offspring are sterile. Who's more reproductively successful at the second generation? Oops. I'm a loser.

(I guarantee that this precise situation has happened at least once with some pair of sister animals in the history of life on earth.)

Basically you're right that selection only works at the individual level, but the results of selection can be very different if conditions change and depending on whether you look at the short-term, or the long term and assume significant environmental changes, including splits of populations - which I think is fundamental to speciation. Sometimes you have to look at multiple generations, and then the opposing conclusions of what's good for the individual vs for the whole population start to blur.

All your principles do support the following behaviour: animals in overcrowded environments would do well to kill, even better eat, their neighbour's offspring. Effective and adaptive, and not uncommon.

When we try and apply these principles to humans, there's a new dimension I'd like to explore. Do you regard population-control measures like the Chinese "one child" policy as the outcomes of evolution? If not, what are they? Sure, they aren't dictated by our genes because I don't suppose the Chinese of 100 years ago were genetically different, but it's a behaviour that the Homo sapiens genome codes for. (I have a knee-jerk reaction against treating this kind of thing as some kind of different order from evolution.)

Thinking about this another way, it's a result of being genetically disposed to comply with social structures - to obey authority - and in return for the benefits that gives us, we take on some behaviours that are opposed to our individual reproductive success. I think this applies to a lot of species; not just us. When a selective pressure works on one mutation or gene or feature that has multiple effects, it can result in apparently adverse traits being selected for. A similar thing happens with sickle cell anemia (http://en.wikipedia.org/wiki/Sickle_cell_anaemia) (having one copy of the gene gives some immunity to malaria, having two gives you the disease).

I wait patiently for you to show me how mistaken I am! :popcorn:

joe

[The Lone Ranger]

. . . don't tell me "selection only works at the individual level".
Actually, as I pointed out earlier, there are cases in which a form of group selection can occur.

What you’re describing can (in a modified form) work; it’s called demic selection. Since a deme is a population of closely-related individuals, they’ll share a common gene pool – as such, they can be viewed as a single evolutionary entity, almost a “superorganism.” Demic selection can and does occur, since one deme can and will overwhelm and replace nearby demes if the average fitness of individuals in Deme “A” is higher than the average fitness of individuals in nearby demes.


As for your example, as mentioned earlier, in cases of extreme resource scarcity, it may be in the individual’s best interest to reduce his/her reproductive output, and so such mechanisms do occur. Here’s the rub: such mechanisms do not (and as far as we can tell cannot) evolve to prevent the population from reaching the point that it’s overcrowded in the first place. If the population is overcrowded, it may be in an individual’s best interest to forego reproduction (which, especially for mammals, greatly increases the mother’s chances of dying) until conditions improve – because if your kids are going to starve anyway, it’s better not to have any and improve your chances of surviving ‘til conditions are better.

Though demic selection can and does work under some circumstances, the situation you propose is highly unlikely, to say the least. The fatal flaw is that natural selection would never favor the persistence of any mutation that acted to prevent the population from reaching the point of overcrowding.

Theoretically, it’s possible that mutation would create a gene that caused its bearers to reduce their reproductive output when the population began to push its environmental limits. Theoretically, it’s also possible that genetic drift could “fix” the gene in the population so that all the individuals in that population had it – astronomically unlikely, but possible.

What happens when a mutation occurs in this population and an individual is born who does not lower her reproductive output in such times of stress? She’ll have more children, on average, than her fellows, and after each crash that “selfish” gene will make up a larger and larger percentage of the gene pool, until the “prudent” gene is reduced to very low levels or disappears entirely.

So, it’s extremely unlikely that a gene for “prudent” behavior could ever become fixed in a population in the first place, and it could only remain predominant in the population under the condition that a “selfish” mutation never occurs.



What happens if population A has a mechanism that limits population growth when there's overcrowding and population B doesn't
This is the key point. Perhaps I haven’t been sufficiently clear: once the population has exceeded its carrying capacity, it very often is in the individual’s best genetic interest to reduce her reproductive output. Therefore, it’s not at all unlikely that mechanisms will evolve to cause individuals to lower their reproductive output under such circumstances, since individuals who forego reproduction under these circumstances are more likely to survive the coming population crash.

So population “A” is less likely to undergo a crash to 0 than is population “B” and so less likely to go extinct. In the long run, you expect the evolution of mechanisms to limit reproduction once the population has already become overcrowded – there’s no controversy there, I think. What will not happen, though, is the evolution of mechanisms to prevent the overcrowding from occurring in the first place since, so long as the population is not overcrowded, it’s always in the genetic best interest of individuals to out-reproduce their competitors. (Actually, it is in times of overcrowding too; it’s just that under those circumstances, the best way to do so may well be to forego reproduction for a while, since an individual who has starved while trying to support her fetus won’t produce any offspring.)


So, just to make sure we’re all on the same wavelength: mechanisms to limit reproduction in times of overcrowding can and do occur, but mechanisms to prevent overcrowding from occurring in the first place almost certainly do not evolve. Is everyone agreed on that?



When we try and apply these principles to humans, there's a new dimension I'd like to explore. Do you regard population-control measures like the Chinese "one child" policy as the outcomes of evolution? If not, what are they? Sure, they aren't dictated by our genes because I don't suppose the Chinese of 100 years ago were genetically different, but it's a behaviour that the Homo sapiens genome codes for. (I have a knee-jerk reaction against treating this kind of thing as some kind of different order from evolution.)
Humans are smart-enough to have figured this sort of thing out for ourselves – that’s a handy “side effect” of selection for intelligence. Certainly, our brains are evolved structures, and so you could say that our genome codes for it in that sense, but that seems kind of pointless to me.

Selection has favored the development of great big brains – one side-effect is that those brains allow us to figure out that population control might be a good idea. But how does it help to say that our genome codes for population control? Our great big brains also allow us to build computers. Does it make any sense to say that our genome codes for computer-building?

As E. O. Wilson pointed out, in all cultures studied, people tend to throw spears pointy-end-first, not blunt-end-first. Is this evidence for a gene that tells people how to throw spears? No: it’s just evidence that humans aren’t stupid.



Thinking about this another way, it's a result of being genetically disposed to comply with social structures - to obey authority - and in return for the benefits that gives us, we take on some behaviours that are opposed to our individual reproductive success. I think this applies to a lot of species; not just us.
In the case of humans, our intellects do indeed allow us to adopt behaviors that reduce individuals’ reproductive outputs – otherwise, there would be no such things as condoms or celibate priests. We are not slaves to our biological drives – at least not entirely. That is a side-effect of consciousness, and it’s a very recent “innovation” indeed on an evolutionary timescale. As far as we know, we’re the only species in which this occurs. If anyone has found examples in a species other than Homo sapiens of individuals deliberately adopting behaviors that lower their inclusive fitness, I’ve yet to hear of it. (And I guarantee that it would be considered big news in the field of evolutionary biology!)


When a selective pressure works on one mutation or gene or feature that has multiple effects, it can result in apparently adverse traits being selected for. A similar thing happens with sickle cell anemia (having one copy of the gene gives some immunity to malaria, having two gives you the disease).
That’s the point: in an environment where malaria is present, having the gene even if it causes you to develop sickle cell anemia is advantageous. People who are homozygous for the sickle-cell gene tend to live to be 30 or so before dying of anemia – plenty of time to pass it on. Before the advent of modern medical techniques, people who lacked the gene entirely tended not to live long-enough to reproduce at all. From a genetic perspective, it’s certainly better to live to 30 and reproduce than to live to 5 and not reproduce.

In places where malaria has been brought under control, the sickle-cell gene is not advantageous, and is being eliminated from the gene pool. So long as the overall effect of the gene is advantageous, it will be selected for; if its overall effect is harmful it will be selected against, even if it happens to have some beneficial effects.


Hope this clears some things up. I’m probably not being as clear as I could be.

Cheers,

Michael

[The Lone Ranger]

Thinking about this another way, it's a result of being genetically disposed to comply with social structures - to obey authority - and in return for the benefits that gives us, we take on some behaviours that are opposed to our individual reproductive success.
As in the malaria example, what matters is that the genetic benefits (either in an individual's direct reproductive output or in how well s/he gets genes into the next generation by helping relatives reproduce) of social structures outweigh the genetic costs. So long as the benefits outweigh the costs, you'd expect selection to favor the behavior in question.


There's another thing to consider, though, and that's maladaptive traits. What's a useful trait now may not be in the future. Certainly, human society is currently changing at a far faster rate than our genes are able to keep up. So, it's entirely possible (quite likely, actually) that we have evolved features that were quite useful to our ancestors, but that are useless or even detrimental in today's world -- simply because our biological evolution hasn't had time to "catch up" with our cultural evolution.

Some have argued that our aggressive natures may ultimately prove suicidal in a world with nuclear weapons, for instance.

Cheers,

Michael

[JoeP]

Thanks Michael. I think you're being very clear! I think we are agreeing, in fact. Especially:

So, just to make sure we’re all on the same wavelength: mechanisms to limit reproduction in times of overcrowding can and do occur, but mechanisms to prevent overcrowding from occurring in the first place almost certainly do not evolve. Is everyone agreed on that?Yes.

maladaptive traits ...
Some have argued that our aggressive natures may ultimately prove suicidal in a world with nuclear weapons, for instance.In other words, traits that evolve (independently, i.e. unlike the sickle-cell gene) are adaptive at the time, necessarily, but when conditions change, they can't be eliminated from the gene pool at the rate conditions change.

Joe

[The Lone Ranger]

In other words, traits that evolve (independently, i.e. unlike the sickle-cell gene) are adaptive at the time, necessarily, but when conditions change, they can't be eliminated from the gene pool at the rate conditions change.


Yup. Traits spread through populations because they're adaptive at the time (or at least, they're not harmful). But when conditions change, a trait that was previously quite useful might suddenly turn out to be very disadvantageous indeed.

Cheers,

Michael