Originally, altruism and self-sacrifice were thought to be incompatible with natural selection, even by Darwin. Now we have several explanations for how altruism can increase an individual's fitness. One is kin selection, or the idea that helping relatives can help increase one's genes in the population. Another involves ecological constraints and punishments. Here, individuals contribute to the group and wait their turn to reproduce.
Krebs, John R. and Nicholas B. Davies. An Introduction to Behavioral Ecology, chapter 11
April 24, 2009
Professor Stephen Stearns: Okay, I think it's probably appropriate that the course finishes on this note. These are some of the most interesting and most nuanced and deepest issues that we've addressed in ecology and evolution, and they bring all these fields together: evolution, ecology and behavior.
So the outline of the lecture is given here, and basically what I'm going to tell you is that the initial idea on how altruism and cooperation could evolve was kin selection; later alternatives were proposed, and the current situation is a bit more nuanced. We currently have pretty good explanations for why unrelated individuals can cooperate. You've seen a bit of that perhaps with the vampire bats, and you'll see some more examples in other cases.
Now, this line of research was really provoked by Darwin's honest admission; something that I think every transparent and intellectually honest scientist ought to do. He stated the conditions under which he should be willing to abandon his theory. If ever it could be shown that individuals repeatedly and reliably sacrificed their own fitness to increase the fitness of others, the theory of natural selection would be refuted. Well, naturally that attracted attention. The logic is given here, and the conclusion that one would come to from that is that if you just take a simple-minded view, mid-nineteenth century view, of the theory of natural selection, altruism and cooperation should be impossible.
And an early alternative explanation focused on group selection, and the iconic example there was the red grouse in Scotland; it's a kind of ptarmigan. And the group selection claim was that they would curtail their reproduction to keep their population from over-cropping the food supply and thereby preserve the population as a whole. And so the issue was why shouldn't an individual reduce reproduction? Why should one not reproduce reproduction and thus avoid over-exploiting the food supply?
And the problem is that if you look at the decision made by any single individual, it's benefiting the whole population, but at the cost of its own offspring. Okay? And everybody benefits, but the costs are paid by that single individual. And so any selfish individual who decided to defect, who wouldn't reduce its reproduction, would benefit. The others would cut back, it would get more.
And so group selected altruism, as we've seen from evolutionary game theory, is not going to invade when rare. It's not an evolutionarily stable strategy, and it doesn't resist invasion by selfish alternatives. So the group selection explanation is logically faulty. That doesn't mean that group selection never works; we'll pretty soon state the conditions under which it might, but it certainly doesn't work in this case. So this is very similar to the tragedy of the commons.
And the reason that the red grouse reduce their reproduction when the population is dense, I think you already know. Can anybody tell me why? Population density goes up, what happens to fertility, and why? Yes?
Professor Stephen Stearns: Right, exactly. They have a very straight physiological response to that. Population density goes up, they have less to eat, they can't make as many babies. That's why the red grouse don't make as many babies in the fall, when the population is dense. So let's go back to the basics, to see what the critique of group selection is. Because this had to be worked through before people could really understand it and abandon it and seek other alternatives. We're back to day one of the course; we're on the last day of the course, we're going back to day one.
So if there's any selection process going on, its ability to produce change is determined by how much variation in reproductive success there is, among the things being selected; by the correlation between the trait under selection and the variation in the success of the unit, which would be in this case either an individual or a group, a whole population of red grouse; and the genetic variation of the trait among the units.
Well let's just step through all of those conditions. Usually the correlation between a trait and reproductive success is much stronger for the individual agents for the group than for the group in which they're embedded. And you have to think of the reproductive success of an individual as being its offspring, and the reproductive success of the group as being the number of groups that it might found.
The variation in reproductive success itself is greater among individuals than among groups, usually, and that's primarily--I mean, you can just see it statistically. You're taking--the reproductive success of the group is going to depend upon the average of the reproductive successes of the individuals in it, and so normally the variation will be greater among the individuals than it would be among the groups.
The amount of genetic variation in a trait that could be accounted for by differences among individuals is also a lot larger. So inter-individual variation is usually much greater than variation among groups. We saw that, by the way, when we did the analysis of genetic variation among ethnic groups and we saw that 85% of human genetic variation is among individuals, and only about 15% is elsewhere.
The generation time of individuals is a lot shorter than that of groups. When you get into things like species selection--remember, the average lifetime of a species is one to ten million years. If you want to talk about reducing the extinction rate of species on the basis of group selection, you've got a real problem, because you only get a selection event once every million years to once every ten million years. So anything that's cranking along at the level of the individual is going to happen many, many, many times before a single event will happen at the level of a group. And that is this final point, which is the number of incidents of selection on individuals, in any given unit of time, is normally much, much greater. Okay?
So it's the combination of these six conditions that makes it possible for a selfish mutant to invade a resident population of self-sacrificing altruists. So that's the more precise analysis of why the original nineteenth century Darwinian theory said we expect individuals to be selfish. But look what we find. We find communal colonial living, group hunting and sharing food, alarm calls, reproductive helpers; you just saw three examples of them in the video. And these are the leading alternatives to group selection: kin selection, punishment, and mutualism. So let's go through them.
The basic idea of kin selection I think you're already familiar with. It has interesting and unsettling philosophical implications. If what matters isn't the survival of the adult phenotype, of the whole organism, but an increase of frequency of genes that that individual carries, then it will pay for that individual to sacrifice itself if more copies of its genes get into the next generation than if the individual did not sacrifice itself.
So the costs and benefits have to be weighed in genetic currency. The genes are not only in it, but they're also in its relatives. So if it can help its relatives to survive and reproduce then--and if the increase that it gets by doing that is greater than the cost that it suffers by doing that--then that behavior will be selected. So to formalize that, just that the benefit is the increase in the relative's fitness, as a result of the act. The cost is the decrease in the donor's fitness.
R is the coefficient of relationship. So you are .5 related to full sibs, .5 related to mom, .5 related to offspring. Then calculating out: .125 to first cousins and things like that. That's what R is. And that is calculated as the probability that a gene in the donor and a gene in the recipient are so-called identical by descent from a common ancestor. So R varies between 0 and 1. Then the condition for helping is that B divided by C is greater than 1/R, or the benefit times the degree of relationship is greater than the cost. Or to put it into words, the increase in the relative's fitness as a result of the act, times the relationship to the relative--right here--is greater than the decrease in the donor's fitness as a result of the act, times its relationship to itself; which is 1. Okay?
So that is a very simple inequality, and it's a very powerful idea. This is the guy that had it; this is Bill Hamilton. And Bill is a remarkable man. He died in 2000, after having--I think I've told you--after having gone to the Congo to see whether or not AIDS got into humans through a polio vaccine. And he was a very creative and iconoclastic scientist; certainly one of the major minds in evolutionary biology in the last hundred years.
Now some of the traits that people think might be kin selected are alarm calls, guarding behavior, helping at the nest, and suppressed reproduction. By the way, if you printed out the lecture, I've taken out the two slides on social hymenoptera, just to save a little time, because I wanted a little more time for Sir David. And I'm not really sure I'm going to get through all of this. But these are the kinds of things that people think can be explained by kin selection.
So one of the early ones was the contrast between ground squirrels and marmots. And this is based on a difference in their social organization, and in the probability that a given sex will be close to relatives. So in the ground squirrels--these guys live in Yosemite--the males wander off, the females stay near their burrow, and it's the females who give alarm calls, and they usually do so when offspring are threatened, and sometimes they give alarm calls when a niece is threatened. And that's because the females don't move so far, and the daughters and nieces and so forth are living in adjacent burrows. So an adult female will watch and will give the alarm call. It's quite different in marmots. In marmots they hold harems. The male who is sitting on a pile of rocks, with a bunch of females in it, knows that all the babies in that pile of rocks are his own. And so in this case it is the male that is giving the alarm call.
And the contrast between the two species--I think you can see some of the power of the comparative method here. They're very similar in their biology otherwise, but in one case the male gives the call and in the other case the female gives the call, and the contrast tells you that it seems to be pretty tightly associated with how close they are in space to their offspring. So you might want to consider whether or not this evidence of parental care really is evidence of kin selection, or is it simply the act of the individual completing reproduction. So I'll leave that up in the air. But this is some of the early evidence that was used.
Now perhaps a little bit more convincing are helpers at the nest. There are quite a few birds in which there are helpers at the nest. Here are four of them; so Pied Kingfisher, Florida Scrub Jay, Acorn Woodpecker, and White-fronted Bee-eater. We'll do the Pied Kingfisher example, which was done by Ueli Reyer. And he did this in East Africa, contrasting colonies living on Lake Victoria and Lake Naivasha.
So basically the reason he contrasted them is that one of them has rough water and one of them has smooth water. And when the water is rough it's harder to hunt and you need more help to feed your babies, and when the water is smooth it's easier to hunt and you don't need so much help to feed your babies. And indeed the percentage of nests with no helpers was higher at Lake Naivasha, with smooth water. Primary helpers were not so frequent. Primary and secondary helpers were pretty rare; almost no secondary helpers at all at Lake Naivasha. Primary helpers are older sibs and secondary helpers are unrelated birds. And these are the proportions of encounters in which mated males are attacking or greeting potential secondary helpers. So this relates for unrelated birds coming into the nest. Do they attack them or greet them? And if you have two to three in the nest, they usually get attacked at Lake Naivasha, but if you have four or five babies, and you really need help, then they are greeted.
And at Lake Victoria, with two to three in the nest, they are greeted, and if you have a reduced clutch they get attacked. So this was a case in which there was a clutch manipulation. So basically if you need help you greet helpers, and if you don't need help you chase them away. And what's going on here is that there's both degree of relationship for the primary helpers who are sibs, but then there's also the issue of can I get a nest site? And why is it if I'm not related to these guys am I helping them out? And the answer is that the helpers inherit the nest sites.
So nesting is done in cliffs. There's not very much available cliff habitat. They dig a meter-long burrow, which is a lot of effort, back into the cliff--so if you can take one of these over, you don't have to dig it yourself--and they lay their eggs way at the back of the burrow. So there's an ecological constraint on the system, and it's interacting with degree of relationship.
So overall, if you look at total fitness of the different kinds of roles that are being played in the population--the breeding birds, the primary helpers, the secondary helpers, and those not helping--and you calculate both direct fitness and indirect fitness through relatives, over the lifetime of the birds--so this takes years to gather; this is a summary of a lot of work--mating is better than helping. We see that here.
Helping's better than not helping, because it puts you in a position to inherit a nest burrow. If you do help, it's better to help a relative than a non-relative. And if you are a breeding bird, you only should accept help if you really need it, because the helpers might get a little frustrated and restless and try to kick you out, so they can breed.
Now in social carnivores the situation is interesting because they are cases of reproductive suppression. And in African hunting dogs, in hyenas, in mongooses, there will usually be, in social mongooses, there will be a social group that forages together, and usually the dominant female does all the breeding.
The group living brings with it protection from things like predators. And if I go back, these guys are usually all related in a family, but they do accept non-relatives; the majority will be in a family but they will accept non-relatives coming in. And so kin selection is one of the things that's going on in social carnivores, and it's implying sacrificing self for relatives.
And when we ask what is the best evidence for this gene-centered interpretation of evolution, we see that wow, there's some of it from kin selection, and that does seem to work some of the time, although often there are alternative explanations.
But the really convincing evidence that it is genes that are being operated on by natural selection, and not individuals, is the evolutionary theory of aging and all of the evidence that now supports it; which is extensive. And that is that the soma, which is us, is sacrificed for the germ line, which is our offspring going forward.
And here we have lots of experimental proof that this is really the case. Okay? It's been done now in possums, fruit flies, worms, bacteria. So we have experimental evolution confirming this idea. And therefore I would say that overall this view, that genes really are what evolution is operating on, is pretty well supported. Now that doesn't mean that kin selection is wrong, it just means we're more certain that the evolutionary theory of aging is correct.
I would say that virtually everyone in behavioral ecology today accepts that kin selection is an important thing. I emphasize these points because I actually lived through the period in science when these were all alternative explanations and evidence was accumulating, and it was clear that things like the comparison of the ground squirrels with the marmots wasn't necessarily convincing evidence, because there was the alternative that they were just completing the act of reproduction. Okay? Since then I would say there have been enough confirmations of kin selection effects to make us think that it's very probably correct. Okay.
It does have some problems. There are highly specialized societies that have fine division of labor, extensive cooperation, and the individuals in them are no more closely related than are individuals in simpler societies. There has been a tendency to over-estimate indirect fitness benefits; so kin selected fitness benefits. And that has been done by including direct descendants of the pair rather--which could be, for example, parental care--rather than looking at nieces, nephews, aunts, uncles, things like that.
The direct fitness benefits have often been under-estimated. So there has been more or less a tendency, once people learned about kin selection, to try to take the world and fit it into that theoretical construct, and in the process people ignored some simpler explanations.
So if we want to explain social living and cooperation, it might be good to take a look at some of the alternatives, because it doesn't look like kin selection is going to do all of it. And Tim Clutton-Brock has made a career out of coming up with simple alternatives for current bandwagon explanations, and this paper here is one of them.
So let's take a look at social carnivores. If the only possibility of surviving at all is to be in a group, and if that group's policed by a dominant female, then she has the opportunity of telling subordinate females that they've got to stay in the group, even if they don't have offspring, and help her out. Because even if it's going to take them a long time to grow up and wait for her to die, their chances of reproducing, by doing so, are greater than if they left the group.
I've been with the meerkats on the border between South Africa and Botswana. They live--you saw the Cape Cobra. There are also various eagles--Batteleur Eagles and Harpy Eagles and things like that--flying around. And I would say that the expected survival time--not the reproductive success but just the survival time of a meerkat that leaves a group--is on the order probably of about twenty-four hours; maybe it might, a few might make it for a week, but they're not going to make it for the months that they would need to reproduce.
So being in a group is so important that even if the group is being policed by a big bully it pays to stay in it. And if they do make a mistake and they get pregnant and they have offspring, she'll kill them. And they're only allowed to stay in the group if they help.
So what's running this system basically is a combination of ecological constraint and punishment, not kin selection. Now in a case where you have tight interactions within a group, and the success of everyone in the group depends upon the degree of cooperation, then you can have cooperation arising for reasons of win-win kinds of interactions, even if they're not related, and if the overall success of the group can only be improved by having the rising tide lifts all boats effect. Okay?
So there is also--this is something else which is going on with these social carnivores; one could conceive of it like this. Group size increases the capacity of group members to catch, produce or defend food. By the way, it also increases their ability to detect and repel predators. They can then, when the group splits up and a reproductive module is moving off--it could be five or ten of them, rather than one or two of them; so the offspring group is then safer. They do better at raising young. They do better at competing and defending territories against other groups.
And it has been noticed that--by the way, this isn't just meerkats, this is social carnivores in general. So individuals that live in smaller groups have slower growth rates. They have low survival, low breeding success. Small groups frequently become extinct. And it's thought that actually individuals that are living in this social circumstance might even advertise to solitary individuals wandering the landscape, "Hey, come and join us, because if you come and join us, even though you're not related to us, then we will all do better."
So that gets us into reciprocal altruism--okay?--and the conditions under which unrelated individuals will cooperate with each other. The basic idea here is win-win mutualism; you scratch my back, I scratch yours. And so the payoff in equality is really simple. If the benefit that I get from engaging in this behavior is greater than the cost, then I'm going to do it, and if it's the same for you, then you're going to do it, and if that is a cooperative thing then we're both going to do it and we will get cooperation.
You'll notice that this is basically selfish cooperation; both individuals are gaining. It works best if there are repeated encounters between the two agents. You need to have--for reciprocal altruism you need a cognitively fairly gifted kind of species. Okay? It's got to have good memory. It's got to have individual recognition. You need the spatial contiguity that will result in the repeated interaction. And the result is long-term self-interest. Okay?
And often this is coupled with a disincentive to cheat. So you saw that with vampire bats in the movie; that if they remember that that individual did not share blood with them when they needed it, they won't share back. And this is very close to the tit-for-tat strategy in the Prisoner's Dilemma.
So here are vampires again. And you saw the nice overview. So here is the hungry bat and the satiated bat. And the hungry bat is more likely to get fed if she has cooperated in the past; so she's getting rewarded for that behavior. And this is the kind of cost-benefit analysis that they're on. This is the amount of pre-feeding weight that they are losing over time; which is interesting. You can see that they're losing about oh, fifteen to twenty percent of their body weight per day, if they're not eating. And you can see that the donor is losing weight and time, and the recipient is gaining weight and time. If it got down to here, they would starve and die. So they are really daily kind of balancing on a nutritional knife's edge, and the cooperative behavior is making a life or death difference to them.
This is one of the few cases in which a non-human species has been observed to engage in reciprocal altruism. And I think that if you were to go into the literature you would find that some people think that they actually, the donor and the recipient, might be related a bit more frequently than one had thought. So there may also be a kin selection element to this.
So helping behavior can be explained in a number of different contexts. There's straight out manipulation, which benefits the person who is doing- the individual doing the manipulating, and it's bad for the helper. So, in fact, you should think of those social mongoose nurses, who are sitting there waiting to reproduce, as basically paying a cost. They are getting some indirect fitness benefit probably, because they're probably nursing their nieces. Okay? So they are getting some benefit that way. But they'd be doing a lot better if they were the dominant breeding female.
In a mutualistic encounter, as with the vampire bats, there is a positive benefit for both in the interaction. The long-term reciprocal interaction is a case of mutualism, but the short-term reciprocal interaction is not. The short-term interaction is very much like the Prisoner's Dilemma. If those two vampire bats were only going to encounter each other once in their lives, there wouldn't be any incentive to give the other blood; the life would not be saved; and the behavior would not be selected.
With kin selection the effect on direct fitness--direct fitness is direct individual fitness; okay, indirect fitness is fitness through relatives. In kin selection the beneficiary gains and the donor loses. So there is actually a cost in terms of numbers of children per lifetime, or numbers of grandchildren in the kin selection scenario.
With Belding's ground squirrels, or with marmots, the cost should be conceived of as the cost of drawing attention to oneself when one gives an alarm call when a predator is swooping in; so that in fact the probability of dying in a predatory attack is greater if you give an alarm call, or if you're on guard behavior. And, of course, evolution has acted to increase the speed and intelligence of these animals so that they reduce those costs to a minimum. But that, in principle, is the source of this minus sign, right here.
So to summarize the explanations for cooperative behavior and altruism, it is now widely accepted that kin selection does occur, and it can explain altruistic sacrifice. A lot of people have seen it as the biggest deal in evolutionary biology in the twentieth century; at least among people who are concerned with behavior and the evolution of intelligence and things like that.
But the number of cases in which kin selection has really been pushed to the limit, and tested against alternatives, is a lot smaller than the number of cases in which it's been proposed. So there was a bandwagon effect, and people went out and placed this theory on the world, and then tried to cram Nature into it. And the fact that they didn't doesn't mean that it's false, it just means that not all of the cases that were advanced were truly logically convincing.
So the main alternatives to kin selection are long-term individual self-interest, and this would include punishment and reactions to punishment. And we've seen this kind of thing, both with the vampire bats and with the social mongooses and meerkats; meerkats, by the way, are also a kind of social mongoose. And that is really a very strong direct effect, and it's quite measurable, and it's stronger in terms of effects on reproductive success than the weaker effects of kin selection. And then there is win-win mutualistic cooperation; and we haven't really gone into that in detail.
But there are quite a few cases out there where organisms from different species, which by definition are not related to each other very much at all, are creating a mutualistic interaction which benefits both of them. And not every symbiosis is that way, but there are certainly some cases in which this is a win-win for both. Okay.
So I'm going to have a review session tonight, in here at 7:00, and I'm going to hold another review session on Sunday night at 7:00, because I know some of you have conflicts tonight, for various reasons. And remember these, especially the May 4th deadline. That's the one that we can't bend. That's the one where you need a Dean's Excuse if you're going to go past that point. So it's been fun. Thank you all. See some of you tonight.
[end of transcript]