so here's here's the problem if altruistic behavior benefits the whole group the population or the species but decreases the fitness of the individual you know diving into the into the ocean to decrease overpopulation of lemmings is the legendary behavior as it or if warning of of predators or whatever if that benefits the group but decreases your own fitness then the frequency of the altruistic allele should decrease and be eliminated and that's illustrated uh here we have each of these circles represents a population a distinct population of the same species the red represents the selfish alleles these are individuals that watch out for themselves the blue represent the altruistic alleles so the problem here is that if you have in the population say in this population a mixture of selfish and altruistic individuals the selfish individuals are going to win out within that population the selfish alleles are going to win out because selfishness is going to be uh there's going to be an advantage they will more likely pass their alleles on to the next generation and ultimately selfishness will be fixed in the population but there is this idea of group selection that states that populations that are fixed for the altruistic allele so we have these populations that only have the altruistic allele these are less likely to go extinct they all get the warning call when the when the hawk comes and so they dive for cover they are those populations benefit from getting from not getting eaten by the hawk these other populations where it's every every ground squirrel for himself those populations are more likely to go extinct and we call this group selection so this is the concept of group selection that selection favors certain populations over others now the problem is that group selection as as explained there isn't going to work and the reason is first of all selection is going to act more quickly on individuals than it does in populations and the reason for that is that individuals are being born and die and reproduce more quickly than populations are being born and go extinct and so as as these individuals are being born and die that those selfish alleles are going to be fixed in the population more quickly than the population will go extinct furthermore there's going to be gene flow represented by these gray arrows as these selfish individuals represented by the the red alleles they're going to migrate over from one population into another and even though this population is fixed for altruistic individuals they all give the warning call eventually some grounds some selfish ground squirrel is going to migrate over from a population here and then one's going to migrate over from here and they're going to come you know once a selfish squirrel comes over there that benefits from everyone else giving the call all those other ground squirrels give a warning call and he dives for cover but he doesn't give the call he gets so the benefit of everyone else's altruism but doesn't expose himself so that allele for selfishness then has an advantage and that's going to be fixed in the population and so this is you know the williams uh criticism of group selection so the the selection within the population is going to favor the selfish allele more quickly than this group selection this whole population selection can act and so this still this selfish allele is going to be fixed into the population and so these cheaters are going to benefit from the altruism while avoiding the cost the altruistic allele is going to be eliminated from the population before it can be fixed and confer an advantage to the population and so you know it's going to be all the selfish alleles ultimately still going to be fixed in the population so the end result is group selection as generally conceived is not going to work so we're back to where we started selection works at the level of individuals to change populations selection does not work at the level of a whole population being more fit than another whole population however there is an explanation for altruistic behavior and that is the concept of kin selection so kin selection depends on the concept of inclusive fitness now you know what fitness is fitness means the ability to reproduce now inclusive fitness involves not only your own fitness but also the in the fitness of your close relatives so your your brothers and sisters are likely to have the same alleles by descent we've talked about that you know that from genetics they have a certain probability of having the same alleles that you do because they inherited them from your parents if you have half siblings then they have a slightly less probability of having the same alleles that you do your first cousins have slower probability of having the same alleles because they got them from your common grandparents depending on how closely related you are there's a benefit if you can improve the probability of your close relatives reproducing more efficiently so this can explain altruistic behavior you can explain individuals that sacrifice their own personal fitness perhaps if it can benefit their close relatives so if that ground squirrel that you see there in the middle is there giving a warning call if some of the ground squirrels that benefit from that or the squirrel's brothers and sisters and cousins and nieces and nephews and that increases their fitness we can show mathematically that that's going to increase the probability of the same alleles being increased in frequency in the population the same thing could be said of social organisms like the naked mole rats you see on the on the left some of which do not contribute to the population or social insects like honeybees which are largely sterile workers if they can increase the fertility of the queen that benefits their own fitness because the queen shares some of the workers own alleles we'll look at mathematically how we we can describe the idea of inclusive fitness and how it affects the overall fitness of the organism and this is described in hamilton's ruler this is an all altruistic trait can increase in frequency in the population if the benefit received by the donors relatives the donor is the one that is doing the altruistic behavior if the benefit received by the donor's relatives weighted by the degree of relationship to the donor exceeds the cost of that trait to the donor's fitness so again it's a trade-off there's a cost to the altruism what does it cost you to give that warning call versus what benefit do your relatives get from it weighted by how closely related are those relatives how likely is it that those relatives share your alleles that's hamilton's rule in english here's hamilton's rule in in mathematics so hamilton's rule altruism increases if rb is greater than c where r is the degree of relationship b is the benefit to your relatives and c is the cost to you so your cost for altruistic behavior benefits your relatives by the amount b weighted by the degree of relationship r now what is r r is one half for parent offspring if you can benefit your parents or if you benefit your child or your full siblings it's one quarter if they're half siblings or grandparent or grandchild or uncle nephew or aunt nephew and niece or uncle niece or double first cousins butter double first cousins double first cousins or your cousins on both sides of the of the family you and your sibling marry a pair of siblings your children and your siblings children would be double first cousins because they're cousins on both sides of the family and one eighth if you are normal first cousins first cousins and only one one side of the family so here's an example of that if you're talking about just the benefit of parental care degree of relationship for a parent to offspring is 0.5 one half and let's just say that in the extreme case the cost is one the cost is the death of the parent you die if the benefit is greater than two more than two additional offspring are going to survive then the frequency of the behavior will increase in the population the there's a benefit to that behavior in the population and so remember jbs haldane was the geneticist story goes that he said that uh based on this relationship that he wouldn't sacrifice his life to save a brother but that he would do so for two brothers or eight cousins because the genetic benefit for saving two siblings or eight cousins equals the cost keep in mind as a rule natural selection acts on individuals an individual that is more fit better able to reproduce than another one in the same population is going to have its alleles increase in frequency inclusive fitness and kin selection is just an extension of that it's another way of looking at it if your alleles increase in the population it doesn't matter whether they were actually your alleles or the identical alleles in your relatives but evolution is not about one whole population or one whole species being better adapted than another it's about changes in allele frequencies within a population due to differences in fitness among individuals