Harry Potter Science # 5: Kin selection

Continuing on with the series (is anyone else's anticipation about the release of Book 7 growing by the day?). If you're just now tuning in, you the first four posts have covered genetics, fossil dragons, wildlife conservation, and the botany behind wands. Now we're moving on to a more theoretical aspect of science involved in the Harry Potter series: kin selection.

Kin selection is a much-talked about idea in biology, usually invoked to explain acts of altruism, in which an organism does something that results in a decrease of its own fitness but an increase in fitness for one of its conspecifics. This appears to happen most frequently between individuals that are closely related, which led William Hamilton, a leading figure in sociobiology, to develop the idea of kin selection, presented in a much-cited paper, Genetical Evolution of Social Behavior published in 1964 (notably, though John Maynard Smith is the one that actually coined the term).

In this paper Hamilton presented the idea of inclusive fitness, basically that the potential costs and benefits of performing an action must take into account not only the individual's fitness, but those if its genetic relative as well. For example, let's say an old man must decide whether to jump in front of a car to save his grandson from being hit (we are assuming that, although aged, his reflexes are well-preserved enough to do this effectively, 60 is the new 50, you know). This is where Hamilton's Rule comes into play. It is one of the few equations used in introductory biology courses:
the infamous C <> (I appear to be having formatting issues, it should read C is greater than r times B)
C = the reproductive fitness cost to the individual of interest
B = the reproductive benefits to the individual's genetic relatives as a result of the potential action
r = the coefficient of relatedness. This is usually defined as "defined as the probability that a gene picked randomly from each at the same locus is identical by descent.", but for basic purposes you raise 1/2 to the power equivalent to the degree of removal in the family tree. So, for example, my brother is one step away from me, so in that case r = 1. My grandmother is two steps away from me, so r = 0.5 x 0.5 = 0.25.

So, going back to the example, is it an evolutionarily good decision for Grandpa X to jump in front of the car? This depends on the grandson. Their r value is 0.25, which means that according to the theory, we must expect the grandson to produce more than four offspring (remember B must be something that produces a product greater than 1 when multiplied times 0.25, according to the Rule) in order for this to be a good decision, because in losing his life he also loses all reproductive potential, which means C = 1. So, the judgment must be based on what is known (to that point) of the grandon's stud factor. Is he likely to attract a mate and produce a large clan, or is his personality/physique/general life plan such that this future is unlikely? If he can't meet the quota, it isn't a smart evolutionary mood to sacrifice one's entire reproductive potential for him.

Wait a minute wait a minute, something isn't right here. While 60 may indeed be the new 50, and men technically do produce sperm for their entire lives, it is highly unlikely that Gramps is going to sire any more offspring before the end of his life. He is enjoying his retirement immensely, and even if Grandma were still fertile they have no plans to muddy up their lives with a new round of diapers and teething rings. So Grandpa already has a reproductive potential of zero: as far as evolution is concerned, he is done passing on genes whether he jumps in front of the car or not. In this case, even if the grandson only produces one offspring in his entire life, it would still be beneficial for Grandpa to save his life, because that one offspring is Grandpa's only chance to pass his genes on to further generations.

So this is how kin selection works: sometimes individuals sacrifice their own immediate fitness if it ensures the continuation of their gene pool through the success of a relative. The case above would have been much different if it had been an uncle instead of a grandpa. The r value would still be 0.25, but the uncle could still have been in his prime reproductive years, and he would have suffered a much higher cost in fitness than the grandfather that was finished creating offspring. In this case, the decision basically pits the boy's future reproductive prowess against his uncle's. But then we must also take into consideration: in the not-so-distant past, the death of a father could throw his existing family into destitution, and the resulting physical/social ramifications could damage existing children's fitness as well as preventing any future offspring for the uncle. Obviously situations aren't as simple as the equation seems at first glance, but the principle holds: greater success in passing on genes is the trump card.

Now, all of this is very fascinating, and explains many observations about animal behavior. But what does it have to do with Harry Potter?

It has everything to do with Harry Potter. As you might recall, he is revered throughout the wizarding world as "The Boy Who Lived." In a nutshell, Harry Potter's parents, James and Lily, were powerful wizards working to defeat the evil sorcerer Lord Voldemort. When Harry was a year old, Voldemort discovered their home in Godric's Hollow (the location was supposed to be concealed with a spell entrusting it in a "secret-keeper," it could never be known as long as the secret was kept, but unfortunately they were betrayed). He attacked the family in the middle of the night. James Potter tried to hold him off, telling Lily to take Harry and run, but Voldemort struck him dead with the worst of the Unforgivable Curses, the Avada Kedavra killing curse.

Next, Voldemort turned his attention to Lily. He offered to spare her, on one condition: that she hand Harry over. Lily refused, and Voldemort had no hesitations about using Avada Kedavra on her also. Two green flashes of light, and Harry Potter was an orphan.

When Voldemort tries to destroy Harry as well, though, things don't go as planned. He casts the killing curse once again, but this time it rebounded on the evil wizard and came very close to destroying him. He went into hiding, in a semi-human state, and disappeared from the wizarding world for over a decade. The tiny Harry was left with a scar on his forehead, but was otherwise physically fine.

A common theme throughout the series is to cite Lily's sacrifice as the source of Harry's unique powers against Voldemort. According to Albus Dumbledore, who served as Hogwarts headmaster but also--and more importantly, Harry's mentor:

". . .I am speaking, of course, of the fact that your mother died to save you. She gave you a lingering protection he never expected, a protection that flows in your veins to this day. I put my trust, therefore, in your mother's blood. I delivered you to her sister, her only remaining relative."
~Harry Potter and the Order of the Phoenix, p 836

While a common theme throughout the series is to attribute Harry's unique powers to the love of his mother as she died in her efforts to protect him, as biologists we can glean another truth from Dumbledore's words. There is indeed part of Lily (and James) flowing through Harry's veins to this day: their genes (although, technically mammalian blood cells are anucleate, so the "blood" in the metaphorical "bloodlines" isn't a viable source of DNA, but I digress). While their sacrifices were no doubt noble in a moral sense, they can be looked at from an evolutionary perspective as well.

In giving their lives to save Harry (although James may have thought he was saving Lily as well), the Potters ensured the survival of their only offspring. Although they were young, they may not have planned on having any more children (fertility rates, both worldwide and in Britain, are on the decline, after all). In this case, Harry is the only vehicle remaining to perpetuate their gene pool, and it would make sense for them to give their lives in order to preserve his future. (Lily also shares genes with the Dursleys through her sister Petunia, but anyone who knows Dudley will know that his potential to garner a mate is NOT something to gamble your life on).

This is not the end of the story of kin selection for Harry Potter, though. While his parents altruistic acts saved his life, kin selection also turned around and bit him in the ass for much of his childhood. As noted in the quote from Dumbledore, Harry was sent to live with his only relatives, the Dursleys, embarrassments to those of us consigned to the world of muggledom.

The Dursleys agree to raise the boy (although I think it was not as simple as this, and revealing the circumstances of the "deal" between Petunia and Dumbledore will be an important part of Book 7). They do not, however, raise him well. Their son Dudley is the same age, and the dote on the boy, indulging his demands and spoiling him to a disgusting degree. Harry, meanwhile, is treated like little more than a slave. The "cupboard under the stairs" serves as his bedroom, he is often given little to eat, does a grueling schedule of chores, and is never allowed to participate in any family outings. He is treated like a hated imposter, despite being smart, sensitive, and good-natured, while the piggish, dull-witted Dudley is treated like a prince.

Why do Vernon and Petunia spend so much energy spoiling Dudley and discriminating against Harry? Kin selection, of course. Dudley is their full son, while Harry is the nephew (having an r value of 0.25 with Petunia and of 0 to Vernon, since they are only related by marriage). Therefore Petunia has a biological incentive to treat Dudley at least twice as well as Harry (although she obviously goes to greater lengths than this) and Vernon has no reason to give Harry any nurturing at all.

Throughout the series, it is mentioned again and again that Harry has his mother's eyes. (They are said to be a striking green color, one of my major rants about the movie versions of the books is that Daniel Radcliffe, who plays Harry, has gorgeous baby blues. I think the eye resemblance is going to be highly significant to the plot, and Hollywood screwed it up. Really, folks, colored contacts aren't that pricey). Although we're not quite sure of the genetic basis of magical ability (see the first post in this series), the fact that Harry displays such an obvious resemblance to Lily is a constant reminder to the Dursleys that he is not "one of them," not fully at least. Since they associate him with Lily, the also associate him with her magical ability and whatever genetics that entails, further reinforcing the fact that he is not part of their clan and doesn't warrant treatment as such.

It is also notable that the pureblooded wizards in Harry's world are "all related somehow," according to Ron, and this could explain why the Order of the Phoenix has taken such an interest in protecting Harry, beyond the fact that he could serve as a vital weapon in the war against Voldemort. We don't know much of James Potter's background, but we are never told that he had any muggles in his lineage. If he was part of the pure-blooded world, that would explain why important wizards from pure-blooded lineages, such as the aurors in the Order of the Phoenix and many teachers at Hogwarts, seem to have a strong interest in protecting Harry and helping him through all of the trouble that constantly seems to find him. There are many pureblooded families (the Malfoys immediately come to mind) that definitely are not Harry-friendly, but they are historically associated with Voldemort, and turning their back on him after pledging allegiance is a sure way to put their nuclear families in grave danger, not advisable if we are thinking in terms of kin selection.

So there you go, although I don't think Hamilton had wizards in mind when he suggested inclusive fitness fifty years ago, the principles he presented are nevertheless at work in the Harry Potter series.

I think this theme will be interesting in the last book: what if Ron has to choose between saving his sister, Ginny, or saving his best friend, Harry? What about Hermione? Coming from a Muggle family, she probably shares very few genes with any of the wizards, even if she does have a few magical alleles....also, it is notable that many of the "evil" characters are single children families. Losing Draco would strike a much larger blow to Lucius Malfoy's reproductive success than if Arthur Weasley lost one of his seven children. With that much on the line, the stakes are not equal, and the character's actions in these situations will definitely warrant careful attention.