Evolution fun facts 1/3 – Hamilton’s Rule and Inclusive fitness

I want to start a brief new post series about this pretty cool branch of biology, Ethology. This post may sound technical to some, but bear with me, I’m doing it only to be able to explain some cool stuff later on the series.

We said multiple times that evolution tends to maximize reproductive success. In simple words, to maximize this parameter any organism looks to fulfill the following: have sex ( reproducing in some way) frequently, live long and find food to do that. These 3 things will allow an individual to have as many offspring as possible. Offspring means transmission of one’s genes, the ultimate goal.

Once this is all clear, we can make some observations around us. For example we can easily explain why lions monopolize females in harems. Throughout the years we found explanations for the weirdest things like the peacock tail, suicidal behaviour and siblings killing each other (see 2/3 and 3/3). But why would we observe collaboration or altrustic help? Think about it. If I want to maximize my own fitness, I would never help others, because just for the fact that I’m spending time for doing stuff not for myself, is giving me less time to reproduce (or maximizing the fitness in other ways).

Unless, in doing so, I also gain something. It’s interesting to notice that in most of the cases, this is actually true. Pure altruistic behaviour is almost absent in nature. And it doesn’t have to surprise, it just makes sense.

What is definetly not bringing me any benefit is to choose not to reproduce at all, or even less risking my life for others. This, and many others, are extreme cases that we simply wouldn’t expect, since not reproducing, like workers in ant and bee colonies or bird helpers (that decide to help raising siblings instead of reproducing), will never maximize my reproductive success (it will actually always be zero right?).

Belding’s ground squirrel scream a danger call when they see predators. The caller is generally killed by the predator itself because it attracts its attention, but screaming will alert its family to run away safely. And inside family everybody is related no?


Well, that is the reason why we introduce here the idea of inclusive fitness. It’s defined as the total fitness (or offspring) I can produce either personally or by helping my kins in doing so. So, compared to the normal fitness, this includes also the possibility that I help other individuals to produce their children, and in some way I gain fitness. How can that be?

Think about it. I want to maximize the genes I want to transmit to the next generation. So if I’m my sister’s brother, we share 50% of genes right? We come from the same parents after all. So would it make difference having a children of my own or helping my sister having two of them? No it does not. It may seem weird, but statistically the proportion of transmitted genes is the same (50%+50%=100% of my genes I would transmit personally).

Here it comes Hamilton’s rule (thanks to W.D. Hamilton, a great guy that had a couple of great ideas). If two individuals are highly related, a behaviour that gives your kins advantage in producing offspring can be promoted by evolution even if that means not reproducing directly yourself (Ant workers and soldiers never reproduce for all their lives. They just help defending and gathering resource for the colony. Only the queen will mate and have offspring. But workers are also queen’s offspring right?.

The formula goes as it follows: B×r>C i.e. The behaviour (such as worker in a colony) will be favored if Benefits (B) multiplied by the coefficient of relatedness (r) are greater that the costs (C). Makes sense no? What exactly is r? Is a measure of how closely related two individuals are. In case of brothers we said before, it would be 0.5, because they share 50% of genes. Mother and offspring? Also 0.5. Me and my cousin? 0.25 (1/4). It is defined as the proportion of genes trasmitted by common descent. So the inequality will hold when either B is high, r is high or both B and r are.

Selection for this kind of traits that benefit kins more than the individual itself is called kin selection.

Now that we know what inclusive fitness is, what Hamilton’s rule predict and how r is defined we can proceed with the next blog post!

If you read until here, congratulations! Let me tell you another story..

P.S. Inclusive fitness and high relatedness is not generally accepted for explaining behaviour in insect colonies, but that woul lead into technical details. My point is that we can explain counteruintive behaviours looking from another perspective.


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