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Science Brainwaves are also on twitter. @SciBrainwaves. They are doing good regular updates of interesting bits of science news.
“By virtue of exchange, one man’s prosperity is beneficial to all others.”
Frederic Bastiat
Mutualism is quite an alien concept to us humans. In evolutionary terms it is not a good-for-group idea. Nor is it a “scratch my back and I’ll scratch yours” arrangement (I will discuss this in the next post). Surprisingly, it is also completely selfish.
Mutualism is any process or behaviour where both parties make gains which immediately outweigh the costs. Sometimes this is because the costs are virtually nothing. I am struggling to think of more than one example of mutualism in human society, but it is in fact so common it is hard to see it in this light. Trade in humans is nearly always mutualistic. If it is not mutualistic we think of it as either a con or charity, depending on who benefits. So, how is trade mutualistic? The fact that both parties benefit instantly means they must have different needs and assign different value to whatever service or good is being traded. So, a shopkeeper buys chocolate bars in bulk. Lets say they cost £10 for 100 at a cash-and-carry, bulk buy type place. So the bars cost 10p each and the shop keeper sells them for 50p. As the consumer, 50p isn’t much to pay for a chocolate bar. You want a snack and it would be more expensive to buy 100 bars from the cash-and-carry even though they are cheaper per bar. So you are getting a good deal, you would probably pay 60p for the chocolate bar, but it is only 50p. The shopkeeper paid 10p for the chocolate bar so is also getting a good deal. For both parties the benefits (50p to the shopkeeper or a chocolate bar to the consumer) immediately outweigh the costs (10p to the shopkeeper or 50p to the consumer). This then is a true mutualism.
Mutualism then is actually an extremely simple idea. Both parties benefit instantly. Easy. However, the situations in which it occur are harder to understand. There are other mutualisms in human society. When bands play together, especially at smaller gigs, often a out of town headliner will play with a local support band. The support band get to play with a more famous band and get more exposure in their own town. The headliner gets a guaranteed crowd due to the fans of the local band. Everyone wins.
Above: (left) A hornbill eating some fruit and so dispersing the seeds. (right) Mycorhizzal fungi on a plant root.
What about the natural world then? There are many, many examples of mutualisms in the natural world. Humans generally need the same things. Food, money, a house. As organisms are all so different, with different food sources and different needs, mutualism is more common. For example, plants need their seeds to be dispersed (but often have plenty of food, as they make it from carbon dioxide and sunlight). Animals on the other hand often need food. So plants and animals have a form of mutualistic trade. Plants produce fruit, a good food supply for animals. When the animal eats the fruit the seeds get carried around and dispersed. Plants have plenty of food and animals move around anyway, so the costs are minimal. The benefits to both (food for the animal and seed dispersal for the plant) are enough to instantly outweigh the costs.
Many plants have mycorrhizal fungi attached to their roots. Again, plants have plenty of food because they make it themselves. The fungi are particularly good at absorbing water. Plant growth is often limited by the amount of water they can get while fungi are often limted by the amount of sugar. So they trade. The fungi live on the plant roots and give up some of the water that they absorb. In return they get some of the sugar that the plant makes. The cost of water is low to a fungus and high to a plant while the cost of sugar is low to a plant and high to fungus. Because they have different needs, the trade is beneficial to both parties.
We humans also have mutualisms. The bacteria in our gut (the ones that yoghurt adverts insist on calling “friendly bacteria”… pah!) can digest cellulose. Cellulose is the carbohydrate that makes up most of the structure of plants and human cells can’t digest it. We provide the bacteria with a warm, wet place to live. In return the bacteria digest our cellulose for us. Once again, everyone wins.
So that is mutualism. I’ve said that science is made interesting by paradoxes. Mutualism is not a paradox, and so is not the most exciting thing in the world. It is however all around us and some of the most important groups of organism have got where they are due to mutualisms. Animals and bacteria, plants and fungus, termites and digestive fungus, coral and algae. Wherever you look mutualisms abound. However, in the next post I will discuss another paradox. How does evolution evolve when the benefits don’t immediately outweigh the costs, when some individuals can cheat or when the quest for quick gains disrupts long term cooperation.
“Do you live for the good of the tribe, Stilgar?”
“There is no other way.”
“And for the good of the tribe would you let me stab this knife into your heart?”
Frank Herbert – Dune
Normally I wouldn’t bother writing about group selection because as an idea it is dead and gone as far as I, and most evolutionary biologists are concerned. However, I started reading this blog about group selection. I thought I would write about group selection for a few reasons. Firstly, it’s on my mind at the moment so hopefully the post will be quite good. Secondly, the link provides a good “other side of the story”, so hopefully a couple of readers might immerse themselves in the debate. Finally, many members of the public might intuitively see group selection as plausible. This is not something to be ashamed of. A certain Charles Darwin thought it was the best explanation for some social behaviour. Do a biology degree however, and group selection is blasphemy. Because of this it is easy for biologists to lose sight of what an intuitive idea it is.
Two groups of people go into two shops. One of the groups are well versed in the wonderful British institution that is queuing. People politely accept their position in the queue for the tills and everyone gets served quickly and efficiently. In the other shop it is a free-for-all (you might have experienced this in France/Italy. It is very disconcerting.) and because of all the scrummage everyone actually takes longer to pay. However, if anyone in the free-for-all shop tried to form a queue, they would be the last to pay. In the polite shop, if someone skipped the queue they would get their goods quicker than if they joined the queue, but this would increase the time it takes for everyone else to do their shopping. This is the crux of group selection. Group selection says that a group with a behaviour that is for the common good will reproduce faster than those without this behaviour. As long as the selection between groups is stronger than the selection within groups the cooperative behaviour will spread through the population, even though anyone who behaves selfishly will do better than the rest of their group.
The counter argument is that whenever an individual is selfish in a non-selfish group (pushing to the front of the queue), this selfish behaviour will spread through the group. Soon there will be no cooperative groups to even compete with the selfish groups. The selection within groups is stronger than the selection between groups.
Despite what some might say, there is an issue of semantics now. Originally, group selection was supposed to work in the same way as individual selection. Instead of the best individuals surviving and reproducing, the best groups survived and reproduced. Now however, the thinking goes that genes are the unit of selection but if group living allows a social behaviour to evolve, this is also group selection. In other words, group selection can only work when gene level selection says it can work anyway. Some say this is group selection and a highly useful scientific advance. Others say, whats the point in worrying about group selection when gene level selection tells you exactly the same thing? The important addition to this is that gene level selection will never give you the wrong answer. If you make a model and gene level selection says something can evolve, it can evolve. With group level selection however, a model might give you the wrong answer. The only times it will give the right answer in fact, is when you arrive at the gene level selection model, but from a different standpoint.
Furthermore, the “group” is a very fuzzy concept. Since the 60s the idea of an individual has been very carefully scrutinised to the point where an individual or even a cell counts as group. What used to be a group is now a lose nit group. Bigger units, all the way up to the entire earth, can be considered losely bound groups. Everything down to chromosomes can also be considered a group, albeit a tightly nit one. The gene on the other hand has a quite precise, consistant definition. This just leads to more mistakes and fuzzy thinking. Most group selection models consider selection between groups and compare it to selection between individuals. If selection between groups is stronger than selection between individuals group selection can occur. But individuals are now another type of group. So we need to consider individuals as groups as well as our original group (as kin selection essentially does). Eventually you end up considering genes to be “individuals” and everything else a group. Then you have a gene level selection model, albeit in an interesting social setting. Why not start with the gene level model? The maths ends up being easier and you can’t make mistakes.
I intend to write more on the definition of an individual soon. Until then I hope you read DS Wilsons blog and see what you think. The section on Dawkins is an interesting insight into the slightly more personal side of science.
“And I will smite the inhabitants of this city, both man and beast: they shall die of a great pestilence.”
Jeremiah 21:6
Why do diseases make us ill? Why do some diseases kill us outright? As with many biological questions this is really two questions. Firstly, “What chemical and biological processes are happeningin our body that makes us feel ill?” Secondly, “What evolutionary gains does a disease get by making us ill?” These two questions are asking about different things known as the proximate and ultimate answer. The proximate question is looking for the mechanism that makes something happen. The ultimate question is asking why that mechanism has evolved in the firstplace.
Why are humans so intelligent? Here again we have a proximate and ultimate question. Proximately, humans are intelligent because we have large brains. Our brains fold in a way that allows more connections to be made and this also increases our intelligence. You can find the answer to this proximate question by finding a willing volunteer, cutting their head open and having a look inside (or maybe looking in a text book). For the ultimate question, we have to look into our past and try to understand whether higher intelligence perhaps allowed humans to hunt and forage better? Or maybe it helped us live in groups, which meant we could fight rival groups better? The point is, there are often two questions hidden in a single biological question.
So then, why do diseases make us ill? I am most interested in the ultimate question here because it is a paradox, and that’s when science gets exciting. It is a paradox because diseases are spread by their human host. If the human dies, or sits in bed with a lemsip, they are not out and about spreading the disease to new hosts. If a new disease arrives in a country, infects one person and promptly kills them, that is the end of the disease. If instead the disease arrives in a country and keeps the one host alive, the infected person wanders around, sneezing near people, kissing their partner(s), shaking hands with work associates etc. etc. The disease spreads.
Only diseases that keep their hosts alive will spread. All the diseases that have ever infected just one person are gone and forgotten. So, this is the paradox. We see many diseases that kill their host quite quickly, and a few that kill their host very quickly. The best thing a disease can do however, is never kill its host (this is called low virulence). So what’s going on?
The classical answer for this problem is that highly virulent diseases are just less evolved. Diseases like colds that do not kill their host have had a long time to evolve so they have evolved low virulence. More deadly diseases such as malaria must therefore have starting infecting humans much more recently. However, the very high speed of disease evolution makes this theory quite unlikely. However, slightly turned on its head, this theory becomes useful. Maybe diseases aren’t adapted to living in humans at all. It seems likely that many diseases that kill humans are not really ‘human diseases’. The Ebola virus is a good example. In humans it is fatal. However, it is likely that it is not a human disease at all, but a bat disease. It is not lethal to bats, and so is a successful disease. On occasions however, it gets spread to humans. It is not adapted to living in humans and so ‘accidentally’ kills its host.
Another theory comes from the idea of trade-offs. Trade-offs are very common and very important in biology for much the same reason as they are important to humans; we have limited resources. I like reading, but I also like walking in the Peak District. However, I have limited time. Going for a walk indirectly reduces the amount of reading I can do. So I try to find the right balance. I still don’t read as much as I would like, or do as much walking as I would like, but I do the best I can. What then are the trade-offs that affects virulence?
Transmission is how well a disease can spreaditself to other hosts. It depends on many things such as how many copies a disease makes of itself in its host. The more malaria bacteria in the blood,the more likely it will be picked up and spread by a mosquito. However, these bacteria need nutrition, and so the more bacteria, the more of the bodies resources they use and the more ill the host becomes. Here then is a trade-off between transmission and virulence. The balance found by diseases depends on many things such as how densely populated the area is and whether the disease is spread by contact, by animal vectors or some other means of transmission.
This then explains why some diseases are quite fatal, whereas others are benign. It is an ultimate explanation. I don’t know much about the specifics of how malaria kills people or how the common cold avoids killing its host, but I know that they are both struggling with a trade-off between transmission and virulence. This trade-off is (ultimately) why some diseases make us slightly ill and some diseases kill us very rapidly.
