MEC1960

Tantulus - you're correct that for most purposes of understanding selection the distinction is not critical though for pedagogical reasons I do think it is important to always keep it in mind. Too many "just-so" stories are told about selection because this key idea is forgotten. Ophiolite - I don't think that pendulum has swung at all. Quite the opposite, in fact. There is still a lot of heated arguments about it, particularly recently among physiologists but also from those still clinging to ideas like group selection, but most of those stem from a misreading (or misunderstanding) of the selfish gene concept. That's not to say that the debate is settled - far from it - just that the gene-centrist view of selection has the upper hand. FTR - I don't have a dog in this fight. I do accept the evidence of the gene-centric view but accept also that there may be exceptions.

We must be careful here. The idea of fitness has been debated for a long time with it (very roughly) boiling down to two concepts, one championed by Gould and Eldrege (via G.G. Simpson and others) and the other by Richard Dawkins, W.D. Hamilton and others. The Gouldians (if I may) argued that an individuals reproductive success is selected based on a variety of traits, such as the tendency to engage in "risky behaviors" cited above. However, the dominant idea today is that it is not individuals that are the unit of selection, it is the genes themselves.

The lymphatic system's primary role is to distinguish self from non-self and is only one of several lines of defense we use to combat pathogenic (disease causing) organisms. Your question revolves around the colonization of the skin and mucosa (the linings in our GI, excretory and respiratory tracts) with organisms which do not normally cause disease. These "good" germs occupy niches that "bad" germs might otherwise. Our skin and mucosa are barriers to our insides - think of the space in those tracts I mentioned as the "outside"- and thus are a kind of passive immunity. This is unlike the lymphatic system which is an active form of immunity - our immune cells must undergo an active process of recognition (self v non-self), signalling to other constituents of the response, activation (and then deactivation) and then partake in various mechanisms for clearing the pathogen (the same process is used to clear cancer cells). This internal system of immunity is able to recognize and then "remember" the pathogens so that in future, should the germs breach the external barriers again, it can respond. The lymphatic immunity you refer to is called "adaptive" for this reason. There are few pathogenic organisms that can cross the passive barriers themselves - usually they must be breached. A cut, tear or some other means to gain access to the "inside". We don't really need the "good" germs in this sense as our barriers are quite effective. But they are not inviolable; we are all subject to things that can damage those barriers. Three features govern a germ's ability to successfully cause disease in a host; it must be be infectious (able to grow in us), pathogenic (able to cause disease) and it must be present in sufficient amount to make a beach head, so to speak. This last is called an innoculum - the number of germs- too small and it will not result in disease even if it is infectious and pathogenic (typically because our adaptive immune system clears it, but there are other passive means that can handle a small innoculum). The presence of "good" germs on our skin, for example, effectively reduces the size of the innoculum of "bad" germs on us and thus addresses one of the those three features. So having conditions which favor the growth of "good" germs can be seen as adaptive. It is likely that we have evolved to favor those conditions. They are not perfect (thus the need for an adaptive immune system) but our skin and mucosa have evolved to not just be barriers but to also provide favorable growth conditions for "good" germs. I should point out that some "good" germs can turn bad. Most of us have streptococcus growing on our skin that, under normal conditions are "good", but under the right conditions can switch to being "bad".