Biological Perspectives on Friendship Bread
If you think about it, "friendship" recipes (possibly including sourdough bread) are actually very bizarre hybrid biological/memetic symbiotes, which use their biological qualities (fermenting fungal culture) to provide a benefit (cake) to the host organism (us) which results in the genetic (the culture) and memetic (the recipe) components of the "organism" being spread to new hosts (our friends).
The parallels are not perfect. While wholly biological symbiotes evolve as a result of random mutation and natural selection, one of these hybrids can experience changes in its genome and especially its "memome" as a result of conscious actions on the part of the host. In layman's terms, this is when you change the recipe. These changes are often self-correcting (ie. if the new recipe makes bad cake, you won't use it again or give copies to your friends) but "neutral mutations"* or positive ones (new recipes which taste better) may be passed on to new "hosts" along with the starter.
* such as my friend's fortuitous "eggnog friendship cake" variation, which adds an extra egg and substitutes nutmeg for cinnamon. Very tasty :-) I now pass on both recipes with the starter, having effectively doubled the memome size of the hybrid.