Suppose that, among related host species that carry related symbionts, the relationship is mutualistic in some pairs and parasit
ic in others. How would you determine (a) which relationship is mutualistic and which is parasitic (0.5 pt), (b) what the direction of evolutionary change has been (1 pt), and (c) whether the change from one to the other kind of interaction has been a result of evolutionary change in the symbiont, in the host, or both (1 pt)? (Hint: for (b) and (c), think phylogenies!)
a) a) Mutualistic relationship differs from parasitism because the first one benefits both of the involved organisms. Otherwise, parasitism benefits only one of the organisms and it can harm the host. Therefore, the parasite would have to be smaller than the host and it would be somehow extracting energy from the host.
b) The direction of evolutionary changes would follow some kind of pattern, like coevolution. Therefore, analyzing the phylogenies individually and then contrasting them, would help to understand how the host, as well as the parasite, have been evolving.
c) The phylogeny would also help to understand what changed first, or if they changed together as a result of coevolution.
