Lineage selection in plasmid evolution

Question

I've been reading through Paulsson (2002) and I am not sure what he means by "lineage selection" in the second to last section. The paper deals with plasmid replication, and mostly concentrates on the contrasting pressures from two levels of selection:

1. intracell selection - competition between plasmids in a single cell. A plasmid can undergo a cis-mutation and over-replicate, resulting in a higher intracell fitness than the focal plasmid. The plasmids reproduce.
2. intercell selection - competition between cells. Cells with a heavier load of plasmids will take longer to reproduce, and plasmids that produce a heavy load will have lower intercell fitness. The cells containing plasmids reproduce.

In this context, what does the third level of selection -- lineage selection -- mean? What reproduces? How do the lineages split or interact?

My guess

Does lineage selection simply mean group-selection on separate colonies of cells? In that case, how are new lineages formed? I would expect this group level to select for zero levels of plasmids (since they place no loads on the cells and thus these groups of cells will grow the fastest), but Paulsson (2002) suggests the opposite:

lineage selection could favor plasmid traits that help the population of plasmid-containing cells to fight plasmid-free cells.

Is there a more detailed discussions of this available than the one section in Paulsson (2002)? Neither the unit of selection nor either the evolutionary or genetic lineage Wikipedia articles address my question. The first only mentions lineage in passing, and the second two don't discuss models of selection.

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References

Paulsson, J. (2002). Multileveled selection on plasmid replication. Genetics, 161(4): 1373-1384.

Answer 1

He defines lineage selection as selection for traits which increase the fitness of a group of plasmids, rather than an individual plasmid with in a cell or a particular cell containing plasmids. He says that the unit of selection are "plasmid-host clades" : in other words the unit of selection is the group of closely related plasmids in separate cells. It is an example of a kin-selection though I haven't seen the specific terminology in widespread use. He likely doesn't use kin selection because the plasmids don't have well defined offspring, so he uses a broader term for kin--lineage. I might have preferred clade selection, but this term has its own baggage. I'm not sure (and neither is Paulsson) that kin selection is need to explain 'spitefully low loss' rates, since intra-cellular and inter-cellular selection both favor lower loss rates.