Did group selection play a role in the evolution of plasmid endosymbiosis?

plasmidBacterial plasmids are nucleotide sequences floating in the cytoplasm of bacteria. These molecules replicate independently from the main chromosomal DNA and are not essential to the survival or replication of their host. Plasmids are thought to be part of the bacterial domain’s mobilome (for overview, see Siefert, 2009), a sort of genetic commonwealth which most, if not all, bacterial cells can pull from, incorporate and express. Plasmids can replicate inside a host and then move to another cell via horizontal genetic transfer (HGT), a term denoting various mechanism of incorporation of exogenous genetic material.
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Continuing our exploration of group selection

This Tuesday, I gave the second of two presentations for the EGT Reading group, both focused on the theory of group selection. Though I am currently working outside of academia, it has been a pleasure to pursue my interests in ecology, and our group discussions have proven to be both enjoyable and challenging.

The first presentation [pdf] is a review of a 2011 paper written by Marshall. It argues that when the models underlying inclusive fitness theory (IFT) and group selection are formally identical. However, as I tried to show during the presentation, this formal equivalency only holds for one specific type of group selection – group selection as the partitioning of selection between groups from selection within groups. It no longer holds when we consider the more restrictive definition of group selection as “natural selection on groups” in strict analogy to individual selection (this, incidentally, is the definition of group selection I gave in my last blog post)

Marshall J.A.R. (2011). Group selection and kin selection: formally equivalent approaches, Trends in Ecology & Evolution, 26 (7) 325-332. DOI:

The second presentation [pdf] is a review of a paper by Paulsson (2002). That paper presents an interesting case of multi-level (group) selection, where the “individuals” are plasmids – self-replicating gene clusters in the cytoplasm of procaryotes – and the “groups” are the plasmid-hosting cells. It’s a nice illustration of the basic dilemma that drives group selection. Inside a cell, plasmids which replicate faster have an advantage over their cell mates. But cells in which plasmids replicate too fast grow slower. Thus, at the level of individuals selfishness is favored, but at the level of groups altruism is favored. Paulsson’s paper explains the mechanisms of plasmid replication control; sketches up models of intra- and inter-cellular selection gradients; and explains how conflicts between individual- and group-selection are resolved by plasmids. He also considers a third level of selection on lineages, but both Artem and I were confused about what exactly Paulsson meant.

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