Pierrick Bourrat on levels, time, and fitness, part 2: collective fitness

Last week, I posted some thoughts on Pierrick Bourrat’s new paper in Philosophy and Theory in Biology, focusing on his criticism of Rick Michod’s ‘export of fitness’ framework. This week, I’ll take a look at the second of Bourrat’s criticisms, regarding the transition from MLS1 to MLS2, as first defined by Damuth & Heisler, during a transition in individuality.
MLS1 and MLS2 refer to two different versions of MultiLevel Selection. As Bourrat describes it (and this is pretty much in line with other authors), fitness in MLS1 is defined in terms of the number of particles (or lower-level units, or cells) produced, while in MLS2 the fitnesses of the particles and collectives (or cells and multicellular organisms) are measured in different units. Cell-level fitness (for example) is defined in terms of the number of daughter cells, organism-level fitness is based on the number of daughter organisms. (As with last week’s post, I’ll generally stick to cells and organisms, though the principles apply equally to any two adjacent levels.

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Pierrick Bourrat on levels, time, and fitness, part 1: zero fitness?

Pierrick Bourrat’s new paper in Philosophy and Theory in Biology criticizes aspects of the influential ‘export of fitness’ framework developed by Rick Michod and colleagues and extended by Samir Okasha (Bourrat, P. 2015. Levels, time and fitness in evolutionary transitions in individuality. Philos. Theory Biol., 7: e601. doi: 10.3998/ptb.6959004.0007.001). According to this view, an evolutionary transition in individuality, for example from unicellular to multicellular life, involves a transfer of fitness from the lower level units (e.g. cells) to the higher level unit (e.g. nascent multicellular organism). Fitness is defined as the product of viability and fecundity, and the emergence of a division of labor between reproductive (germ) and non-reproductive (somatic) units at the lower level exports fitness to the higher level. Full disclosure: Rick Michod was my Ph.D. co-advisor, and he has had a huge influence on my thinking about this topic.

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Who are you calling lower?

Weismann Fig. 62

Fig. 62 from Weismann, A. 1904. The Evolution Theory. London: Edward Arnold. Pandorina morum; after Pringsheim. I, A young colony, consisting of 16 cells. II, Another colony, whose cells have reproduced daughter-colonies; all the cells uniformly alike. III, A young Volvox-colony; sz, somatic cells; kz, germ-cells.

I needed to cite some information from August Weismann’s 1904 book The Evolution Theory1 yesterday, so I did something I rarely do anymore: walked over to the library and checked out a physical copy. The University of Montana library has a first edition, two-volume set of the translation by Arthur Thomson. I’m always interested to see how biologists thought about Volvox before people like Richard Starr, David Kirk, and Rüdiger Schmitt came on the scene. All of the quoted text is from pages 257-261 in Volume I.
Among the lower Algae there is a family, the Volvocinæ, in which the differentiation of the many-celled body on the principle of division of labour has just set in; in some genera it has been actually effected, though in the simplest way imaginable, and in others it has not yet begun.

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The most important time in your life

The New York Times has picked up a recent article in Physical Review Letters by Stephanie Höhn and colleagues (Höhn, S., Honerkamp-Smith, A.R., Haas, P. a., Trong, P.K. and Goldstein, R.E. 2015. Dynamics of a Volvox embryo turning itself inside out. Phys. Rev. Lett., 114: 1–5. doi 10.1103/PhysRevLett.114.178101).

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