Simplified model assumptions artificially constrain the parameter range in which selection at the holobiont level can occur
Van Vliet & Doebeli (1) (henceforth VVD) present a multi-level selection framework simulating host-microbiome evolutionary dynamics. The model explores the conditions under which the association between hosts and their "helper" microbiome - microbes that developed a trait that provides a benefit to the host at a cost to themselves - is strong enough to allow for selection at the holobiont level. The model is an important contribution to the holobiont debate, making the notion quantifiable and highlighting the involved parameters. In the study, VVD conclude that the parameter space in which selection occurs at the level of the holobiont is extremely restricted (Fig 1). Here we show that this result stems from specific model assumptions. Although these assumptions are reasonable as a modeling starting point, slight biologically-reasonable modifications of the assumptions lead to qualitatively different outcomes. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=110 SRC="FIGDIR/small/988766v1_fig1.gif" ALT="Figure 1"> View larger version (33K): org.highwire.dtl.DTLVardef@29fe1org.highwire.dtl.DTLVardef@4ea220org.highwire.dtl.DTLVardef@75e515org.highwire.dtl.DTLVardef@12e9d6_HPS_FORMAT_FIGEXP M_FIG O_FLOATNOFig. 1.C_FLOATNO VVDs original results - Selection can only act at the holobiont level under stringent conditions. The mean helper frequency is shown as a function of the timescale of the evolutionary dynamics at the microbe level relative to the host generation time and the ratio of vertical to horizontal transmission. Two different levels of sampling variance are shown: the SD of the sampling distribution is 0.02 in A and 0.1 in B; A (top) and B (top) show the results of single simulations, and A (bottom) and B (bottom) show the same data, averaged within discrete bins. C_FIG