LAUSR

21 Identifying what drives individual heterogeneity has been of long interest to ecologists, evolutionary 22 biologists and biodemographers, because only such identification provides deeper understanding of 23 ecological and evolutionary population dynamics. In natural populations one is challenged to 24 accurately decompose the drivers of heterogeneity among individuals as genetically fixed or selectively 25 neutral. Rather than working on wild populations we present here data from a simple bacterial system 26 in the lab, Escherichia coli. Our system, based on cutting-edge microfluidic techniques, provides high 27 control over the genotype and the environment. It therefore allows to unambiguously decompose and 28 quantify fixed genetic variability and dynamic stochastic variability among individuals. We show that 29 within clonal individual variability (dynamic heterogeneity) in lifespan and lifetime reproduction is 30 dominating at about 82-88%, over the 12-18% genetically (adaptive fixed) driven differences. The 31 genetic differences among the clonal strains still lead to substantial variability in population growth 32 rates (fitness), but, as well understood based on foundational work in population genetics, the within 33 strain neutral variability slows adaptive change, by enhancing genetic drift, and lowering overall 34 population growth. We also revealed a surprising diversity in senescence patterns among the clonal 35 strains, which indicates diverse underlying cell-intrinsic processes that shape these demographic 36 patterns. Such diversity is surprising since all cells belong to the same bacteria species, E. coli, and still 37 exhibit patterns such as classical senescence, non-senescence, or negative senescence. We end by 38 discussing whether similar levels of non-genetic variability might be detected in other systems and 39 close by stating the open questions how such heterogeneity is maintained, how it has evolved, and 40 whether it is adaptive. 41 . CC-BY-NC-ND 4.0 International license peer-reviewed) is the author/funder. It is made available under a The copyright holder for this preprint (which was not . http://dx.doi.org/10.1101/105353 doi: bioRxiv preprint first posted online Feb. 2, 2017;