(while the underlying biology is very different, there are some striking parallels to the migration of bacterial strains across different local gut microbiomes)

We show that these local migrations follow a clock-like process @ a rate of ∼1/50 cell divisions - roughly uniform across lineages & time. Plus, migrant B cells continue to evolve w/in their new germinal centers at similar rates, such that the largest lineages in each GC often originate from another

We show that despite this large mutational influx, rapidly evolving pop'ns naturally cluster into a smaller # of distinct “ecotypes”, even when their genetic diversity is much larger. This non-eq analogue of competitive exclusion is driven by a dynamical priority effect that favors resident strains.

Most existing models of evolving ecosystems assume that evolution occurs very slowly, so that the ecosystem can always equilibrate before the next mutation appears. Here we focus on the more empirically relevant case where ecology & evolution act on similar timescales, as often occurs for microbes.

We use this finding to re-examine models of purifying selection & adaptive reversion in human gut bacteria. After correcting for HGT, we show that most protein-coding variants are eliminated ~10x more slowly than previously assumed. Yet they are still reliably purged on 10-100k yr timescales.

Many studies have found that w/in-species dN/dS decays w/ the genetic distance between strains, which is often attributed to natural selection. Here Zhiru shows that a large portion of this trend can be quantitatively explained by the accumulation of horizontally transferred DNA segments over time.

Interestingly, of the subset of species shared w/ industrialized pop'ns, some show evidence of recent transmission, while others suggest a more ancient association. Among these, the genetic isolation between Tsimane & Hadza strains was generally larger than between comparable industrialized pop'ns

We were surprised to see right off the bat that - despite their long history of geographic separation - most of the microbial species in the Tsimane microbiome were also found in the Hadza (and most of these are either rare or absent in industrialized populations).

Excited to share some new work led by John McEnany. We generalize random matrix approaches from community assembly theory to predict how the fitness benefits & fates of new mutations should scale with the diversity & metabolic overlap of their surrounding community. We'd love to hear your comments!

Yeah I agree – it’s still a bit of a mystery for me too! Our paper suggests that the simplest spatial models can’t lower the Ne by *too* much in the human gut, but it’s possible that unmodeled aspects of spatial structure could contribute to this (e.g. diurnal oscillations)

If it helps, we also looked into this question in Fig 3 of this paper: www.pnas.org/doi/10.1073/... (similar model to the paper above). The answer turns out to depend on selection strength - more strongly beneficial mut'ns can arise in a larger effective pop'n (not always = to # of dividing cells)