Horizontal gene transfer becomes disadvantageous in rapidly fluctuating environments
Horizontal gene transfer (HGT) allows organisms to share genetic material with non-offspring, and is typically considered beneficial for evolving populations. Recent unexplained observations suggest that HGT rates in nature are linked with environmental dynamics, being high in static environments but surprisingly low in fluctuating environments. Here, using a geometric model of adaptation, we show that this trend might arise from evolutionary constraints. During adaptation in our model, a population of phenotype vectors aligns with a potentially fluctuating environmental vector while experiencing mutation, selection, drift and HGT. Simulations and theory reveal that HGT shapes a trade-off between the adaptation speed of populations and their fitness. This trade-off gives rise to an optimal HGT rate which decreases sharply with the rate of environmental fluctuations. Our results are consistent with data from natural populations, and strikingly suggest that HGT may sometimes carry a significant disadvantage for populations.