Ecological collapse and the emergence of traveling waves in transitional turbulence

How a laminar flow becomes turbulence has been a long-standing problem in fluid dynamics and is important in various industrial applications. As Reynolds number increases, a single turbulent puff in a long pipe undergoes spontaneous relaminarization and spatiotemporal intermittency and can expand into turbulent slugs, exhibiting non-trivial statistics in transient times that have been found in experimental observation and hydrodynamics simulations. I am studying transitional turbulence by connecting it to spatially-extended ecosystems. We have found the extinction event and the formation and propagation of spatial patterns in predator-prey systems can be interpreted as the instabilities in fluid systems. I am also studying such connection in other hydrodynamics systems.

Nature Physics 12, 245 (2016),    Journal of Statistical Physics (2016).

Collective co-evolution of cyanobacteria and cyanophages mediated by horizontal gene transfer

Horizontal gene transfer is a powerful driving force for accerlerating evolution.  We study a model for how antagonistic predator-prey coevolution can lead to mutualistic adaptation to an environment, as a result of horizontal gene transfer. Our model is a simple description of ecosystems such as marine cyanobacteria and their predator cyanophages, which carry photosynthesis genes. These genes evolve more rapidly in the virosphere than the bacterial pan-genome, and thus the bacterial population could potentially benefit from phage predation. By modeling both the barrier to predation and horizontal gene transfer, we study this balance between individual sacrifice and collective benefits. The outcome is an emergent mutualistic coevolution of improved photosynthesis capability, benefiting both bacteria and phage. This form of multi-level selection can contribute to niche stratification in the cyanobacteria-phage ecosystem.

Rapid evolution

One of the important questions in ecology is how evolution affects ecological systems. Conventionally, the evolutionary time scale is usually assumed to be too long, such like millions of years, when compared with the life time of species. However, how exactly the evolutionary process occurs and changes ecological systems to become the contemporary structures is unknown. Moreover, the evolution time scale could be comparable to the ecology, such that antibiotic resistance keeps emerging rapidly in the treatment for bacterial infections. In fact, anomalous dynamics of ecosystems under evolution has been observed in lab experiments, which is so-called "rapid evolution". Rapid evolution may arise from fast response to strong selection among distinct strains. By applying statistical mechanics approach and stochastic simulation to a generic simple model based on the individual-level behaviors, we find the intrinsic stochastic nature of the system can lead to the emergence of rapid evolution. We also worked out the phase diagram where fluctuation-determined extinction is considered.

Phys. Rev. E 90, 050702(R) (2014).

Evolution of phenotypic fluctuations

 Phenotype fluctuations are pervasive, and can be important in various scenarios in evolution. Phenotypic fluctuations have been conjectured to be beneficial characteristics to protect against fluctuating selection from environmental changes, the so-called “bet-hedging” strategy. However, it is not well-understood how phenotypic fluctuations shape the evolutionary trajectories of organisms. To address these questions we have performed directed evolution experiments and modeling on the speed of migration phenotype of chemotactic bacteria. We present a theoretical model that recapitulates the observed reduction of phenotypic fluctuations in experiment. Our stochastic modeling on the evolution of migration fronts suggests that whether or not phenotypic fluctuations grow or shrink during successive rounds of selection and growth is determined by both strength of selection and the existence of physical constraints.

Physical Biology 15, 065003 (2018).