Speed of secondary wave in a coupled Fisher wave system

As described last week, the Fisher equation describes the spread of a population (or the spread of an advantageous gene through a population) and is a simple nonlinear equation which exhibits travelling wave solutions. The selection mechanism for the speed of these waves was discussed last week. This time we consider two coupled Fisher equations representing two populations e.g. sub-populations of bacteria which are susceptible or resistant to antibiotic. We show that a subtle coupling between two population waves gives rise to a novelspeed selection mechanism for the secondary trait wave moving through the expanding population.