Buskirk et al. used yeast cells (thin orange circles) that are infected with a virus producing both a killer toxin (green arrows) and resistance to the toxin (thick blue circles) at the start of the experiment (represented by the 12 o’clock position). As evolution proceeds (black arrow), cells that no longer produce the toxin but are still resistant to it, take over. Eventually, these cells are replaced by cells that have lost their resistance (since resistance now provides no benefit). But when cells from the latest generation are pitted against cells from the original generation (grey arrows), the latter emerge victorious as the toxins they produce kill the former (open orange squiggles). However, cells from the latest generation can outcompete cells from intermediate generations, and cells from intermediate generations can outcompete cells from the original generations.