Thus average fitness in the population will be maximal but this does not mean the fitness of the population is maximal. Need to distinguish between individual fitness and population fitness . This is largely a demographic question since drought, predators, parasites, etc. A highly fit population is one that has a high reproductive output or is unlikely to go extinct. Consider wasps living in nest holes .
Population is limited by a fixed number of nest holes.
Now a mutation occurs that doubles the egg output of individuals carrying the mutation. Those individuals with mutation are more fit, but the population will not increase due to limitation of number of nest holes. Soft versus hard selection . See figures 7.4, 7.5, pgs.