Changes

The image below shows two different cases (upper and lower) of individual selection, where the green points with fitness = 1 - between the two lines - will be selected, while the red points outside with fitness = 0 will not. The centre of gravity, m, of the offspring is fat black and ditto of the parents and offspring in the new generation, m*  (according to the Hardy-Weinberg law), is fat red.  

The image below shows two different cases (upper and lower) of individual selection, where the green points with fitness = 1 - between the two lines - will be selected, while the red points outside with fitness = 0 will not. The centre of gravity, m, of the offspring is fat black and ditto of the parents and offspring in the new generation, m*  (according to the Hardy-Weinberg law), is fat red.  

www.evolution-in-a-nutshell.se/image001.gif

http://www.evolution-in-a-nutshell.se/image001.gif

Because the fraction of green feasible points is the same in both cases, Fisher’s theorem states that the increase in mean fitness in vertical direction is equal in both upper and lower case. But the phenotypic variance (not considered by Fisher) in the horizontal direction is larger in the lower case, causing m* to considerably move away from the point of intersection of the lines. Thus, if the lines are pushed towards each other (due to arms races between different species), the risk of getting stuck decreases. This represents a considerable increase in mean fitness (assuming phenotypic variances almost constant).  Because this gives room for more phenotypic disorder/entropy/diversity, we may expect diversity to increase according to the entropy law, provided that the mutation is sufficiently high.

Because the fraction of green feasible points is the same in both cases, Fisher’s theorem states that the increase in mean fitness in vertical direction is equal in both upper and lower case. But the phenotypic variance (not considered by Fisher) in the horizontal direction is larger in the lower case, causing m* to considerably move away from the point of intersection of the lines. Thus, if the lines are pushed towards each other (due to arms races between different species), the risk of getting stuck decreases. This represents a considerable increase in mean fitness (assuming phenotypic variances almost constant).  Because this gives room for more phenotypic disorder/entropy/diversity, we may expect diversity to increase according to the entropy law, provided that the mutation is sufficiently high.