Distribution of non-synonymous mutations with respect to their effect on Fitness: Two theories

Geneticists agree that most non-synonymous nucleotide mutations (those that lead to protein substitutions) have a negative effect on protein function. (b) The traditional Selectionist Theory suggests that a small  fraction will have improved or greatly improved fitness (to the right of the neutral point). (a) The Neutral Theory suggests that most will have little or no effect on organismal fitness, and that a very small fraction will improve fitness slightly.

HOMEWORK: SMC thinks that the labels "Low" and "High" should be replaced with "Negative" and "Positive", respectively. Why?