  Alfred Russell Wallace regularly urged Darwin to jettison the term ‘selection’ as misleadingly anthropomorphic, and substitute Herbert Spencer’s ‘survival of the fittest’. Darwin went half way—in later editions he added ‘or Survival of the Fittest’ to ‘Natural Selection’ in the title of chapter 4. As the theory developed in the mid-20th century, the expression ‘survival of the fittest’ was gradually eliminated from any serious presentation of Darwinian selection theory. On the other hand, the concept of ‘fitness’ has played a prominent, and problematic, role. In the mathematical models used in population genetics, ‘fitness’ refers either to the abilities of the different genotypes in a population to leave descendants, or to the measures of those abilities, represented by the variable W. Here is a rather standard textbook presentation of the relevant concepts: