quote: Nobody 'requires' it. Kimura formulated it and tested his hypotheses regarding it due to his observations of amino acid substituion.
This isn’t entirely accurate, or at least is not telling the whole story. Kimura formulated the neutral theory because the mathematics did not support NeoDarwinian evolution, not because of direct observation of amino acid substitution. I’m sure you’ve seen this important snippet before:
“Under the assumption that the majority of mutant substitutions at the molecular level are carried out by positive natural selection, I found that the substitutional load in each generation is so large that no mammalian species could tolerate it. … This was the main argument used when I presented the neutral mutation-drift hypothesis of molecular evolution” – The Neutral theory of Molecular Evolution, Kimura 1983, p 26 [via W. Remine, p 239]
The neutral theory was formulated to combat Haldane’s Dilemma, plain and simple. Haldane (1957, p 520-521) illustrates clearly the problem. He gives an example showing how increasing selection has a negative proportional impact on fitness. He had to slow evolution to a crawl (n = 300 generations per substitution under positive selection) to keep fitness at reasonable levels. He plugged in 7.5 generations and showed that fitness was reduced to e^-4, which he correctly called “hardly compatible with survival (it means 109 offspring required per breeding couple just to maintain constant population size!).