Comment by Mordisquitos

What you say is correct, and that is why I was referring specifically for what's necessary for "features" (the term I used for "phenotypes" to make it more HN) to become ubiquitous. It is a general rule of thumb in evolutionary biology that the more diversity is observed in a given population for a particular phenotype (e.g. hair colour, height, blood group, etc.) the less relevant it is for fitness within its observed range. When a phenotype is strongly selected for in a given population (e.g. bipedalism, opposable thumbs, the ability to speak) it soon becomes dominant and there is much less diversity.

As to your example about, for instance, neck length during abundant times, that follows the same rule: during abundant times neck length simply does not matter for fitness, therefore (all else being equal) there can be phenotypic diversity in the population.

One caveat though as to how a given phenotype may become ubiquitous without favourable selection is of course genetic drift[0], given a small enough population which is isolated for a long enough timeframe. Eventually that phenotype may become selectively "advantageous" inasmuch as it is no longer compatible with alternatives, and individuals from the isolated population who this phenotype can no longer have successful offspring with individuals of a different phenotype, resulting in speciation. That's what I meant with regards to a "make nerves on important places generate more pleasure/pain in brain" genotype being incompatible with a "have more nerves on important places" one. A hypothetical hybrid creature would be a mess.

[0] https://en.wikipedia.org/wiki/Genetic_drift