[Jean-Papoulos]

>1. It must increase the fitness of the organism that carries it, i.e. the likelihood of its carrier having descendants as compared to non-carriers

This isn't necessarily true. If you map out changes through the history of species, you'll find no significant changes but a lot of diversity for long periods, followed by big changes and low diversity for a short period. That's because during "abundant" times, the population will develop diversity as long as it doesn't significantly hinder reproductive rates. When an environnemental pressure comes up, the diversity dies down because the ones lucky enough to have adaptations that suddenly become useful and reproduce more.

So an animal might get a longer neck, but that doesn't significantly increase reproduction because food is aplenty. It's only when there's a drought that longer necks become an advantage and the trait is now selected for.

[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