Let’s say that there is a set of mutations S = \{x_1, x_2, ..., x_n \} that contribute towards advantageous trait Z, and that each member of the set x_i was acquired by accumulated changes over long periods of time, as you said.
Imagine this scenario: Mutations x_1, x_2, ..., x_{n-1} are all neutral, and were acquired “randomly” and fixed in the species over a long period of time. However, at some point x_n was acquired which in combination with x_1, x_2, ..., x_{n-1} resulted in a much more reproductively advantageous trait Z that also became fixed in the species.
- Is this scenario possible? Is it realistic?
- If this happened, from our genomes, would we be able to identify the individual who first obtained the x_n mutation, or for that matter, the different individuals who first obtained the x_1, x_2, ..., x_{n-1} mutations?
Why is species the defining line? Shouldn’t there be plenty of slightly advantageous and/or neutral mutations (or set of mutations) within different subsets of the H. sapiens population today which are slowly being accumulated, such that say, 100,000 years from now, there will be more H. sapiens with certain traits than others (e.g. hair color, social inclinations, susceptibility to various diseases, etc.) ?