from this article, first page, middle of the second column:
Even if harmful alleles do not become fixed, they can still reduce the efficacy of selection on neighbouring loci through a process called Hill–Robertson interference8. This effect occurs because individuals bearing deleterious mutations are less likely to survive and reproduce, reducing the number of individuals that contribute genetically to the future population. This reduces the effective population size witnessed by a focal locus
I don't understand the link between the Hill-Robertson effects and the effective population size. Why would the effective population size be greater not two genes were on the same chromosome (if there were no Hill-Robertson effects)? Can you please make this link clear to me?
Here is another question on the same article.