evolution - Hill-Roberston effects and effective population size

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 interference. 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 in absence of Hill-Robertson effects? Can you please make this link clear to me?

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Here is another question on the same article.

Answer

Hopefully this syllogism will answer your question.

Given the following premises:

1. In the absence of selection, fitness of individuals are at a theoretical maximum.


2. If a theoretical maximum fitness is achieved then effective population size is maximum.


3. If there is an allele that confers both increased and decreased fitness you have a genetic conflict (e.g. an allele that turns you green gives you good woodland camouflage but is un-attractive).


4. If a deleterious allele is linked to a beneficial allele this is similar to having a genetic conflict.

The conclusion follows:

Strong linkage between beneficial alleles and deleterious alleles (Hill Robertson effects, i.e. 3. and 4.) prevents a theoretically maximum fitness to be attained (1.) and thus a reduced effective population size (2.). Which can only be overcome by high levels of recombination and mating (i.e. by separating deleterious and beneficial alleles).