...because in helping a close-knit commune it is very possible you're helping copies of genes you already have in other hosts.
I think in the 60's some biologists were proposing that the process of natural selection might try to preserve larger gene pools in that manner but so far there's been no actual evidence of such a thing. Richard Dawkings proposed the single gene -view in his 70's book The Selfish Gene that has as one of its main thesis that you must always look at evolution from the point of view of a single gene, which became the standard view and has stood up to challenges so far.
I'm quoting a summary of the book here taken from another
site:
"Natural selection acts on the individual's genes or rather, on the phenotypic effects of an individual's genes - not on the group as a whole. Genes build 'survival machines' or individual organisms. Genes are selfish in that they build survival machines to increase the number of copies of themselves, thus survival machines tend to be inherently selfish. Individuals that form groups do so for the benefit of their selfish genes (e.g. there is safety in numbers; safety for genes). An individual may act altruistically, but does so for its own gene preservation -- not group preservation."