I can't figure out the meaning of the idiom "friends of a feather", as in:<br/><h2>A new <a href='http://www.pnas.org/cgi/doi/10.1073/pnas.1011687108' target='_blank' rel='nofollow'>study</a> suggests that when it comes to certain genes, friends of a feather flock together—but with others, opposites attract.</h2>I'd grateful for any hints.