Though not biologically related,friends are as“related”as fourth cousins,sharing about 1%of genes.That is_（1）_a study,published from the University of California and Yale University in the Proceedings of the National Academy of Sciences,has__（2）_.The study is a genome-wide analysis conducted_（3）__1,932 unique subjects which__（4）__pairs of unrelated friends and unrelated strangers.The same people were used in both_（5）_.While 1%may seem_（6）_,it is not so to a geneticist.As James Fowler,professor of medical genetics at UC San Diego,says,“Most people do not even_（7）_their fourth cousins but somehow manage to select as friends the people who_（8）_our kin.”The study_（9）_found that the genes for smell were something shared in friends but not genes for immunity.Why this similarity exists in smell genes is difficult to explain,for now,_（10）_,as the team suggests,it draws us to similar environments but there is more_（11）_it.There could be many mechanisms working together that_（12）_us in choosing genetically similar friends_（13）_”functional Kinship”of being friends with_（14）_!One of the remarkable findings of the study was the similar genes seem to be evolution_（15）_than other genes Studying this could help_（16）_why human evolution picked pace in the last 30,000 years,with social environment being a major_（17）_factor.The findings do not simply explain people’s_（18）_to befriend those of similar_（19）_backgrounds,say the researchers.Though all the subjects were drawn from a population of European extraction,care was taken to_（20）_that all subjects,friends and strangers,were taken from the same population.
13选？
上下文语义衔接+介词短语辨析所填词为逻辑关系短语，空前说选择基因相似的朋友，空后说具有“实用关系”的朋友，可知前后为对立关系，并且根据文章主题，可知肯定前者而否定后者，可锁定B选项rather than。A选项according to（根据）没有体现出对立及取舍关系，因此排除；C选项regardless of（不管）表让步关系，干扰性较强，但文章更强调取前舍后，因此排除；D选项along with（伴随）表顺接，故排除。故答案为rather than。
