Japanese and some other races have even their 10th rib as a floating rib. Does that have any relation to evolution?

While all races have two pairs of floating ribs, some ethnic groups, especially Japanese have three pairs of floating ribs or tenth rib is a floating rib. No, it is not related to evolution. It is just that Japanese have high tendency to choose or to get tenth rib as true floating rib. There is a chance of having tenth rib as floating rib in other groups as well but Japanese are highly inclined to this chance. It is a normal variant in Japanese. There are four possibilities of intercoastal structures present between 9th and 10th ribs, one of which results in making 10th rib as floating rib. With the variation in these structures, the relation in 9th and 10th ribs is elucidated.