Different women will have different numbers of daughters and there's a tendency for some lines to reproduce more prolifically than others. There's a neat little mathematical trick which demonstrates that if you take the total number of women in an isolated group, (say 40), within the same number of generations (40) every female in the group will most likely be descended from just one original female. Using that formula, a group of 160 women could reduce to just 4 lines in 1,000 years. Of course, in the article, it says 40% came from just 4 lines which means there were a lot more than 160 in the original migratory group and there's still plenty of reducing yet to go before you get down to just one line. It's estimated that every woman living today is descended from just one female who lived about 160,000 years ago.
Fascinating.