In 9 percent of all mammal species, males and females will share a common territory for more than one breeding season, and in some cases bond for life. This is a problem — a scientific one — because male mammals could theoretically have more offspring by giving up on monogamy and mating with lots of females.
In a new study, Dr. Lukas and his colleague Tim Clutton-Brock suggest that monogamy evolves when females spread out, making it hard for a male to travel around and fend off competing males.
On the same day, Kit Opie of University College London and his colleagues published a similar study on primates, which are especially monogamous — males and females bond in over a quarter of primate species. The London scientists came to a different conclusion: that the threat of infanticide leads males to stick with only one female, protecting her from other males.
http://www.nytimes.com/2013/08/02/scien ... .html?_r=0
Most ape species live in matriarchal societies, where the females stay in the group, and the males scatter out to the neighboring clans to live, cutting ties with who they grew up with to join new groups.
However, most human societies are traditionally patriarchal, in that lineage of the father is the foundation of the family, and females are spread out to the surrounding groups. This has brought society into a clannish setting, with families exchanging women and forming ties/alliances with each other.
BUT, bonobos are also apes whose females leave the group to join others after they mature, and doing such is linked to a higher X:A ratio. A higher X:A ratio implies that a larger portion of females were able to pass on their genetics than males of a population.
Differences in female and male population history, for example, with respect to reproductive success and migration rates, are of special interest in understanding the evolution of social structure. To approach this question in the Pan ancestor, we compared the inferred ancestral population sizes of the X chromosome and the autosomes. Because two-thirds of X chromosomes are found in females whereas autosomes are split equally between the two sexes, a ratio between their effective population sizes (X/A ratio) of 0.75 is expected under random mating. The X/A ratio in the Pan ancestor, corrected for the higher mutation rate in males, is 0.83 (0.75–0.91) (Fig. 4 and Supplementary Information, section 8). Similarly, we estimated an X/A ratio of 0.85 (0.79–0.93) for present-day bonobos using Ulindi single nucleotide polymorphisms in 200-kb windows (Supplementary Information, section 9). Under the assumption of random mating, this would mean that on average two females reproduce for each reproducing male. The difference in the variance of reproductive success between the sexes certainly contributes to this observation, as does the fact that whereas bonobo females often move to new groups upon maturation, males tend to stay within their natal group20. Because both current and ancestral X/A ratios are similar to each other and also to some human groups (Fig. 4), this suggests that they may also have been typical for the ancestor shared with humans.
http://www.nature.com/nature/journal/v4 ... 0120628#f4
Graphs to the left (CEU) are European females, while graphs to the right (YRI) are West African female:
http://www.nature.com/ng/journal/v43/n8 ... g.877.html
Interestingly, the authors come to the conclusion that the disparity in X:A ratios are due to the bottleneck effect on the population that migrated out of Africa. I think it is due to pressure to be politically correct. Discussing behavioral differences of the different races are politically incorrect and could cost scientists their livelihoods.
In short, these graphs show a higher success rate of a higher number of European men to women passing on their genetics compared to African men to women, which could easily be explained by a higher level of monogamy in the former's culture.
Another study starts out as:
Comparisons of chromosome X and the autosomes can illuminate differences in the histories of males and females as well as the forces of natural selection. We compared the patterns of variation in these parts of the genome using two data sets that we assembled for this study that are both genomic in scale. Three independent analyses show that around the time of the dispersal of modern humans out of Africa, chromosome X experienced much more genetic drift than is expected from the pattern on the autosomes. This is not predicted by known episodes of demographic history, and we found no similar patterns associated with the dispersals into East Asia and Europe. We conclude that a gender-biased process that reduced the female effective population size, or an episode of natural selection unusually affecting chromosome X, was associated with the founding of non-African populations.