Sexual selection is a topic that invoked some controversy among evolutionary biologists. Darwin distinguished sexual selection as the difference in the ability to produce offspring, and natural selection, on the other hand, is about the struggles for existence.
Sexual selection is a combination of many factors. It could be a male-male struggle to reach a female, females snubbing males with certain features, or simply mating with certain males leading to weaker or no offspring.
mtDNA and NRY know it all
Whatever may be the precise reason, it has been established now using complex DNA analysis and computation that historically, more females and fewer males have participated in the development of the human race. In other words, throughout human history, leave the modern time when females started moving with their partners, fewer men participated in the reproduction process, although there are no reasons to believe that their respective numbers in the population were different. It went to such a low around 8000 years ago when the female-to-male effective population ratio was about 17!
References
Lippold et al. Investigative Genetics 2014, 5:13, Human paternal and maternal demographic histories: insights from high-resolution Y chromosome and mtDNA sequences
Genome Res. 2015 Apr; 25(4): 459–466, A recent bottleneck of Y chromosome diversity coincides with a global change in culture
