Sexual reproduction carries some advantages and disadvantages. Dividing a population of organism into males and females creates a fitness disadvantage because males cannot have their own offspring T. Morran et al. state "Every male takes the place of an offspring-bearing progeny (female or hermaphrodite) that could have been produced. The systematic loss of offspring-bearing progeny can reduce the numerical contribution of a lineage by as much 50%." However, out-crossing (reproduction between different individuals) encouraged by sexual reproduction has an advantage of genetic variability. This genetic exchange between individuals creates a possibility for evolution of new traits, which may be vital for survival and adaptability in a novel environment.
Let's go back to the example of the world with no men. It is not hard to notice how "un-novel" our environment is. Modern medicines and vaccines keep us from getting diseases from most parasites and pathogens. The word "survival" seems very distant from our lives (unless you think making money is directly related to it). Moreover, sexual reproduction in humans seems even deleterious; mistakes in the genetic exchange mechanisms lead to children born with genetic diseases such as Autism and Down's Syndrome. At this point, no man has ever felt this guilty about his existence.
< Above: The 1998 study by Dybdahl and Lively on New Zealand Snails and its parasite trematodes revealed that the parasite will infect the snails with highest genetic frequency in a time-lagged manner.>
The Red Queen Hypothesis may be the answer to our men's despair. It underlines that the "evolutionary arms race" between hosts and their pathogens may be an endless chase. The presence of a pathogen puts a selective pressure on its hosts; the selected hosts that are resistance to the pathogen survive and reproduce. We might then think it is the host that is an eventual winner, but what we don't usually see is the fact that this evolution of hosts also puts the selective pressure on pathogens. As the resistant host increases in population and becomes the most common host, the pathogen counter-evolves for that host, which then has to evolve again to be resistant to the new strain of the pathogen. In the end of the day, the pathogen is endlessly chasing the host. Since there must be a time gap between each selection process, neither one "gets" the other.
How does sexual reproduction link to the Red Queen Hypothesis? 2011 study by T. Morran et al. compared the relationship between the nematode C. elegans and its bacterial parasite S. marcescens in three types of mating conditions: strictly selfing (asexual reproduction), strictly out-crossing (sexual reproduction), and wildtype (control). Each conditional population was exposed to three types of S. marcescens: ancestral strain (no new strains), non-coevolving strain (new strains without selection) and coevolving strain (new strains with selection). Each population was allowed to reproduce up to 30 generations.
The results on the left highlights that all of the selfing population under coevolving parasite (graph A: c and f) were eliminated within 10 generations (hence the comparison is between ancestral and 10th ). Host mortality for this population also doubled after 10 generations. To the contrast, when the population was allowed to have sexual reproduction (graph C: o and r) the host mortality went down and the population sustained itself after 30 generations. Therefore, we can conclude that sexual reproduction is vital for survival from the continuously evolving pathogens.
As safe as our world today seems like, we are actually still under the threat of pathogen evolution. Development of groundbreaking medicines and antibiotics have also led to the discovery of superbacteria, one that is resistant to the most antibiotics of today. While it may not be possible to fight the bacteria off with reproduction alone, men will still represent the reason why we were able to survive and thrive for the long human history.
Dybdahl, Mark F., and Curtis M. Lively. "Host-Parasite Coevolution: Evidence for Rare Advantage and Time-Lagged Selection in a Natural Population." Evolution 52.4 (1998): 1057-066.
Hampikian, Greg. "Men, Who Needs Them?" The New York Times. The New York Times, 25 Aug. 2012. Web. 20 Feb. 2013.
Morran, L. T., O. G. Schmidt, I. A. Gelarden, R. C. Parrish, and C. M. Lively. "Running with the Red Queen: Host-Parasite Coevolution Selects for Biparental Sex." Science 333.6039 (2011): 216-18.