"So the question is simple, is the sex determinant ratio exactly 50 : 50?"
No, there are known cases when the population's gender ratios are not 50:50. When is it an advantage not to have a 1:1 gender ratio?
Adactylidium mites are born pregnant with their eggs already fertilized. The female matures then her fertilized eggs hatch inside her. Her offspring ratio is a single male to all the females. Since there is no option or mate selection one male is sufficient to ensure optimal reproductive success. The male fertilizes his siblings, the mother dies, and the female young emerge but the male has completed his job and does not emerge. Group selection favors female biased gender ratios for greater reproductive efficiency but it only works when mating is not random.
http://en.wikipedia.org/wiki/Adactylidium
This indicates there are selective pressures on a population to have a relatively balanced gender ratio based on the populations habitat.
If there were no selective pressures then females who bore mostly female offspring would have a huge advantage in ensuring their genetic survival. This trait for bearing more female offspring would continue to spread until males were in too short a supply to cover all the females in each generation.
At this point the rare males are all reproducing but not all the females are. Now the benefit is with the females that produce more male offspring. At this point alleles for bearing male offspring would spread in the population. The population stays in equilibrium because the reproductive benefit of producing male and female offspring is roughly equal.
http://en.wikipedia.org/wiki/Fisher%27s_principle
'An Experimental Demonstration of Fisher's Principle: Evolution of Sexual Proportion by Natural Selection '
http://www.genetics.org/cgi/content/full/148/2/719
Edit-
The mites live on a single thrips egg. Their habitat varies very little. There is much less selective pressure on the mite populations so a small range of genetic variation is sufficient in a stable, consistent environment. They do have sexual reproduction to mix the array of new infidelities that arise in every individual in every generation. DNA replication introduces a level of new alleles for sufficient genetic variation here for the degree of change they have faced in past generations. However with fewer males the population can increase much more rapidly that when the females spend resources producing half the population as males.
Aphids alternate between being asexual and being sexually reproductive.
Stable environment => parthenogenesis => rapid population increase with genetic fidelity
Changeable environment => sexual reproduction => genetic variation with winged emigrants
While the environment continues stable the aphids skip the genetic recombination of sexual reproduction and the effort of producing males that cannot increase the population. Only when the aphids must face a variable environment do they forgo rapid population increases for sexual recombination with males. Variety is what enables the species to have some members in the next population able to use a different environment than their parent's occupied.
When the aphid's preferred soft plant tissue is plentiful, aphids reproduce in the wingless, asexual stage. In this stage they reproduce with a more rapid increase in population since all offspring can produce more offspring. In this way a single aphid, finding a good food source, can produce millions or billions of descendants well suited to consuming this food source.
When food starts to become scarce they switch to the winged sexual life cycle stage in order to ensure the genes are recombined so there is more genetic variation in the dispersing generation. Of those that emigrate and locate a new food source that is slightly different there will be a few with a better greater fit to the new conditions. All it takes is one or two to find a new suitable food supply within the range of their genetic variation for the line to continue.
‘Family Planning Aphid Style’
http://indianapublicmedia.org/amomentofscience/family-planning-aphid-style/
Terrestrial animals, the metazoan vertebrates face some of the most unstable environments possible. This selective pressure comes from short generations of microbes that constantly adapt to our immune responses and from the ecological interactions as well as the abiotic conditions. Thus the vertebrates have balance gender ratios for maximal recombination despite the limitation it places on population growth rates. Further vertebrates have evolved the highly mutable adaptive immune response so metazoans can directly compete with the bacterial pathogen's rapid rate of genetic change.
