Those of you who were naturally tempted to click on this article by your well honed instincts, which drive evolution and survival, are being set up for a sore disappointment due to a complete lack of titillation, the norm on the forum of this website.
There is an unacceptable waste of time and effort involved in sexual mating which is inefficient and non-productive and could be better utilized by an organism able to reproduce by cloning or a hermaphrodite unencumbered by the problem of finding a date! Why then does evolution not eliminate this redundancy of this bi-sexual mating behavior?
There are three accepted explanations.
First, the recombination of genes allows the mixing and propagating of beneficial mutations and elimination of harmful mutations by sequestering them in the same successful, aborted or extinction destined progeny. Secondly, the gift of a set of genes from each parent provides a spare tire to compensate for any defective inheritance from one parent. Third and finally the process of recombination during meiosis allows the marked variation, that leads to every sib being different except monozygotic identical twins and provides a way to combat faster proliferating parasites with short life cycles and faster generational variation.
Now that we have successfully answered, why sex has been preserved and propagated by evolution, it brings us to the next enigma as to how it is determined. In humans the presence of a single gene called SRY and located on the Y Chromosome determines the development and thus birth of a male child. Those with two X chromosomes and no Y develop as females. Those with an XY combination develop as males. A non-functioning SRY gene on the Y chromosome leads to development as a female with a blind vagina and an absent uterus. Thus the natural tendency of a human fetus is to develop as a female unless interfered and stopped by the product of the SRY gene on the Y chromosome.
The process is not as simple and in birds the development is reversed. It gets even more radical in our amphibian ancestors who first deserted the seas for a partial life on land. In them it is the ambient temperature, which determines the sexual development path of an egg, and even more astonishingly it is the reverse between turtles and crocodiles. Thus the temperature that pushes the development of turtle eggs to female does the opposite in crocodiles. It gets even more bizarre in insects like the bees. Here the mother Queen Bee determines the sex of the egg, depending on whether her specialized organ, the spermatheca, where she stores the sperms from her multiple matings with different drones, is squeezed or not by the size of the egg-space of the cells of the hive. Males are born of unfertilized eggs and are haploid or have only half the chromosomes of their sisters, who are diploid and have double the number of chromosomes. There are certain wasps, which propagate by injecting their egg into the live bodies of other caterpillars or fruits and if they sense the presence of another egg in the host, they use their spermatheca to deposit a male egg instead of a female one to increase the chances of propagating their species.
It gets even stranger in a fish called the blue-headed wrasse, where the normal social life is organized like the Mormons, with a single large dominant male and a harem of females. Surprisingly, if the male dies by natural causes or predation, the largest female in the harem swiftly changes into a male and captures the harem. Aggression is a typically male characteristic and best correlated with size and testosterone levels, confirming the frequently quoted adage, that size does matter. In the matriarchal society of hyenas, where females rule and are uniquely masculinized by high testosterone levels that lead to a marked enlargement of the clitoris to a mini-penis and altered secondary sex characters to make birthing difficult. It is the dominant female and her privileged princess who rule the clan.
In the seahorse, the male cares for the eggs and progeny while the promiscuous female philanders. To further validate that size matters, in a marine worm called Bonellia Viridis, the hormonal secretions of a proximate female, if they reach the eggs lying in the sand of the water, determine the development pathway and result in a miniscule male, who resides in the womb of the female, feeding on her body and periodically fertilizing her eggs. Those eggs, which are not within the hormonal secretion territory of the larger female, develop as females to guide and capture their own nearby subservient resident male.
The fanatically suicidal zealot like some Islamic terrorists, but with a far more benign and divine agenda, is the male Black Widow spider, who is smaller and thus naturally less aggressive than the larger female. He approaches her web, lured by his fatal instinct and serenades her by twanging the spokes of her web, till she is sufficiently soothed, and mates with her. More often than not, he fails to escape her predatory nature after mating and she eats him. He has his revenge by his broken off penis plugging her vagina, thus precluding future mating and sacrifices his life and body to serve as nutrients for his children, that she will spawn and earns his permanent visa to partial immortality at a grave and high price.
Was there ever a more loving father?