Why are there two genders of humans and animals?

Answer 1

There are multiple questions here:

• Why did sexual reproduction evolve?
• Why did sexual dimorphism (ie. the differences between genders) evolve?

To answer the first question, we must take a close look at what sexual reproduction is, in its most basic form. The answer I provide here is: sexual reproduction is the exchange of genetic materials between organisms, whereby some of the exchanged genetic traits are inherited by offspring. Now we know that even simple single-celled organisms exchange genetic materials. For instance, bacteriae store part of their genomes in plasmids, which are freely exchanged between bacteriae in the same population. There are various scenarios that describe how such simple lateral gene transfer may have evolved into more complex sexual reproduction involving gametes; scenarios which I will not outline here. Suffice it to say that despite the costs involved with sexual reproduction, there are distinct benefits that result in an increased overall fitness of the offspring, which is why sexual reproduction would be promoted if it were to arise. It is these costs, by the way, that explain why there have not evolved more than two genders.

To answer the second question again requires that we ask what the most fundamental differences are between male and female. Richard Dawkins explains in his book The Selfish Gene: "However, there is one fundamental feature of the sexes which can be used to label males as males and females as females, throughout animals and plants. This is that the sex cells or 'gametes' of males are much smaller and more numerous than the gametes of females." Again, there are numerous hypothetical scenarios by which such an asymmetry might result in the evolution of pronounced behavioural and morphological differences between males and females. The most frequently occurring pattern in male-female relationships, ie. the promiscuity of the male vs. the chastity of the female, is easily explained considering that it costs males next to nothing to produce countless sperms and impregnate as many females as he can find willing - while the female has a limited supply of eggs, in which she has invested much more resources, and therefore much choose carefully what genes to combine hers with.