If you have an organism that already works, and you introduce a random change, the chances are that the change will disrupt some system and make it worse, rather than by fortunate accident making it work better. But remember that even though most mutations are harmful, some are beneficial. It's like winning a genetic lottery. Sometimes you get lucky. Without beneficial mutations, evolution could not happen.
Luck. There is no way of having identical twins, let alone choosing the sex.
There are two primary types of twins, Identical and Fraternal.
Identical twins occur when the egg splits after fertilization. To date no indication of a genetic link has been found. Anyone has a chance, though small, of having identical twins.
Fraternal twins occur when more than one egg is dropped during ovulation. This trait can be inherited from the mother's side of the family. Studies have shown that the chances are also increased by being overweight or older. Fertility drugs also make this more likely to happen.
The true about species is that Sexual Selection is the skill of speciation acquired in the process of natural selection (Eukaryotes). This implies that diverse depends on opportunities to permeate Sexual Selection, and because opportunities aren't frequent, there is a lack of a diverse gene pool.
A membrane potential is basically the difference between the inside and outside of the cell. Ions are charged, and so will change the membrane potential (the difference between charges on the inside and outside) when they move.
Please see the related link below which includes a diagram of how ions affect membrane potential.
They are transported from the ovaries by the fallopian tubes to the uterus