ما علاقة تطعيم شلل الأطفال بالتطور ؟

This process of weakening is called attenuation, and it is an evolutionary process. The attenuated vaccine strains came from wild, virulent strains of poliovirus, but they were evolved by Albert Sabin to become attenuated. Essentially, he grew the viruses outside of humans, and as the viruses became adapted to those non-human conditions, they lost their ability to cause disease in people.

This method of attenuation has been used to create many live vaccines. Evolution was the good guy here because it helped us make the vaccine. But the role of evolution and evolutionary biology does not end here — evolution becomes the bad guy too.
Evolution can also destroy the effects of vaccine.

When a person eats the attenuated virus, it infects his/her gut cells and starts doing what viruses do — making copies of itself.
These viral progeny infect other cells in your gut, those in turn make other viral progeny, and so on, until you have a population of poliovirus growing inside your gut.
Some of these viruses carry mutations, and some of those mutations (one or two in particular) restore most of the virulence to the virus.
In your gut, these restored viruses may have a selective advantage over the weakened viruses, and in the course of a week or so after eating the vaccine, you begin shedding virus with restored virulence. In short, an evolutionary process inside your gut undoes Albert Sabin’s attenuation of the virus.