Describe how feedback inhibition works

Say you have a pathway 
A-->B-->C-->D 

So your A leads to production of B which leads to production of C which leads to production of D 
In feedback inhibition (or a negative feedback loop), the final product of a pathway (in this case, D) will usually inhibit or stop the first step in that pathway. So in this instance, D would do something to stop A-->B from occurring. It could bind A or the enzyme that makes that step occur. 
Basically, the end product somehow stops one of the earliest steps in the pathway

It is evolution, because we all have some of the same traits like eye color or shape or ears.