How do symbiotic bacteria act as a barrier to pathogenic bacteria?

Symbiotic bacteria act as a barrier to pathogenic bacteria and promotes the development of the host's immune system in the gut and the skin in a number of ways; These includes; they prevent growth of harmful pathogens by altering the pH, consuming nutrients for pathogen survival, secreting toxins that inhibit the growth of pathogens. Additionally,some may release ant-inflammatory products, other commensals promote the development of B cells that produce protective antibody (IgA), others trigger development of isolated lymphoid follicles in the intestines. 

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