Hi adair,
Since pierre has gave you some basic information about your question I would just like to add few more facts.
First of all yoga is physical, mental, and spiritual discipline and this is very important. The goal of yoga is to make you balanced in all these three states. It is all connected and you really need to give yourself into it if you want results. Since you are looking for scientific evidence it is very unlikely that you will accomplish this.
Basically yoga will improve your health and it will help you to liberate yourself from the worldly suffering.
So try to relax if you are interesting in practicing yoga and if you can’t I advise some other discipline.
yes, performing yoga is very good for health, it will shabe your boby stucture especially for women