Are the bubbles in the sourdough starter caused by yeast cultures? I'm new to sourdough starter, so I'm a little confused regarding a statement I read recently which said that the yeast is formed AFTER you see bubbles forming in the starter.  Yet, it makes sense to me that the yeast is the cause of the bubbles in the first place.  Would someone please explain? 

Thanks.
The answer is actually both! Yes, the bubbles are caused by the yeast, they are converting sugar to carbon dioxide, among other things. Its also an indicator that the yeast colony is multiplying. So, technically, there is (more) yeast forming.