Oy.
First off, speciation is an observed and proven phenomenon. You'll find many examples here: http://www.talkorigins.org/faqs/faq-speciation.html
The sections are numbered and you'll find the evidence under section 5.0: Observed Instances of Speciation.
Furthermore, the mechanisms which cause these small changes are the same mechanism which cause larger changes. What theists refer to as "macro evolution" is simply an accumulation of changes that happens over a tremendous amount of time.