I will provide you will an explanation that someone at your level of education (or lack thereof) should be able to grasp:
Evolution Frequently Asked Questions
1. How can one species "turn into" another?
One species does not "turn into" another or several other species -- not in an instant, anyway. The evolutionary process of speciation is how one population of a species changes over time to the point where that population is distinct and can no longer interbreed with the "parent" population. In order for one population to diverge enough from another to become a new species, there needs to be something to keep the populations from mixing. Often a physical boundary divides the species into two (or more) populations and keeps them from interbreeding. If separated for long enough and presented with sufficiently varied environmental conditions, each population takes its own distinct evolutionary path. Sometimes the division between the populations is never breached, and reproductive isolation remains intact purely for geographical reasons. It is possible, though, if the populations have been separate for long enough, that even if brought back together and given the opportunity to interbreed they won't, or they won't be successful if they try.