I have always used Google as my search engine, but this past few weeks Yahoo has replaced it. I've checked preferences and Google is my default search engine.
MacBook Air (11-inch, Early 2014), OS X Yosemite (10.10.1), kaspersky
Please disable all Safari extensions and test. If the problem is resolved, re-enable the extensions one or a few at a time until you find the culprit.
I did as you suggested, the only Extension I had was for Safari Genius. When I clicked to disable, it went to trash. So, all's working ok now.
search changes from Google to Yahoo
