BOSTON ( TheStreet) -- Judging from the ink that Apple's ( AAPL) iPhone gets, one would think the company has a hammerlock on the smart-phone market. That's far from the case.

According to research firm Canalys, Apple's iPhone OS accounted for about 15% of smart phones sold last year, a distant third behind phones using Nokia's ( NOK) Symbian OS and Research in Motion's ( RIMM) OS. Those own 47% and 21% of the market, respectively. While Apple has grown much faster than those two brands, it still lags by almost 10 million units.

Nokia's Symbian is selling more than twice the units of its closest competitor. But given that sales growth is in the low single digits, the big movers could overtake it sooner rather than later. Research in Motion and Apple could overtake Nokia in about three years based on current growth rates. Sales may slow, of course, but the point is that market share is shifting and competition is intensifying.

As an example, Microsoft's ( MSFT) Windows Mobile platform lost about 5% of total market share of new phone sales in 2009 as Google's ( GOOG) Android and Apple's iPhone swooped in and gobbled up the low-hanging fruit with little resistance.

Nokia, despite big sales numbers, hasn't been a relatively attractive investment. While Apple and Research in Motion gained 116% and 68% over the past year, respectively, Nokia climbed 31% on a beta value of 1.5, indicating a good deal of market risk and little reward. Because analysts expect growth for Apple and Research in Motion to be nearly 10% higher than that of Nokia, it's likely that this trend will continue.

Google's Android OS is the newest entrant in the smart-phone war. As with all things, Google has taken no prisoners and is projected to be the second-largest smart-phone operating system by 2012, according to Gartner Inc. Like with Symbian, Android has a huge advantage by being available on several phones from many manufacturers. With Motorola ( MOT), Dell ( DELL), HTC and Samsung offering Android-based phones, the OS casts a much larger net than Research in Motion or Apple can. Those companies create the only devices to use their software.

The ability of the Android system to be dumped into a plethora of devices, including tablets and netbooks, makes it nearly impossible for Apple to beat it in head-to-head sales. Investors interested in playing the smart-phone revolution should consider Google as the most likely heir to the market-share throne. As Microsoft and Nokia crumble under the weight of new competition, Google is their likely successor.

-- Reported by David MacDougall in Boston.
Prior to joining TheStreet Ratings, David MacDougall was an analyst at Cambridge Associates, an investment consulting firm, where he worked with private equity and venture capital funds. He graduated cum laude from Northeastern University with a bachelor's degree in finance and is a Level III CFA candidate.