This article is part of our Rising Star Portfolios series.

The black turtleneck will no longer dominate the stage. Steve Jobs, founder of Apple (Nasdaq: AAPL), finally bowed to the inevitable and resigned his CEO position last night.

I've purchased a share of Apple twice before for my Messed-Up Expectations portfolio. This will be the third time. While the stock took just a bit of a hit today -- down 1% as of this writing -- that's not the only reason why I'm increasing the portfolio's position.

The Apple of today is the same company as it was yesterday, the last quarter, and much of the last year. Tim Cook, Apple's new CEO, has held that position in all but name since January, when Jobs took a medical leave of absence. As chief operating officer, he made Apple into a lean machine. Plus, the team Jobs built includes designer genius Jonathan Ive, marketing chief Phillip Schiller, and operating platform head Scott Forstall. These men and others are the brains behind Apple's phenomenal success. With them, I expect Apple to continue to sell a lot of products and produce a ton of cash, expanding the reach of its ecosystem.

Apple is a strong company, no doubt, but it does have its share of challenges, which I'll be keeping a careful eye upon. (Nasdaq: AMZN) has a successful cloud system and is expected to release its own tablet computer to compete with the iPad in the not-too-distant future. Google (Nasdaq: GOOG) is in the middle of purchasing Motorola Mobility (NYSE: MMI), aiming to increase the competitiveness of its Android products. Even Microsoft (Nasdaq: MSFT) is not out of the picture, remaining the dominant operating platform for personal computers and bidding for cloud dominance. How Cook and his team -- don't worry, you'll get used to saying that -- respond to these threats over the next few quarters will give a clear picture of Apple's ability to continue its growth.

Furthermore, how well the next new products from Apple do is important, as we move away from Jobs' influence. Success will show that Apple is in good hands, while flops might be reason to sell. Remember, Microsoft faltered pretty badly after Gates left. Will Apple follow the same path? Only time will tell.

Despite these concerns and even because of them -- uncertainty presents opportunity -- I'll be purchasing another share of Apple tomorrow for the Messed-Up Expectations portfolio.

After you add Apple to My Watchlist, come discuss the decision on my Messed-Up Expectations discussion board, or follow me on Twitter.

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Jim Mueller owns shares of Apple and Amazon, and has an option position on Microsoft. He's an analyst for the Motley Fool Stock Advisor newsletter service. The Motley Fool owns shares of Apple, Microsoft, and Google. Motley Fool newsletter services have recommended buying shares of Amazon, Microsoft, Apple, and Google, as well as creating a bull call spread position in Microsoft and Apple. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool's disclosure policy is never messed up.