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Bertrand Serlet, one of the leading figures in the development of Apple's Mac OS X operating system, is stepping down.

Apple on Tuesday announced Serlet's departure and Serlet's replacement, VP of Mac software engineering Craig Federighi.

"I've worked with Steve for 22 years and have had an incredible time developing products at both NeXT and Apple, but at this point, I want to focus less on products and more on science," said Serlet in a statement. "Craig has done a great job managing the Mac OS team for the past two years, Lion is a great release and the transition should be seamless."

Prior to working at Apple and NeXT, Serlet spent four years at Xerox PARC.

Serlet's departure comes as iOS products -- the iPhone, iPad, and iPod -- have eclipsed Mac OS products in terms of revenue. Apple's iOS line now accounts for about 75% of the company's revenue, while the Mac OS line accounts for about 20%.

However, Jean-Louis Gassée, a partner at Allegis Capital who previously headed Mac OS development at Apple and founded Be Inc., dismissed the notion that Apple's shifting focus played any role in Serlet's decision.

"Apple has made very clear, even if we talk about the post-PC era, that PCs have a long, fun and profitable future," Gassée said in a phone interview. "I know tablets are eating into part of the PC business, but as you say in English, 'C'est la vie.' It's natural for markets to fall like that. It would be really superficial to say that PCs were everything and now they're no longer everything."

Gassée believes that Mac OS X will continue to play a foundational role at Apple. "In the recesses of iOS there is a lot of OS X DNA," he said. "The two have never been totally separated." He suggested this common software infrastructure will be helpful to Apple if ARM processors supplant Intel's x86 processors as the dominant chip architecture for computers.

Carlos Icaza, co-founder and chief evangelist of Ansca Mobile, believes Apple is preparing for a transition to ARM chips, particularly given Apple's acquisition of chip maker Intrinsity last year. "We could actually start seeing the lower end MacBook models and the MacBook Air running A5 chips instead of Intel chips," he said.

Icaza said that Serlet's departure is certain to have some impact on Apple, but noted that Serlet's successor, Federighi, is an insider who also worked with Serlet at NeXT.

Looking at Apple's success selling Macs while Windows PC makers have done less well, Gassée said he sees a great opportunity for Apple to make a lot of money by continuing to steal market share from Windows PCs.

Praising Serlet for his ability to interact with Apple's demanding CEO Steve Jobs and for his accessibility to company engineers, Gassée said other talented leaders are waiting in the wings.

"One thing that Steve Jobs has accomplished in Apple 2.0 is building an impressively solid and cohesive management bench," said Gassée. "It's a deep bench, so to speak. Bertrand has had an amazingly long, successful, distinguished career working with Steve, which is no mean feat, and this has given everyone time to put together an organization that will continue to perform."

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