A principal challenge in testing the role of natural selection in speciation is to connect the build-up of reproductive isolation between populations to divergence of ecologically important traits.

检验自然选择在物种形成中的作用的一个主要挑战是，将生殖隔离的出现与有着生态学重要性的特征联系起来。

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These are the principal spiritual handholds I have found to possess enduring value. They offer both an exciting challenge and a calm assurance. They are the things I believe.

这些便是我所发现的精神支撑点，它们具有永恒的价值。它们给予我们的不仅是刺激的挑战，还有令人安心的承诺。这就是我信仰的一切。

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The principal, Peter Walker, wanted a building that would challenge and uplift his students.

NEWYORKER: Lady Zaha

In late June Kappe was one of 43 principal scientists to win a chunk of the multiyear Grand Challenge awards, funded largely by the Bill and Melinda Gates Foundation.

FORBES: Chutzpah Science

Energy efficiency is the most significant challenge standing in the way of continued miniaturization of electronic systems, and miniaturization is the principal driver of the semiconductor industry.

ENGADGET: Stanford engineers find work around for barriers to carbon nanotube computers