I'm currently trying to change careers to technical writing.  I know that, often, technical writing openings request a portfolio of writing samples.  Similarly, many arts and programming jobs ask for art and code portfolios.

If I were contributing to a collaborative or open-source project, how would I show my contributions in a portfolio?

As an example, say I worked on some random Wikipedia article.  I may not have started the article, I may not have finished the article, but I may have contributed heavily to the development of the article - but I can't just stick the article in my portfolio, because it's not just my work.