The last S&P 500 company with an all-male board adds a woman

• The lone S&P 500 company with an all-male board of directors said it's adding a woman to its board.
• The business, car auction business Copart, added CyrusOne executive Diane Morefield to its board.
• Still, only 1 in 4 board seats at S&P 500 companies is occupied by women, and all-male boards are still common among hundreds of smaller public companies.

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Finally: The last S&P 500 company with an all-male board of directors has added a woman to its ranks, marking a milestone in the push to diversify the corporate boardroom beyond the domain of men. 

The online car auction company Copart on Tuesday said it added Diane M. Morefield, chief financial officer at real estate investment trust CyrusOne, to its previously all-male board, the last among the nation's largest public companies. The milestone took decades, with more than 50 companies in the S&P 500 still sporting all-male boards as recently as 2009.

Copart had been singled out earlier this month in a Washington Post article about the slow progress of gender diversity on America's corporate boards, although Copart at the time said it planned to add a "highly qualified, accomplished woman this year."

Despite reaching this milestone, only 1 in 4 board seats at S&P 500 companies is occupied by a woman. And the realm of the all-male board is hardly dead, given that more than 500 companies on the Russell 3000 index don't have any women sitting on their boards, according to the Equilar Gender Diversity Index. Only 37 businesses in the Russell 3000 have reached gender parity, meaning half the seats are held by women, Equilar said.

The Russell 3000 represents the largest 3,000 publicly traded businesses in the U.S., which means it therefore includes hundreds of smaller companies than the S&P 500 index, whose components are selected by criteria including market capitalization. 

Proponents of diversity argue that the issue isn't only one of fairness and good corporate governance, but that creating more diverse management ranks helps businesses perform better. Studies back that up, such as a global analysis of almost 22,000 companies across 91 countries that found organizations with more women in leadership roles tend to be more profitable. 

Businesses where at least 30% "C-suite" executives are women boost their profitability by 15% compared to those with male-only executive suites, the Peterson Institute for International Economics found.