  Street demonstrations occur across the world. In Rio de Janeiro, June/July
2013, they reach beyond one million people. A wrathful reader of \textit{O
Globo}, leading newspaper in the same city, published a letter \cite{OGlobo}
where many social questions are stated and answered Yes or No. These million
people of street demonstrations share opinion consensus about a similar set of
social issues. But they did not reach this consensus within such a huge
numbered meetings. Earlier, they have met in diverse small groups where some of
them could be convinced to change mind by other few fellows. Suddenly, a
macroscopic consensus emerges. Many other big manifestations are widespread all
over the world in recent times, and are supposed to remain in the future. The
interesting questions are: 1) How a binary-option opinion distributed among
some population evolves in time, through local changes occurred within
small-group meetings? and 2) Is there some natural selection rule acting upon?
Here, we address these questions through an agent-based model.
