  Inspired by recent works on evolutionary graph theory, an area of growing
interest in mathematical and computational biology, we present the first known
examples of undirected structures acting as suppressors of selection for any
fitness value $r > 1$. This means that the average fixation probability of an
advantageous mutant or invader individual placed at some node is strictly less
than that of this individual placed in a well-mixed population. This leads the
way to study more robust structures less prone to invasion, contrary to what
happens with the amplifiers of selection where the fixation probability is
increased on average for advantageous invader individuals. A few families of
amplifiers are known, although some effort was required to prove it. Here, we
use computer aided techniques to find an exact analytical expression of the
fixation probability for some graphs of small order (equal to $6$, $8$ and
$10$) proving that selection is effectively reduced for $r > 1$. Some numerical
experiments using Monte Carlo methods are also performed for larger graphs.
