While the screensaver is running, you can use the up (or down) arrow key to vote for the sheep you like (or not), and so influence the evolution of the flock.
Furthermore, you can design your own sheep and post them into the gene pool. If your sheep are popular, they will interbreed with the rest of the flock and produce children. By mutating and crossing the genomes, variations of your sheep will appear and themselves evolve.
To design your on sheep, Windows users can run Fr0st or Apophysis. Mac users run Oxidizer. Linux users have their choice of Apophysis under Wine or Qosmic, a native Qt application. There's also a portable Java version for all platforms.
There's a Sheep Edition of Apophysis that has exactly the right variations you'll need.
For more information, see the Fr0st blog and the Sheep Design Tips on the wiki.
After you've got a hang of the software and are ready to share, join the genetic design email list and ask for the password for connecting to the server.