If a patent claims a broad genus, can a reference including a single species anticipate the claim?

Yes. For example, a claim that specified a fuel composed of (among other things) "10 to 50 percent methane" would be anticipated by a prior fuel that met all of the other claim elements and was composed of 25 percent methane.  The converse, however, is not true: If a claim calls for a fuel composed of exactly 25 percent methane, and a prior publication discussed such a fuel with a methane concentration of anywhere from 10 to 50 percent, the claim requiring exactly 25 percent would not be anticipated. However, the claim may or may not be invalid for obviousness.