In each main mongo executable (mongo, mongos, mongod, mongoq, etc.) there is a function that initializes the WireSpec parameters:

https://github.com/10gen/mongo/blob/6a836ac18b9530f4a5e0f87a048497eb0686a77d/src/mongo/s/mongos_main.cpp#L742-L751

https://github.com/10gen/mongo/blob/cf78d87ee503ecb877d261c9138d786b0b3b399f/src/mongo/db/mongod_main.cpp#L1393

https://github.com/10gen/mongo/blob/cf78d87ee503ecb877d261c9138d786b0b3b399f/src/mongo/embedded/embedded.cpp#L127

https://github.com/10gen/mongo/blob/cf78d87ee503ecb877d261c9138d786b0b3b399f/src/mongo/shell/mongo_main.cpp#L167

(enterprise links in comments)

We should move these default parameters out of the middle of the setup code and into defaults somewhere, where they are more easily discoverable.