A population of descendents of the individual in which the original mutation event occurred. The mutation is now inherited by the regular reassortment and recombination mechansisms, as are other alleles. In practice, this is hard to measure. This is easy to measure. All existing alleles are the result of the past 4 billion years of mutation events.
Mutation events are rare.
Observation of large numbers of progeny. For traits which prevent survival, under a given condition. Replica plate bacteria colonies on agar lacking a nutrient which the wild type strain can make with its own enzymes. Compare the advantages and disadvantages of each. Gene mutations vs. Chromosomal mutations.