Which process involves a change in genetic variability of a population due to physical events?

The process that involves a change in genetic variability of a population due to physical events is genetic drift. Genetic drift refers to the random fluctuations in allele frequencies that occur in small populations, which can lead to significant changes in the genetic makeup of a population over time. This process can occur due to random events such as natural disasters (like floods or earthquakes) that dramatically reduce the population size, thus leading to the loss of certain alleles purely by chance.

This randomness means that some alleles may become more or less common irrespective of their effect on survival and reproduction, which can decrease genetic variability over time. Unlike natural selection, which is driven by the differential survival and reproduction of individuals based on their traits, genetic drift is entirely a stochastic process.

Other processes mentioned, such as natural selection and artificial selection, involve mechanisms that favor certain traits based on their adaptive value or human intervention, respectively. Mutagenesis refers to the process of genetic mutation and does not directly relate to the changes in variability caused by external physical events.