Microevolution, on the other hand, is a change in allele (or gene) frequencies within a population from one generation to the next. A population is an interbreeding group of individuals of the same species. Microevolution takes place on a much smaller time scale than macroevolution—generally, between two generations. Evidence of microevolution is commonly found in the resistance of weeds to herbicides, pathogens to medicines, and pests to pesticides. For example, a population of bacteria may have a mixture of some gene versions that are resistant to an antibiotic and some that are not. When the antibiotic is introduced, those that are resistant will survive, with natural selection favoring those gene versions. In a generation or two (which, for bacteria, can occur in a matter of minutes), a population of bacteria will evolve into only those that are resistant.Although survival and reproduction of individuals determines evolution, individuals themselves cannot evolve. Continuing with the example of a population of bacteria, no individual bacterium that was not resistant to the antibiotic evolves into one that is. Those that are not resistant perish. Those that are resistant survive and reproduce, passing on the genetic variation that is resistant. Over the course of a couple of generations, only resistant bacteria remain in the population. The population is the smallest unit of evolution.