How does antibiotic resistance arise in bacteria? The following animation illustrates the effect of natural selection on a bacterial population. We can observe evolution directly in many existing species, but bacteria provide an excellent opportunity to view evolution as it occurs due to bacteria's short generation time. All populations exhibit genetic variation, which may be introduced by mutation or influx of organisms from neighboring populations. In bacteria, new gene forms are introduced frequently through a high mutation rate. Although selective pressure affects the frequency of genetic traits, a genetic trait must already exist for it to spread through a population. In bacteria, if no individual organisms carry a gene from that provides antibiotic resistance and no random mutations convey resistance before the antibiotic kills the organism, then all bacteria would die, and no evolution would occur. This animation simplifies the process of bacterial growth and antibiotic effects in order to demonstrate the principle. In reality, different antibiotics kill bacteria in different ways and with differing levels of effectiveness. Additionally, bacteria undergo many rounds of division each day, usually dividing about once every 20 minutes.