Natural selection is such a powerful force in evolution because organisms or cells with even a small reproductive advantage will eventually outnumber their competitors. To illustrate how quickly this process can occur, consider a cell culture that contains 1 million bacterial cells that double every 20 minutes. A single cell in this culture acquires a mutation that allows it to divide faster, with a generation time of only 15 minutes. Assuming that there is an unlimited food supply and no cell death, how long would it take before the progeny of the mutated cell became predominant in the culture? (Before you go through the calculation, make a guess: do you think it would take about a day, a week, a month, or a year?) How many cells of either type are present in the culture at this time? (The number of cells N in the culture at time t is described by the equation N = N₀ × 2t/G, where N₀ is the number of cells at zero time and G is the generation time.