Directional Selection of Bacteria
Directional selection can be seen in the changes in a bacteria population. Initially, when the predominantly non- resistant bacteria is exposed to the antibiotic, Penicillin, the non- resistant bacteria die and decline in numbers mostly before they reproduce. Penicillin acts as an environmental pressure in which the bacteria that has a mutation that helps it to develop resistance to Penicillin have an advantage over the non- resistant bacteria. Because this type of bacteria has a higher ability to survive and reproduce, this type of resistance to Penicillin becomes an allele that becomes prevalent in the bacteria population. However, things become complicated when another antibiotic, Ampicillin, is added to the equation. Even thought the current bacteria population has a resistance to Penicillin, it doesn't have a resistance to Ampicillin, and therefore, is vulnerable to dying. The rate of decline in population depends on how deadly the antibiotic is to the bacteria population. However, some bacteria that have a mutation, in which the bacteria have a resistance to both Penicillin and Ampicillin, possess a higher advantage than bacteria that have a resistance only to penicillin. Therefore, since that type of bacteria is able to survive and reproduce more easily with the presence of both Penicillin and Ampicillin, the allelic frequency is again changing towards resistance to both Penicillin and Ampicillin. Since, the allelic frequency is going in one direction from no resistance to a progressively higher amount of resistance in the bacteria population, directional selection is occurring.

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