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.

This model simulates a version of fixes that fail.  The research that produced this model can be found by clicking the Source Document button.

From PLOS One Article April 2012 Worni, M et al System Dynamics to Model the Unintended Consequences of Denying Payment for Venous Thromboembolism after Total Knee Arthroplasty

This insight displays some of the main factors effecting the decreasing koala population in South East Queensland, the measures put in place to stop their extinction, and the possible measures that could be taken to further help the conservation effort.

The perils of internships

Second Draft

This model theorizes that increasing charter school enrollment is having a negative impact on neighborhood schools.  Specifically, increasing the concentration of high needs students in these schools which in turn, makes teachers' jobs more difficult and lowers standardized test scores.

Describes the unintended consequence of Governor Schwarzenegger performing a line item veto to California's Public Health's Domestic violence Program.

A clone of the first model with the addition of a converter to describe the competition between rabbits for available vegetation based on the relationship between rabbit density and rabbit birth rate

This model simulates a version of fixes that fail.  The research that produced this model can be found by clicking the Source Document button.