Chapter 2. EVOLUTION II—SELECTION

Learning Objectives

Pre-Lab—Replica Plating
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General Purpose

Conceptual

  • Gain an understanding of how and why replica plating can be used to test a population against different selective pressures.

Procedural

  • Be able to use replica plating technique to transfer identical populations to Petri plates.

Replica plating allows the same populations of organisms to be tested by varying amounts (and/ or different kinds) of selective pressure. This technique can allow the identification of organisms within the population that have genotypes which confer a survival or reproductive advantage (i.e., individuals with increased fitness). The process of replica plating can be used for single-cell organisms that can be grown on growth media in a Petri plate.

Please view the following video which explains some of the rationale for using the replica plating procedure.

The technique involves taking a Petri plate which has cells growing on it and transferring via a stamping process some of those cells to a new Petri plate. The growth conditions for this new plate can be altered to apply survival pressure to the organisms. Those organisms which are genetically better suited to survive the pressure will grow whereas organisms lacking the genes needed to survive under the pressure produced by the new growth conditions will not (or will do so at a much reduced rate).

These altered growth conditions can be the result of the presence of a material added to the growth media, the lack of something normally present in the growth media, or even a variation in the incubation conditions normally used for growth. By using the stamping method, the exact copy of the same population can be tested against several different growth conditions.

Last lab you plated a dilute suspension of bacterial cells onto tryptic soy agar growth media in a Petri plate. Those bacterial cells produced clones by binary fission, so that all the bacteria in a colony are genetically identical to a single bacterium which was transferred onto that location on the Petri plate. Optimally, this origin stock plate will have many (20–40) bacterial colonies (and no contaminants). Replica plating will create plates that have the same bacteria as the stock plate, with the identical orientation.

Please view the following video which shows the replica plating procedure.