Adapting evolutionarily: short term benefits & dead-ends

The adaptive processes we have been discussing up to now take place at the level of the individual organism.   However, populations also adapt to changing environments through the processes of evolution, first clearly enunciated by Charles Darwin & Alfred Wallace. 

Since both activity- and gene expression based adaptation systems were generated by evolutionary processes, the theory of evolution provides the intellectual framework for all of biological science.

Evolution is driven by two interacting factors: environmental selection acting on genetic variation.

If the environment were constant, and the population were large enough to minimize the effects of random fluctuations, a population of organisms would settle down to a specific "local optima" of fitness.

Natural Selection would act primarily to eliminate variants with decreased "fitness".

The environment, however, is rarely constant for very long. Predators, pathogens, competitors and prey are themselves searching evolutionary space for an advantage.

Coupled with environmental changes and cosmic events (e.g., plate tectonics, asteroid impacts, changes in solar activity) most populations must adapt continually.

Only a "happy" few find a stable adaptive strategy; "living fossils", such as the horseshoe crab, appear more or less unchanged for tens to hundreds of millions of years.

More commonly, however, changes in the environment leave organisms, once well adapted, tragically maladapted and unable to survive.

The reality of extinction was first recognized by Georges Cuvier.

The changing parade of organisms through the ~3.5 billion year long history of life on Earth is direct evidence for the dynamics of adaptation and change

Under some conditions they even behave as multicellular organisms.

What we learn from them can often be related to biologic systems in general.

Because of their simple growth requirement, rapid doubling time, and clonal (asexual) mode of reproduction bacteria are a great system for the study of real time evolution.

In addition, because bacteria are haploid (i.e. they have only a single copy of each gene), when mutations occur their effects rapidly become visible; they are not masked by the presence of a second (wild type) copy of the gene.

In our studies, we will use two bacterial systems: our old friend, the human intestinal inhabitant E. coli and Psuedomonas fluorescens, which normally grows on the surface of plants.

Both are adaptable and easily grown in the lab.

The simplest of such experiments involves two populations of the same organism. To perform this type of experiment, however, we have to be able to distinguish the these closely related populations.

Since bacteria generally reproduce in an asexual clonal manner, it is easy to generate cultures of genetically similar individuals.

But how do we distinguish two different populations without disturbing their behavior?

We will derive two strains of E. coli. One that is resistant to the antibiotic nalidixic acid, the other resistant to the antibiotic streptomycin.

Mutations in the gene encoding DNA gyrase are responsible for nalidixic acid resistance.

Mutations in the S12 ribosomal subunit protein lead to streptomycin resistance.

In both cases, it is possible to find mutations that lead to resistance but have little effect on cell growth.

The mutations that lead to resistance lower the affinity with which these drug binds their target proteins.

To identify such mutations, we will use the lowest concentration of each antibiotic that efficiently inhibits cell growth. At these concentrations, even a small decrease in the affinity of the target protein for the antibiotic can lead to growth.

To determine the lowest concentration of antibiotic that effectively suppresses cell growth we will use gradient plates.

LB agar containing either 500 µg/ml nalidixic acid or 500 µg/ml streptomycin is poured into 10 cm diameter sterile petri plates, held at a slant.

Once the agar solidifies, the plates are placed flat on the desktop.

LB agar without antibiotic is poured on top and the plates are allowed to sit for 24 hours in order to establish the antibiotic gradient.