Natural Selection Made Easy.

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Example of Natural Selection: Antibiotic Resistance

evolution of antibiotic resistance: a simple view

Bacteria exhibit natural variation in ability to resist antibiotics.
Variation in antibiotic resistance is due to variation in  certain genes carried by the bacteria.
When you take an antibiotic the least resistant bacteria die first.

If you take all  the antibiotic, the chance of any bacteria surviving is very small. Stopping the antibiotic early means that a high proportion of surviving bacteria are antibiotic resistant.

Thus the final population of bacteria after early stopping of antibiotic consists of a greater proportion of resistant bacteria.

Next person may require more or a different antibiotic.

Note that you might get better because your immune system eventually resists the bacteria but some 'escape' to infect someone else!
The bacteria example is not artificial selection even though humans are involved.

In the absence of antibiotic the proportion of genes in the population conferring resistance generally decreases to a low level because the resistant bacteria actually tend to be at a disadvantage relative to those bacteria that are not resistant to antibiotic. This is another reason why overuse of antibiotic is not recommended.

Comparison of Evolution by Artificial vs Natural Selection:

Natural selection can be understood by comparing natural selection to the sort of intentional selection practiced by plant and animal breeders.

  Artificial Selection on dogs: Example of a breed where curly coat is desired: http://www.ukcdogs.com/breeds/gun dogs/curlycoatedretriever.std.shtml

Natural Selection for antibiotic resistance in strep bacteria. http://www.fda.gov/fdac/features/795_antibio.html

Natural Selection for beak size in Darwin's finch: Grant, P. R., and B. R. Grant. 2002. Adaptive Radiation of Darwin's Finches.  American Scientist 22, no. 2.
Source of variation in trait. Mutation gives rise to a curly coat in a certain dog. The curly coat genes which have arisen by mutations are good examples of gain of function mutations. This is an important point because people who are skeptical about evolution claim that mutations cannot generate new functions and that evolution only results in some sort of reduction of information. Mutation gives rise to a small proportion of bacteria resistant to the antibiotic. Genes conferring bacterial resistance can also be transferred from one species of bacteria to another through one of several ways. This sort of "horizontal transfer" may also be important the evolution of other organisms as well. Mutation gives rise to variation in genes influencing beak size.
Agent of Selection
 Intentional selection by breeder desiring more dogs showing curly coats.  Presence of  antibiotic in the bacteria's environment - your throat. 
Climate change- increased dryness  leads to harder to open seeds available to the birds.
Immediate Result of selection
 Individuals with curly coats are more likely to survive and be used for breeding.
Greater proportion of bacteria that survive are resistant to the antibiotic.
Birds with larger beaks survive and leave more offspring since they are better able to use harder seeds.
Result after a number of generations assuming a constant environment! Genes causing curlier coats desired by the breeder become wide spread. Average coat curliness in the breeding population  increases. Greater proportion of curly coated dogs in the population. Genes causing antibiotic resistance become wide  spread as long as individuals with those genes leave more offspring on average compared to those individuals that lack the resistance genes. Average level of antibiotic resistance in the bacteria population increases. Greater difficulty in treating strep throat. Genes causing larger beak size become more common. Average beak size in the finch population becomes larger.

In all three cases if the agent of selection acts differently with time then the population will follow these changes. For instance antibiotic resistant bacteria are typically at a disadvantage when antibiotics are not used. In the finch example if the climate becomes wetter, seed size increases favoring smaller beaked finches again. There is no goal direction in natural selection. Instead the genetic make up of the population changes in response to the environment! Likewise with artificial selection, certain breed characteristics may fall out of fashion or no longer be useful. For instance Standard poodles have a naturally curly coat because poodles were originally bred as water retrievers and the curly coat resists water. Poodles are rarely used today for that task so perhaps curly coat could fall out of fashion if not maintained by breeders.

 


pgd 04/10/05