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Name: Dece M.
Status: Student
Age: 18
Location: N/A
Country: N/A
Date: 2001 


Question:
Hi, im researching micorevolution, and was wondering if you could give me some specific examples of populations in which gene flow, non-random mating, and genetic drift have resulted in microevolution. and secondly, how has microevolution come about and what are the consequences for alleles and genotype frequencies?



Replies:
Hi Dece,

You did not indicate whether you are interested in higher organisms or not. May I point out microorganisms as a model to study microevolution. Bacteria can exchange DNA (by natural transformation, conjugation or by transfecting phages), their genes mutate by random drift, and sometimes recombinations occur within their genome. Since they have such a short generation time, the effect of these genetic changes can be detected rapidly, and studied in real time. One bacterial species that comes to mind is Helicobacter pylori, which lives in the stomach of humans and which displays a wide variation in genotypes between strains. In fact, if you zoom in at the polymorphisms within genes, one could state that every individual has his 'own' adapted strain of H. pylori, but that would still be highly related to the strain of another individual who was infected by a common source.

The relative importance of genetic exchange (horizontal gene transfer) versus mutation (genetic drift) is a current subject of research. Not all bacterial species have the ability to spontaneously take up DNA, but H. pylori does, and it seems that gene transfer significantly contributes to micro-evolution.

The mechanism of these events is studied in the laboratory: how do the bacteria protect themselves against incoming DNA that would be harmful; how is the DNA taken up; do mutations (genetic drift) follow a pattern, these are some of the questions addressed with H. pylori as a model.

One also has to think what the effect of mutations will be. It is most likely that any mutation with a phenotypic effect (that is, a genetic change that has 'some' effect on the properties of the bacteria; mutations that do not change the amino-acid of proteins are considered to be 'silent' mutations) will be selected for the best fit. In other words, mutations occur all the time but there is selection for those forms that will be adequately fit for the niche they occupy. And when the selection pressure changes (in pathogenic bacteria this can be the onset of an immune response; or in H. pylori the passage from one host to another) the best-fit organisms will survive and multiply.

Microevolution is a fascinating but complex system to study. If you browse PubMed for 'microevolution and bacteria' you will find many interesting papers. Two recent reviews (Ziebuhr et al, Cell. Mol. Life Sci. 1999 Nov 30;56(9-10):719-28. and Schloter et al, FEMS Microbiol Rev. 2000 Dec;24(5):647-60) are a good starting point to explore the subject.

Dr. Trudy Wassenaar



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