Hi! Manoj,

Follow the link and read patiently, you get answers for all your questions.

http://en.wikipedia.org/wiki/Evolution

at a molecular level one of the several possibilities exist i could detail a few

1. Genetic diversity: Bacterial genomes do contain errors that under normal conditions do or dont produce a phenotypic trait in this case resistance against certain drug. When you are treating with that drug you tend to select that clone which is resistant.

2. Increase in mutation rates as said above when bacterai sitch off dna repair this creates a situation to increase mutants of which few can survive the catastrophe.

3. plamids and uptake of genetic material. plasmids can be transferred from/to bacteria by sexual conjugation

4. Bio films: they are probably the most interesting way to sheild off from environmental genotoxins and chemicals

@Manoy

I think that resistence -development in bacteria is in fact a question of plasmid-DNA transfer and therefore a moderate good example for evolution. More interesting answers are coming from the evolution of multicellularity and animal development by reorganization of genetical mechanisms preexistent in ancient prokaryotes and evolved in basal eukaryotes before the origin of animals (transcription factor genes ans so on).

Any change in the environment (e.g. adding antibiotics) will cause the selection of those individuals which have any genetic variation (probably not very frequent before the environmental change) which confers and advantage in the new conditions. Hence those individuals will produce more offspring and the ratio of that genetic variation will increase in the population (e.g. genetic variation which encodes an aminoacid change in an enzyme that degrades the antibiotic).

Hi, Manoj -

When Bacterial population in an are exposed to antibiotics most of the Bacteria get killed due to that antibiotic as they don't have resistant against that antibiotic but out of total population some bacteria (for example 1 in 10^6) may have the gene for resistance against that antibiotic and such bacteria survives and increases their number following rules of natural selection they survive in given conditions and further they increases copy number of that resistance coding gene. Such resistant bacteria spreads in population mean while there is removal of non resistant bacteria due to use of that antibiotic . Finally a new strain having resistance is observed in population. Thus now a days combinations of antibiotics are given to avoid such problems if 2 antibiotics are given then the chance of getting resistant bacteria decreases to 10^36 and for combination of 3 antibiotics it decreases 10^216 this will go on by following rules of probability.

Are we ignoring the fact that the antibiotics may act as elicitors that triger in-built resistance in the bacterial cell? Depending on the duration of development of resistance, I believe the first line of defense is the development of protective cover i.e. bio-film. I appreciate the fact of 'suicide', a few sacrfice to die to save the lineage.....however, if the antibiotic is applied continuously, it is likely that extinction may occur. But if the antibiotic is applied intermittently, then a cope up mechanism is developed including transfer of genetic material to mutate sites of interactions with molecules of antibiotics leading to resistant.

@Himanshu

From your answer, I infer that the genetic code that protects the bacteria is already present in some members of the species. When exposed to hazardous condition, the members of the species that contain this particular genetic code survive. Does it not mean that the genetic variations among the members gradually decrease if they are exposed constantly to one or the other harsh conditions?

@Manoj-

Ya i mean;

Natural selection can decrease the genetic variation in populations of organisms by selecting for or against a specific gene or gene combination (leading to directional selection).

OR-

It can increase the genetic variation in populations by selecting for or against several genes or gene combinations (leading to disruptive selection or balancing selection). And what we r talking about is an example of directional selection in which a single phenotype is favored, causing the allele frequency to continuously shift in one direction as an out come of this genetic variations among the members gradually decreases in such harsh conditions,

But we cant ignore two major forces responsible for Evolution:

1) Mutations which r the creators of variations and

2) Recombination events which brings variations among organisms of same species.

It is important to note that genetic code may stay unchanged through cosmological time (billions of years). And its interactions rely also through self-organizing behaviors once evolving.  Thus, evolution from self –organizing behaviors, take place from simple rules, many interactions, many agents and decentralization; where, the emergence of complex patters are difficult if not, impossible to predict or repeat. Life, appears as an emergent property of the system rather one imposed in the system by external influences. Each specie will become unique while evolving with its own characteristic from genotype to phenotype (The article that refers to all this topics appears at the bottom).  

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