2011-05-24, 06:51 PM
The environment is a part of NS. What drives NS is the fitness of the phenotype arising from a change in the genotype i.e. mutation.
NS leads to the selection of a particular phenotype while putting pressure on those with less viable phenotype at that period of time and place, and gradually as more genotypic variations occur you eventually come to a point where a whole new species is formed because so many changes in genotype has occured. This is one of the processes of speciation. Lots of other processes too.
A note for you is that mutations can be neutral, can be positive or can be negative. neutral mutations have no selection pressure for them, so if you e.g. have blue eyes instead of brown eyes, it's not going to make a hell of a diff to your survivability (unless there are madmen who hunt blue-eyed things). Positive mutations are those that confer greater fitness in the local environment e.g. you have 5 digits on your hand, and some miraculous mutation gave you 6 digits that allows you to have greater dexterity blah blah, so because of NS you "survive better" blah blah blah. Same thing, if you have some miraculous mutation that gave you 4 digits... blah blah blah. Not everything that is present now is necessarily 100% due to NS! Mutations are RANDOM, and NS depends on the local environment [and any changes to that local environment]. A mutant phenotype may be neutral in one environment, or may be selected for/against in another. And likewise, we have such things as convergent and divergent mutations.
A common example to show NS (accelerated) is in antibiotic resistance in bacteria. Say you have an infection. You see the doc, he gives you ampicillin. You take most of the pills. The ampicillin kills MOST OF THE BACTERIA (beta lactams only kill bacterial cells that are actively growing) and the remaining little bit is left for your immune system to finish off. You didn't finish the entire course of antibiotics. The little bit of bacteria that managed to survive the antibiotic treatment is said to have increased fitness for some reason or another e.g. has a mutated gene that ampicillin does not work on. Your immune system did not manage to eradicate all the remaining bacteria with increased fitness. This fitter bacteria now grows and multiplies. So you fall sick again, and you see the doc. So he gives you Amoxiclav. And so on... So what did you learn today? EAT THAT BLOODY PILL OR ELSE YOU FALL MORE SICK AND INFECT OTHER PEOPLE WITH YOUR SUPERBUG THAT'S FKING HARD TO TREAT!
A better example would be in the lab. You have a bacteria with no resistance to Kanamycin. You take a colony of bacteria and attempt to transform them so that they now have an antibiotic resistance gene. You culture them in an LB plate with Kanamycin. So of all the bacteria that has been plated, those transformed bacteria [i.e. "mutated"] now has been conferred resistance to Kanamycin, and so has greater fitness. Those that didn't get transformed will be susceptible to Kanamycin, and will face selection pressure. In this case, Kanamycin is acting as the selection pressure. The process of selecting the transformed bacteria against the non-transformed one is called NS.
So, in that plate with Kanamycin, mutations can occur. Negative mutations would be selected against e.g. mutating the Kanamycin resistance gene making it inactive, so obviously these negative mutants will have decreased fitness and will therefore die, because the selection pressure, Kanamycin, is still present. Neutral mutations would probably include inserting random DNA bits into non-important parts of the chromosome, or mutating an enzyme that is not important in the current environment - no selection pressure for/against. Positive mutation would probably be something like... gaining an extra copy of the Kanamycin resistance gene, making it have even greater fitness in the current environment.
If I, let's say, take the selection pressure off by sub-culturing the bacteria in an LB plate without Kanamycin, then ALL of the mutations can occur, because there is no selection pressure to select for or against any of the variations. So the bacteria could very well mutate and have a defective Kanamycin resistance gene now, and nothing will happen to it because it's neutral in the current environment.
Evolution takes a very long time - it takes so many years before the numbers of one genotype completely swamp an area and a less-fit genotype dies out. That's because mutations are so rare - it's random and caused by the environment, and occurs at a ridiculously low rate. Nobody really knows where evolution is headed, for very obvious reasons like we don't know what's going to happen in the future, what selection pressures would be present etc.
Alright, if you want any further detailed explanation, go to Rubik's Cube please.
Hadriel
NS leads to the selection of a particular phenotype while putting pressure on those with less viable phenotype at that period of time and place, and gradually as more genotypic variations occur you eventually come to a point where a whole new species is formed because so many changes in genotype has occured. This is one of the processes of speciation. Lots of other processes too.
A note for you is that mutations can be neutral, can be positive or can be negative. neutral mutations have no selection pressure for them, so if you e.g. have blue eyes instead of brown eyes, it's not going to make a hell of a diff to your survivability (unless there are madmen who hunt blue-eyed things). Positive mutations are those that confer greater fitness in the local environment e.g. you have 5 digits on your hand, and some miraculous mutation gave you 6 digits that allows you to have greater dexterity blah blah, so because of NS you "survive better" blah blah blah. Same thing, if you have some miraculous mutation that gave you 4 digits... blah blah blah. Not everything that is present now is necessarily 100% due to NS! Mutations are RANDOM, and NS depends on the local environment [and any changes to that local environment]. A mutant phenotype may be neutral in one environment, or may be selected for/against in another. And likewise, we have such things as convergent and divergent mutations.
A common example to show NS (accelerated) is in antibiotic resistance in bacteria. Say you have an infection. You see the doc, he gives you ampicillin. You take most of the pills. The ampicillin kills MOST OF THE BACTERIA (beta lactams only kill bacterial cells that are actively growing) and the remaining little bit is left for your immune system to finish off. You didn't finish the entire course of antibiotics. The little bit of bacteria that managed to survive the antibiotic treatment is said to have increased fitness for some reason or another e.g. has a mutated gene that ampicillin does not work on. Your immune system did not manage to eradicate all the remaining bacteria with increased fitness. This fitter bacteria now grows and multiplies. So you fall sick again, and you see the doc. So he gives you Amoxiclav. And so on... So what did you learn today? EAT THAT BLOODY PILL OR ELSE YOU FALL MORE SICK AND INFECT OTHER PEOPLE WITH YOUR SUPERBUG THAT'S FKING HARD TO TREAT!
A better example would be in the lab. You have a bacteria with no resistance to Kanamycin. You take a colony of bacteria and attempt to transform them so that they now have an antibiotic resistance gene. You culture them in an LB plate with Kanamycin. So of all the bacteria that has been plated, those transformed bacteria [i.e. "mutated"] now has been conferred resistance to Kanamycin, and so has greater fitness. Those that didn't get transformed will be susceptible to Kanamycin, and will face selection pressure. In this case, Kanamycin is acting as the selection pressure. The process of selecting the transformed bacteria against the non-transformed one is called NS.
So, in that plate with Kanamycin, mutations can occur. Negative mutations would be selected against e.g. mutating the Kanamycin resistance gene making it inactive, so obviously these negative mutants will have decreased fitness and will therefore die, because the selection pressure, Kanamycin, is still present. Neutral mutations would probably include inserting random DNA bits into non-important parts of the chromosome, or mutating an enzyme that is not important in the current environment - no selection pressure for/against. Positive mutation would probably be something like... gaining an extra copy of the Kanamycin resistance gene, making it have even greater fitness in the current environment.
If I, let's say, take the selection pressure off by sub-culturing the bacteria in an LB plate without Kanamycin, then ALL of the mutations can occur, because there is no selection pressure to select for or against any of the variations. So the bacteria could very well mutate and have a defective Kanamycin resistance gene now, and nothing will happen to it because it's neutral in the current environment.
Evolution takes a very long time - it takes so many years before the numbers of one genotype completely swamp an area and a less-fit genotype dies out. That's because mutations are so rare - it's random and caused by the environment, and occurs at a ridiculously low rate. Nobody really knows where evolution is headed, for very obvious reasons like we don't know what's going to happen in the future, what selection pressures would be present etc.
Alright, if you want any further detailed explanation, go to Rubik's Cube please.
Hadriel