Evolution Without Selection Pressure?

fishstix · May 17, 2004

For those who believe in evolution: can evolution occur in the absence of selection pressures - natural or artificial? Or does some variation exist just because. The reason I'm asking is because I was just considering herbicide resistant weeds. Genetic traits for herbicide resistance occur in a population of plants which are normally susceptible to the chemical long before the herbicide is even invented. It usually doesn't give the plants any particular advantage unless the herbicide is used; in some cases it may be a significant disadvantage to the plant. So why would it exist in these plant populations before the herbicide is invented?

notto · May 17, 2004

Variation happens just because.
Natural selection picks which variations are better able to survive.

One thing to remember is that populations evolve, not individual organisms.

The environment determines if a mutation (variation) is a disadvantage, and advantage, or neutral to a population.

Evolution will not happen without selective pressure. Variation will, but a particular variation will not become dominant in a strain unless it is selected for due to environmental pressure.

boughtwithaprice · May 17, 2004

fishstix said:
For those who believe in evolution: can evolution occur in the absence of selection pressures - natural or artificial? Or does some variation exist just because. The reason I'm asking is because I was just considering herbicide resistant weeds. Genetic traits for herbicide resistance occur in a population of plants which are normally susceptible to the chemical long before the herbicide is even invented. It usually doesn't give the plants any particular advantage unless the herbicide is used; in some cases it may be a significant disadvantage to the plant. So why would it exist in these plant populations before the herbicide is invented?

A heroin resistant heman being can be formed by continous exposure to the drug at low levels. What is deadly to a heroin naive person, has no effect on an addict. The same is true for alcohol, and is a characteristic that has nothing to do with evolution. It requires no reproduction to take place.
Genetic variations occur spontaneously apparently, but may have been programed at the beginning. We only observe what has occurred and try to make sense of it. Evolution is dependent on reproduction not death.

lucaspa · May 20, 2004

fishstix said:
For those who believe in evolution: can evolution occur in the absence of selection pressures - natural or artificial? Or does some variation exist just because. The reason I'm asking is because I was just considering herbicide resistant weeds. Genetic traits for herbicide resistance occur in a population of plants which are normally susceptible to the chemical long before the herbicide is even invented. It usually doesn't give the plants any particular advantage unless the herbicide is used; in some cases it may be a significant disadvantage to the plant. So why would it exist in these plant populations before the herbicide is invented?

1. Contrary to notto, there can be evolution without selection pressures. In a very small population, new alleles (forms of genes) can be fixed (spread to all the members of the population) by chance. It's called genetic drift. But for there to be reasonable chance of this happening, the population must be very small, 10 or less breeding individuals.

2. Variation exists "just because". Variation is either 1) random recombination during sexual reproduction or 2) errors in copying DNA (mutations). These just happen. It is not possible to make a completely error-free copying system. Such a system would require more entropy than is present in the entire universe.

3. Variation is present in all populations. Most variation is neutral under a particular environment, so it stays in the population at low frequencies (few individuals have the variation). It does so by the laws of Mendelian genetics. There is a law of genetics called the Hardy-Weinberg law that says that the frequency of an allele (form of a gene) remains constant in a population in the absence of any outside force, such as selection. You can see this if you do the diagrams of two alleles A and a and mate individuals with AA with Aa. This is the real important paragraph to answer your question. We can go into the equations if you want.

When the environment changes -- such as applying herbicide -- then those individuals are suddenly very lucky and they have the right variation to survive. However, sometimes the variation appears after the selection is applied. This happened with some of the antibiotic resistant genes. It also happened in E. coli with the appearance of a mutation allowing the E. coli to live in acidic fluids such as apple juice.

fragmentsofdreams · May 20, 2004

fishstix said:
For those who believe in evolution: can evolution occur in the absence of selection pressures - natural or artificial? Or does some variation exist just because. The reason I'm asking is because I was just considering herbicide resistant weeds. Genetic traits for herbicide resistance occur in a population of plants which are normally susceptible to the chemical long before the herbicide is even invented. It usually doesn't give the plants any particular advantage unless the herbicide is used; in some cases it may be a significant disadvantage to the plant. So why would it exist in these plant populations before the herbicide is invented?

Change occurs all the time without selection. Because mutations and which genes are passed on to a child are more or less random, changes in the population would occur even if each organism had an equal chance of reproduction.

Herbicide resistance could occur when a plant has a mutation that, for example, changes the way a particular enzyme reacts with a group of chemicals. Normally, these chemicals are not present, so the trait has no effect on the survival of the plant. When the herbicide is applied, the enzyme allows the plant to break it down and survive when others can't.

The mutation could easily survive if it had no negative effects because there would be no pressure to eliminate. It would even survive for a while if it had a small but significant negative effect. Selection can take several generation to eliminate a negative mutation if the effect is not so big that it prevents all organisms from reproducing.

lucaspa · May 20, 2004

boughtwithaprice said:
A heroin resistant heman being can be formed by continous exposure to the drug at low levels. What is deadly to a heroin naive person, has no effect on an addict. The same is true for alcohol, and is a characteristic that has nothing to do with evolution. It requires no reproduction to take place.
Genetic variations occur spontaneously apparently, but may have been programed at the beginning. We only observe what has occurred and try to make sense of it. Evolution is dependent on reproduction not death.

With all respect, sincerely, it's possible that you just confused a lot of people. As you said, tolerance to a drug isn't evolution and, IMHO, it makes a very misleading example. Evolution is about changes in populations.

I don't see any way to program genetic variations at the beginning. Since mutations are copying errors, how do you program specific copying errors? If the variation results from recombination of chromosomes during meiosis, then again 1) how do you get specific combinations and 2) how do you ensure that that particular gamete is fertilized?

Finally, if a part of the genome isn't being expressed and is not contributing to the organisms, it gets random changes and becomes very different. There is no selection to keep it fit, so if you put possible changes in it to begin with, they won't stay there over time.

boughtwithaprice · May 20, 2004

lucaspa said:
With all respect, sincerely, it's possible that you just confused a lot of people. As you said, tolerance to a drug isn't evolution and, IMHO, it makes a very misleading example. Evolution is about changes in populations.

I don't see any way to program genetic variations at the beginning. Since mutations are copying errors, how do you program specific copying errors? If the variation results from recombination of chromosomes during meiosis, then again 1) how do you get specific combinations and 2) how do you ensure that that particular gamete is fertilized?

Finally, if a part of the genome isn't being expressed and is not contributing to the organisms, it gets random changes and becomes very different. There is no selection to keep it fit, so if you put possible changes in it to begin with, they won't stay there over time.

sorry if I confused people, but I think resistance to herbicide and other drugs or chemicals is a poor example of evolution, if they can be induced without the requirement of reproduction. They are cellular biochemical expresson changes in the organism, not genetic changes in the population that produce the observed effect.

I am just putting forth my random thoughts, sorry I am trying to resolve the fact of evolution in my mind against the argument used by creationists, in that it is based on death.

My way of thinking is that DNA polymerase is the enzyme that developed to drive reproduction. This enzyme is imperfect in that it produces errors or changes in the gentetic code. I believe that these changes, that occur without necessarily selection pressure, produce what we see as evolution and speciation.

I will still respect you if you tell me that I am crazy and must be smoking something , but I am trying to resolve the poor arguments that creationists use against evolution, just give a converted YEC some time eh I am still learning

fishstix · May 20, 2004

boughtwithaprice said:
A heroin resistant heman being can be formed by continous exposure to the drug at low levels. What is deadly to a heroin naive person, has no effect on an addict. The same is true for alcohol, and is a characteristic that has nothing to do with evolution. It requires no reproduction to take place.
Genetic variations occur spontaneously apparently, but may have been programed at the beginning. We only observe what has occurred and try to make sense of it. Evolution is dependent on reproduction not death.

Herbicide resistance is quite different to heroin resistance. The plants don't slowly build immunity by receiving small doses. There are a number of different kinds of herbicide resistance, but I don't know of any that would develop slowly in a plant that was initially not resistant to the herbicide by repeatedly applying small doses of the herbicide to the plant. Herbicide resistance generally involves the plant having somewhat different genes than the other plants in that species, resulting in the plant either being able to break the herbicide down, or not be able to convert it into a deadly version like the other plants do, or having somewhat different enzymes that the herbicide can't bind to, or things of that nature. Quite often, these differences which give herbicide resistance also give the plant somewhat of a disadvantage if there is no herbicide killing off the other plants around it by making it less efficient at doing whatever the usual enzyme or process is supposed to do, such as produce an amino acid or photosynthesize or whatever.

lucaspa said:
1. Contrary to notto, there can be evolution without selection pressures. In a very small population, new alleles (forms of genes) can be fixed (spread to all the members of the population) by chance. It's called genetic drift. But for there to be reasonable chance of this happening, the population must be very small, 10 or less breeding individuals.

The weed populations with herbicide resistance are usually not very small. There would almost always be more than 10 breeding individuals.

2. Variation exists "just because". Variation is either 1) random recombination during sexual reproduction or 2) errors in copying DNA (mutations). These just happen. It is not possible to make a completely error-free copying system. Such a system would require more entropy than is present in the entire universe.

I'm suddenly getting an urge to try to calculate how much entropy there is in the entire universe. That might be interesting to find out. I wonder if it's possible to calculate...

3. Variation is present in all populations. Most variation is neutral under a particular environment, so it stays in the population at low frequencies (few individuals have the variation). It does so by the laws of Mendelian genetics. There is a law of genetics called the Hardy-Weinberg law that says that the frequency of an allele (form of a gene) remains constant in a population in the absence of any outside force, such as selection. You can see this if you do the diagrams of two alleles A and a and mate individuals with AA with Aa. This is the real important paragraph to answer your question. We can go into the equations if you want.

Yes, I vaguely recall the Hardy-Weinberg law from a plant breeding and genetics class I once took. I suppose that could work for mutations that are neutral in nature, but what about for those which are detrimental to the plant as compared to non-mutant plants unless the herbicide is present?

When the environment changes -- such as applying herbicide -- then those individuals are suddenly very lucky and they have the right variation to survive. However, sometimes the variation appears after the selection is applied. This happened with some of the antibiotic resistant genes. It also happened in E. coli with the appearance of a mutation allowing the E. coli to live in acidic fluids such as apple juice.

Wouldn't the plants that don't have the variation in advance already be dead after the selection, in this case the herbicide, is applied?

boughtwithaprice · May 20, 2004

fishstix said:
Herbicide resistance is quite different to heroin resistance. The plants don't slowly build immunity by receiving small doses.

How so? There is no anti-heroin immune globulin produced. It is simply the downregulation of the mu receptor by negative feedback or tachyphylaxis that produces tolerance, not immunity.

There are a number of different kinds of herbicide resistance, but I don't know of any that would develop slowly in a plant that was initially not resistant to the herbicide by repeatedly applying small doses of the herbicide to the plant. Herbicide resistance generally involves the plant having somewhat different genes than the other plants in that species, resulting in the plant either being able to break the herbicide down, or not be able to convert it into a deadly version like the other plants do, or having somewhat different enzymes that the herbicide can't bind to, or things of that nature. Quite often, these differences which give herbicide resistance also give the plant somewhat of a disadvantage if there is no herbicide killing off the other plants around it by making it less efficient at doing whatever the usual enzyme or process is supposed to do, such as produce an amino acid or photosynthesize or whatever.

I thought that alot of herbicides work by inhibiting acetylcholinesterase, which is lethal in high doses, but through receptor downregulation, could be made tolerable over time. Most poisons are labeled with an ED95, or effective dose where 95% of a population will be killed by a given concentration. I would need to study the mechanism of herbicide resistance to give a better answer.
Your answer would explain why resistance goes away when selection pressure is removed, but, if the change is genetic, why does it go away? If it is receptor mediated, then the removal of negative feedback would explain the change. The same goes for antibioltic resistance. Antibiotic resistant bacteria are found in hospitals, where selection pressure is high and host immunity low. Community acquired infections with bacteria are not common with high antibiotic resistant organisms. I do believe that some antibiotic resistance is plasmid mediated; however, that would make it genetic

Wouldn't the plants that don't have the variation in advance already be dead after the selection, in this case the herbicide, is applied?

only if the dose is high enough,.....the old addage, what doesn't kill you makes you stronger. I admit that I am just speculating here (read-babbling). I need more facts to study. My expertise is in human physiology, not plants

Captain_Jack_Sparrow · May 20, 2004

fishstix said:
I'm suddenly getting an urge to try to calculate how much entropy there is in the entire universe. That might be interesting to find out. I wonder if it's possible to calculate...

No it is not possible.

fishstix · May 21, 2004

boughtwithaprice said:
How so? There is no anti-heroin immune globulin produced. It is simply the downregulation of the mu receptor by negative feedback or tachyphylaxis that produces tolerance, not immunity.

I thought that alot of herbicides work by inhibiting acetylcholinesterase, which is lethal in high doses, but through receptor downregulation, could be made tolerable over time. Most poisons are labeled with an ED95, or effective dose where 95% of a population will be killed by a given concentration. I would need to study the mechanism of herbicide resistance to give a better answer.

You're thinking of insecticides. There are a number of different herbicide modes of action. Herbicides can be grouped according to mode of action:
Group 1: Inhibitors of acetyl CoA carboxylase (ACCase)
Group 2: Inhibitors of acetolactase synthase (ALS)
Group 3: Microtubule assembly inhibitors, tubulin protein involved in cell division, interrupts mitosis (root inhibitors)
Group 4: Synthetic auxins
Groups 5 and 6: Inhibitors of photosynthesis at photosystem II, Site A
Group 7: Inhibitors of photosynthesis at photosystem II, Site B
Groups 8 and 15: Conjugation of acetyl co-enzyme A. (shoot inhibitors)
Group 9: Inhibitors of 5-enolpyruvylshikimimate-3-phosphate synthase (EPSPS)
Group 10: Inhibitors of glutamine synthetase, also known as inhibitors of ammonia assimilation
Groups 11 and 13: Inhibitors of carotenoid biosynthesis
Group 14: Inhibitors of protoporphyrinogen oxidase (PPO or Protox)
Group 19: Inhibition of auxin transport
Group 22: Photosystem I - electron diverters
Group 28: Inhibitors of p-hydroxyphenyl pyruvate dioxygenase (HPPD)

Your answer would explain why resistance goes away when selection pressure is removed, but, if the change is genetic, why does it go away? If it is receptor mediated, then the removal of negative feedback would explain the change. The same goes for antibioltic resistance. Antibiotic resistant bacteria are found in hospitals, where selection pressure is high and host immunity low. Community acquired infections with bacteria are not common with high antibiotic resistant organisms. I do believe that some antibiotic resistance is plasmid mediated; however, that would make it genetic

The resistance doesn't go away. Once you have resistant plants, that resistance to that herbicide group is there to stay. It is genetic. Again, I think you are thinking of something other than plants and herbicides.

only if the dose is high enough,.....the old addage, what doesn't kill you makes you stronger. I admit that I am just speculating here (read-babbling). I need more facts to study. My expertise is in human physiology, not plants

Um... With the possible exception of a very, very low dose of the group 4 herbicides (which mimic a plant growth "hormone"), if the plant manages to survive a herbicide application it certainly won't be stronger.

fishstix · May 21, 2004

Captain_Jack_Sparrow said:
No it is not possible.

Hmm. That's too bad. My calculator hasn't been getting much of a workout since my last physics class.

boughtwithaprice · May 21, 2004

fishstix said:
You're thinking of insecticides. There are a number of different herbicide modes of action. Herbicides can be grouped according to mode of action:
Group 1: Inhibitors of acetyl CoA carboxylase (ACCase)
Group 2: Inhibitors of acetolactase synthase (ALS)
Group 3: Microtubule assembly inhibitors, tubulin protein involved in cell division, interrupts mitosis (root inhibitors)
Group 4: Synthetic auxins
Groups 5 and 6: Inhibitors of photosynthesis at photosystem II, Site A
Group 7: Inhibitors of photosynthesis at photosystem II, Site B
Groups 8 and 15: Conjugation of acetyl co-enzyme A. (shoot inhibitors)
Group 9: Inhibitors of 5-enolpyruvylshikimimate-3-phosphate synthase (EPSPS)
Group 10: Inhibitors of glutamine synthetase, also known as inhibitors of ammonia assimilation
Groups 11 and 13: Inhibitors of carotenoid biosynthesis
Group 14: Inhibitors of protoporphyrinogen oxidase (PPO or Protox)
Group 19: Inhibition of auxin transport
Group 22: Photosystem I - electron diverters
Group 28: Inhibitors of p-hydroxyphenyl pyruvate dioxygenase (HPPD)

The resistance doesn't go away. Once you have resistant plants, that resistance to that herbicide group is there to stay. It is genetic. Again, I think you are thinking of something other than plants and herbicides.

Um... With the possible exception of a very, very low dose of the group 4 herbicides (which mimic a plant growth "hormone"), if the plant manages to survive a herbicide application it certainly won't be stronger.

I stand corrected. Knowledge moves on

Evolution Without Selection Pressure?

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