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  1. #21
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    I think this thread is missing some significant points.

    Evolution, through selective pressure, is less about the "form" of the pressure than it is about the "strength" of the pressure. The greater the pressure (in any form), the faster and more strongly selection occurs. The resulting changes are "evolution."

    The selection can either be natural or artificial.

    Secondly, the source of the selective pressure is important. Not the form; the source.

    Let's just take a blood-sucking insect, a mosquito, on a human as an example.

    The mosquito bites the human and withdraws enough blood to fill itself. Selective pressure on the human? Very, very low to none. The amount of blood withdrawn is highly unlikely to pose any threat to the human. The mosquito, and, in most cases, the allergic reaction to the mosquito bite, however, make humans desire to avoid the mosquito. So, humans have developed a defense. Slap.

    Now, if our human misses a mosquito, well, no real harm done. The mosqito escapes with her life and a full meal of blood. The human survives quite nicely, and the mosquito bite confers no real loss of fitness on the human.

    So, the selective pressure from the mosqito on the human is negligible, at best.

    But, if that mosquito is carrying a disease -- let's just say, malaria -- the human's life is at risk because of the malaria. Loss of life, if occurring before offspring can be left, seriously reduces evolutionary fitness. So, the selective pressure is much, much higher on the human. But not from the mosquito; from the plasmodium that causes malaria. As the chances of contracting malaria increase, the selective pressure increases.

    Now, a "reasoned" method of reducing the selective pressure might be to reduce the population of mosquitoes that carry malaria, or a way to reduce the incidence of mosquito bites, but selective pressures don't function that way. The selective pressure is from the plasmodium, not from the mosquito.

    Same thing goes, seems to me, for Varroa on honey bees. As long as the viruses or other pathogens carried by Varroa are the source of the selective pressure on honey bees, the adaptations that will enhance the evolutionary fitness (survival) of the bees will be in response to the direct source of the selective pressure.

    Same ideas apply to resistance to "soft" treatments developing among Varroa. As long as the selective pressure is low (either the mites face relatively little chance of exposure to selective pressure, or the pressure does not significantly reduce the evolutionary fitness of the mites), resistance will develop very, very slowly -- if ever. But if the selective pressure increases, the speed with which resistance (or adaptation) will develop will increase correspondingly.

    Just an interesting note on mites overcoming resistance: T. L. Harvey, T. J. Martin and D. L. Seifers at Kansas State University used experimental pressure to measure the speed with which wheat curl mites (Aceria tosichilla) can overcome plant resistance to the mites. The plant resistance is frequently physical, not chemical, such as "hairy" plants preventing the mites from acquiring feeding sites. Now, these mites are not the same as Varroa, but they are both mites. The wheat curl mites were not given a chance to switch plants -- either they managed to feed on the plants presented to them, or they starved to death. The mites overcame the best forms of host plant resistance available at the time of the study in 60 days. More interestingly, 60 days represents 8 generations of wheat curl mites.

  2. #22
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    Human adaptation to malaria is a perfect example of the random nature of evolution. We developed a gene which confers imunity; great. Why haven't we all got the gene? Because a homozygote develops a dangerous disease; sickle-cell anaemia. In a region with significant malaria, the gene reaches a level in the population where the extra offspring born to people with a single gene (advantage) balance out the offspring not being born to people who die of sickle-cell first (disadvantage)
    RSBrenchley@aol.com
    Birmingham UK

  3. #23
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    Default Response to Jim's Challenge

    "Can anyone name any example of any animal developing some sort
    of "natural resistance" or "ability to deal with" a parasite?" Jim Fischer

    OK, Jim I will take the bait, and take it a step further, just as a heuristic conversation.

    I will go out on a limb and postulate that all animals or organisms that do not go extinct due to parasitism have developed natural resistance and coping abilities to deal with parasites (& I see virus as an obligate parasite) developed through a Darwinian type of natural selection.

    Would honeybees go extinct without human intervention? I would wager they would not, but there would be a huge genetic bottleneck where the susceptible genotypes are culled by Mom Nature.

    Several Varroa and virus coping mechanisms have been observed and well documented in honeybees. For example, the researcher who first documented IAPV found that in 30% of the honey bee samples the viral genome had been incorporated in the the bee genome, thus conferring disease resistance from the virus. This type of genetic phenomena combined with other genetically based traits, such as higher degrees of Varroa Sensitive Hygiene (VSH), slightly shorter larval developmental periods, propensities for high degrees of allogrooming and individual grooming will prevent the extinction of the honeybee due to Varroa and the mites they vector. It is almost impossible to talk mites with a consideration for the virus they help transmit.
    Last edited by JBJ; 11-12-2007 at 03:22 PM. Reason: typos
    John B Jacob www.oldsolbees.com

  4. #24
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    Quote Originally Posted by Kieck View Post
    The selection can either be natural or artificial.
    Define artificial in this context.

    Keith
    Bee Sting Honey - So Good, It Hurts!

  5. #25
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    Quote Originally Posted by JBJ View Post

    Would honeybees go extinct without human intervention? I would wager they would not, but there would be a huge genetic bottleneck where the susceptible genotypes are culled by Mom Nature.
    I agree with the first stament, but question the idea of a huge bottleneck, at least as I read the term huge bottle neck. To me this implies a significant bottleneck, i.e. a narrow one. I think that it would not be as bad as you think. Would there be some loss of genetic heterozygocity? Sure, but nothing akin to say, the cheetah. Why? I think the base is too wide for that and the mites don't act fast enough. Of course I am speculating, and rather wildly at that.

    Keith
    Bee Sting Honey - So Good, It Hurts!

  6. #26
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    Quote Originally Posted by Michael Bush View Post
    Since most external parasites usually don't kill their host, there is no reason for natural selection in this matter. Since Varroa do, there would be natural selection (assuming you aren't treating for them).
    If it feeds off of you, it affects you. Something need not kill you outright to have selective pressure. Now killing you is very effective pressure, but if it does so fast enough and completely enough you will never develop resistance. Think gunshot to the head.

    For a population to develop resistance (and this is always discussed in terms of populations not individuals) you have to have a parasite that is not immediately fatal, and a host population in which there is some variability in susceptibility. Then you can see some changes related to selective pressure.

    People aren't killed by leeches so why would there be any natural selection?
    Who says there isn't. Just because a behavior is A) a behavior and B) has other uses doesn't mean it does not fit the definition of resistance. Pick the darn things off. Primates have been doing this for eons. Come up to the level of the monkeys and pick. All this evolution and someone needs to point out that this is what you do to be resistant to a leech?

    Don't confuse the ability to kill you with the ability to exert a selective pressure One is not the other. It can be, but it is not necessarily so. All it takes is to reduce your fitness to the point where it creates a differential between your ability to propagate relative to your conspecifics. Don't make the assumption that the mechanism of resistance is not already present in some other form, recognized by humans or not. Coming up with something brand spanking new is unusual in the extreme, mother nature tends to borrow and improvise rather than engage in de novo creation. That is apparently the realm of various creator deities.

    Keith
    Bee Sting Honey - So Good, It Hurts!

  7. #27
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    Quote Originally Posted by Robert Brenchley View Post
    Human adaptation to malaria is a perfect example of the random nature of evolution. We developed a gene which confers imunity; great. Why haven't we all got the gene? Because a homozygote develops a dangerous disease; sickle-cell anaemia. In a region with significant malaria, the gene reaches a level in the population where the extra offspring born to people with a single gene (advantage) balance out the offspring not being born to people who die of sickle-cell first (disadvantage)

    Well put.

    Keith
    Bee Sting Honey - So Good, It Hurts!

  8. #28
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    Define artificial in this context. -Keith Benson
    I threw the "natural or artificial" comment in to point out that selection can act through either method, but the amount of pressure is the significant factor.

    This context, as I read it, is that honey bees are likely/unlikely to develop "resistance" to Varroa. Natural selection would be simply that bees that are "resistant" to Varroa have greater evolutionary fitness. "Artificial" selection could be imposed on the honey bees by beekeepers if, for instance, beekeepers deliberately destroyed/requeened/etcetera colonies that showed any evidence of Varroa infestation.

    . . . you have to have a parasite that is not immediately fatal. . . . -Keith Benson
    To me, that is redundant. "Fatal parasite" is an oxymoron. "Parasites," by definition, are not fatal to their hosts. Once they become fatal, they move into the realm of "parasitoids" or "predators," to my way of thinking.

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