The Ant and the Blade of Grass

[Lestat]

Daniel C. Dennett likes to tell the story about an ant crawling to the top of a blade of grass. It turn out that the ant's brain can be invaded by a parasitic worm, which need to get into the belly of a cow for survival. Thus (as Dennett puts it), it drives the ant like an all terrain vehicle up the blade of grass so a cow can digest it.

Well, I certainly have a healthy respect for evolution, but just how on earth did this worm evolve to know not only to invade the brain of the ant, but to then have an exact effect on the brain that causes it to climb up the blade of grass? That just seems too big of a stretch for me. Too wide of a gap to be passed on. Btw- I understand the worm doesn't actually "know" anything, but I'm not sure what DOES know. The DNA of the worm?

I have similar questions regarding the behavior of insects... How many times have you seen a spider crawing on the wall and when you went to kill it, it froze. Now I understand that it freezes, because movement makes it more noticable to a predator. Better to freeze. I can somewhat understand a deer or antelope doing this when sensing a lion in it's presence, but only because they have probably seen other deer and antelope (their parents), do it. Even if this was built into their brain through evolution, I could somewhat understand it.

I get how a wing, or spots, or tails evolve (even bi-pedal mammals). It is a gradual yet, beneficial change that gets passed on. But I cannot see how evolution could've hard wired this freezing behavior into an insect's brain. Insects that didn't freeze were eaten. Those that did, survived, but... How did those that survived pass this trait onto their offspring? Maybe I don't understand it, because I'm under estimating an insects intelligence?

[bunny]

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Daniel C. Dennett likes to tell the story about an ant crawling to the top of a blade of grass. It turn out that the ant's brain can be invaded by a parasitic worm, which need to get into the belly of a cow for survival. Thus (as Dennett puts it), it drives the ant like an all terrain vehicle up the blade of grass so a cow can digest it.

Well, I certainly have a healthy respect for evolution, but just how on earth did this worm evolve to know not only to invade the brain of the ant, but to then have an exact effect on the brain that causes it to climb up the blade of grass?

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(Insert usual "I wouldnt really have a clue but..." disclaimer here)

It would seem more plausible to me that it first evolved to live in an ant's brain. An unexpected side effect was that sometimes the ant got driven up the blade of grass, because of where the worm was nestled. Consequently a lot of them got eaten by cows. A few worm larvae or whatever happened to survive. Repeat many times. Gradually, the "living in a cow for a while" bit becomes essential (which would then provide an evolutionary advantage to those ants which happened to live near the "climb a blade of grass" part of the ants' brain)....

[qwnu]

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I get how a wing, or spots, or tails evolve (even bi-pedal mammals). It is a gradual yet, beneficial change that gets passed on. But I cannot see how evolution could've hard wired this freezing behavior into an insect's brain. Insects that didn't freeze were eaten. Those that did, survived, but... How did those that survived pass this trait onto their offspring? Maybe I don't understand it, because I'm under estimating an insects intelligence?

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I'm unclear on what's not to get. How could those that survived NOT pass on the trait to their offspring?

One point of confusion might be "nature vs. nurture", i.e., which part of an animal's behavior is driven by genes and instinct, and which is driven by learning and culture. I think the behavior you're describing, especially in insects, is completely driven by instinct, and therefore completely heritable and subject to natural selection. Perhaps more complicated with the deer freezing when the lion comes along, but I'd be surprised if there was any evidence to suggest that this behavior was learned through observing other deer, rather than completely instinctive.

Another possible point of confusion might be your perception that this trait is somehow black and white, i.e., all or nothing, i.e., "freeze" or "don't freeze". It's probably more helpful (and accurate) to think of it as a continuum, in conjunction with lots of other factors, that results in small differences in the tendencies of different spiders to do slightly different things when confronted with the perception of movement.

In other words, the evolution of behavioral traits like these can be just as gradual as the physical structures you are more familiar with. All that's needed is a small variation (derived from some genetic difference) that confers the slightest benefit (relative to competitors) and we're off and running.

[Lestat]

Thanks. Your explanation helps. I'm not sure I even understand what I'm asking, so it's hard for me to pose the question correctly. I think you helped with this too....

<font color="blue"> I'm unclear on what's not to get. How could those that survived NOT pass on the trait to their offspring? </font>

Passing on a Trait. That's seems to be it. A physical trait gets passed on, because offspring fair better with the change. So it's retained, right? So I can see how you inherit a physical trait. I can also see how beneficial behavioral traits can be inherited. But...

Freezing, is a behavioral trait that occurs in mid-life. Unlike beneficial mid-life physical traits (like waiting to a certain age to become reproductive), this behavioral trait is presumably "learned" in adulthood. After the fact, so to speak.

I can see how a salmon "learns" to swim upstream at the end of its life, because it completes a purpose. A purpose that generations of salmon have completed before him. Freezing, on the other hand, doesn't necessarily complete any meaningful purpose. In fact, if a spider were never put into a position to have to freeze, it wouldn't. It only freezes under certain conditions. How does it learn what those conditions are is what I want to know. Remember, we are talking about an almost zero level of intelligence. Insects are pretty much automatons from what I understand. I could understand a gazelle learning something later in life from its herd. I can't understand how this ocurrs with spiders.

I know I'm very ignorant here, so I request people have patience with me.

[Lestat]

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Daniel C. Dennett likes to tell the story about an ant crawling to the top of a blade of grass. It turn out that the ant's brain can be invaded by a parasitic worm, which need to get into the belly of a cow for survival. Thus (as Dennett puts it), it drives the ant like an all terrain vehicle up the blade of grass so a cow can digest it.

Well, I certainly have a healthy respect for evolution, but just how on earth did this worm evolve to know not only to invade the brain of the ant, but to then have an exact effect on the brain that causes it to climb up the blade of grass?

[/ QUOTE ]
(Insert usual "I wouldnt really have a clue but..." disclaimer here)

It would seem more plausible to me that it first evolved to live in an ant's brain. An unexpected side effect was that sometimes the ant got driven up the blade of grass, because of where the worm was nestled. Consequently a lot of them got eaten by cows. A few worm larvae or whatever happened to survive. Repeat many times. Gradually, the "living in a cow for a while" bit becomes essential (which would then provide an evolutionary advantage to those ants which happened to live near the "climb a blade of grass" part of the ants' brain)....

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You are very good at reverse engineering! I didn't think of that. Dennett simply states that the worm "needs" to get into the belly of a cow. So I assumed this preceded all, and that evolution somehow figured out the best way for for the worm to achieve this. But yes, if the cow came afterward, it makes much more sense.

[vhawk01]

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Daniel C. Dennett likes to tell the story about an ant crawling to the top of a blade of grass. It turn out that the ant's brain can be invaded by a parasitic worm, which need to get into the belly of a cow for survival. Thus (as Dennett puts it), it drives the ant like an all terrain vehicle up the blade of grass so a cow can digest it.

Well, I certainly have a healthy respect for evolution, but just how on earth did this worm evolve to know not only to invade the brain of the ant, but to then have an exact effect on the brain that causes it to climb up the blade of grass?

[/ QUOTE ]
(Insert usual "I wouldnt really have a clue but..." disclaimer here)

It would seem more plausible to me that it first evolved to live in an ant's brain. An unexpected side effect was that sometimes the ant got driven up the blade of grass, because of where the worm was nestled. Consequently a lot of them got eaten by cows. A few worm larvae or whatever happened to survive. Repeat many times. Gradually, the "living in a cow for a while" bit becomes essential (which would then provide an evolutionary advantage to those ants which happened to live near the "climb a blade of grass" part of the ants' brain)....

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You are very good at reverse engineering! I didn't think of that. Dennett simply states that the worm "needs" to get into the belly of a cow. So I assumed this preceded all, and that evolution somehow figured out the best way for for the worm to achieve this. But yes, if the cow came afterward, it makes much more sense.

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Its a common problem in discussing topics in evolution, this anthropomorphizing of the process, attributing intent, things like that. The reason is, it makes it easier to describe something by saying stuff like "Ok, so the worm needs to get here, so evolution creates this mechanism." It ends up being misleading and is obviously entirely incorrect, but it makes it more approachable. Dennett is probably the worst culprit of this, although I think Gould does it often and Dawkins also, to a lesser degree. Certainly, it is something that is RAMPANT in science textbooks and popular scientific writing. I always cringe a little bit when I read anyone talking about the goals or tactics or strategies that evolution uses to do things, but I also cringe when I read a general chemistry textbook and they talk about electrons in orbits and such. Its a balance, the writer tries his best, and you must consider the audience. Lestat, by posting here on SMP and reading the evolution debate in a very special context, you have a different perspective than the vast majority of readers. You know enough to cringe when you hear agency bestowed upon mindless processes (mindless added for NRs benefit) but perhaps don't have the experience or the repetition needed to translate those pop-speak phrases into what they really mean.

[vhawk01]

And to answer your OP, I really don't know, but I'd imagine it was very complex and took a long, long time. It has the hallmarks of a scaffolding-type scenario.

[luckyme]

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In fact, if a spider were never put into a position to have to freeze, it wouldn't. It only freezes under certain conditions. How does it learn what those conditions are is what I want to know.

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It doesn't. Avoiding detection is a major'trick' used to avoid danger. There is no evolutionary difference between being speckled and being still. Ancestors that didn't freeze under those conditions didn't leave many offspring, just as the ones without camoflage.

Sometimes it hard to appreciate how far back in the chain some of the behaviors or traits can trace their origins. I have no idea if it's true but it wouldn't surprise me to discover that single cells in a pond may have evolved to stop cillating when pressure waves of a certain mix are sensed.

Millions of generations passing through the selection filters can pull of some neat stuff.

luckyme

[Duke]

You can look at any sufficiently complex behavior/situation/process and befuddle yourself trying to figure it out. This is pretty much the antecedent of every religion ever.

[Lestat]

<font color="blue">There is no evolutionary difference between being speckled and being still. Ancestors that didn't freeze under those conditions didn't leave many offspring, just as the ones without camoflage. </font>

Thanks luckyme. I understand that, I just don't think I'm explaining my question properly. One of the things I'm trying to understand is what direction evolution works from. For instance:

Through a random mutation a species of beetle devolops some spots, which it passes to it's offspring. These spots make it harder for a predator to detect, so they become more likely to survive as those without spots start disappearing, because they are now more likely to be the ones eaten. If that's close, then good enough. I have some understanding of how a physical trait is born. We can replace spots with webbed feet, wings, etc. Now let's move on to "freezing".

When the first spider froze at the sight of a predator, it presumably survied because of this. Since this is NOT a random mutation, I don't get how it is first got passed down and evolves. Unlike the the physical characteristic of spots, I don't see how a behavioral characteristic such as freezing is passed down for the firs time. Freezing seems to be a "learned" behavior. I understand how a wildebeast teaches its offspring to freeze at the sight of a lion, and I can even see how this might eventually become hard wired into future generations of wildebeasts. But insects don't possess the intellect to learn from watched behavior of parents (at least I didn't think so).

So exactly how does behavior in insects evolve?

I'm STILL not asking the question the way I want, but hopefully this is close enough.