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Author Topic: The lucky butterfly
thom
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A philosophy of biology thread, for anybody who's interested.

I came across the following thought experiment a year ago, but have recently been reminded of it and I think it raises a number of interesting questions.

Take a population of butterflies with no genetic variation: all butterflies are genetically identical. When this butterfly hatches, it imprints on the leaf on which it hatches, and this imprinting causes it to (a) prefer the taste of that leaf; and (b) to lay its eggs on the same type of leaf when it reproduces. All the butterflies in the population imprint on the same leaf type, and so no selection can act because there is no variation in fitness.

Now lets say one egg gets blown off from the leaf on which it was laid and it lands on a different leaf. The butterfly then imprints on the new leaf, which happens to be a better food source than the other leaf (lets say it makes the butterfly bigger and better at fighting). Thus, the "lucky" butterfly is fitter than the other butterflies, and so is selected for, as it can out compete the other butterflies. Eventually, all the butterflies come to be laid on the new leaf. Thus, natural selection has occurred, but without any variation in genetic fitness.

The argument then goes that we therefore can't privilege the role of genes in the selection process. The orthodox view that it genes have a privileged role in biological inheritance is challenged, since the lucky butterfly shows that the environment is also inherited, in an important sense. The author of the though experiment, Matteo Mameli, goes further, and says that we are as justified in talking of envirotypes as we are talking of genotypes.

So, good people of OTF (and Toro)*: what’d’yer reckon? A challenge to biological orthodoxy, or unnecessary philosophical fooling around?

* [insert emoticon here]

[ 30.11.2005, 15:54: Message edited by: thom ]

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My name is Mumpo
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random thought from someone who's not clued up at all in this biological inheritance stuff:

"The butterfly then imprints on the new leaf, which happens to be a better food source than the other leaf (lets say it makes the butterfly bigger and better at fighting)"

bigger; and therefore better at fighting, but worse at flying. is there such a thing as an advance in natural selection that doesn't have a detrimental side-effect?

sorry. thread hijack.

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thom
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quote:
is there such a thing as an advance in natural selection that doesn't have a detrimental side-effect?
Yeah, though not being a biologist I'm struggling to give a good example. I could suggest some psychological examples, but that really will produce a thread hijack; people suddenly have lots of opinions when you start applying evolution to humans, and the brain in particular.

Anyway, that gets off the point. That the butterfly gets bigger was just an example. If you want, lets say that the food is more nutritious and thus prolongs sexually active lifetime. The argument is not changed.

[ 30.11.2005, 16:22: Message edited by: thom ]

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Ginger Yellow
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quote:
A challenge to biological orthodoxy, or unnecessary philosophical fooling around?
Neither, I'd say. Most biologists accept that parts of heredity are non-genetic, and Dawkins in particular stresses that an organism's environment can be considered part of the phenotype in the right circumstances. Most obviously, humans and other longterm structure building organisms inherit their environments to a certain degree, and there are other areas like the Baldwin effect where plasticity is more important than genetics. Conversely it is a very interesting topic, and far from "unnecessary" or "fooling around".

More importantly, I'm not sure your specific examples works as you or perhaps Mameli thinks. If the butterflies are using exclusively these leaves as a food source, then they're only going to be competing with butterflies on the same leaf type. So you'll (eventually) get a speciation event, not a spread of a new "envirotype" within the species.

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Tubby Isaacs
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quote:
Dawkins in particular stresses that an organism's environment can be considered part of the phenotype
don't use that fucking word in my presence. I don't have a dunce hat to hand.
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thom
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quote:
Most biologists accept that parts of heredity are non-genetic, and Dawkins in particular stresses that an organism's environment can be considered part of the phenotype in the right circumstances. Most obviously, humans and other longterm structure building organisms inherit their environments to a certain degree, and there are other areas like the Baldwin effect where plasticity is more important than genetics.
Of course, no right-thinking biologist would argue that the environment has no role to play in ontogeny. However, biologists are prone to grant genes a privileged role over in the processes of selection, inheritance and development, to pick just three. What Mameli is trying to show (and I'm not sure what I think yet, which is partly why I'm puttint this up here) is that there is no logical justification for doing this. The question is: How can we distinguish between genes and, in this example, the environment? If we can't, how are we justified in saying that selection happens at the genetic level, or that it's genes which are inherited from one generation to the next?

quote:
I'm not sure your specific examples works as you or perhaps Mameli thinks. If the butterflies are using exclusively these leaves as a food source, then they're only going to be competing with butterflies on the same leaf type.
Well, lets say then (I should have made this clear) that all the butterflies in the population hatch on to the same leaf type. What then of the lucky one that gets blown off and lands elsewhere? It can be selected for (or against, even, if the new leaf is no good), but without genes playing any role.

[ 30.11.2005, 17:57: Message edited by: thom ]

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Ginger Yellow
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Fair enough, but as I say, I think you're overstressing the importance people put on genes in selection. Certainly people put lots of stress on the role of genes in inheritance and development (and quite possibly too much), but surely everyone accepts that it's the (extended) phenotype that selection acts on. Dawkins himself would argue that the genes almost exclusively determine the phenotype, I suppose, but he's pretty much out on a limb there. Any species which displays culture is going to have a significant non-genetic component, for a start.
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thom
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quote:
surely everyone accepts that it's the (extended) phenotype that selection acts on
No, not at all. Indeed, Dawkins is most famous for popularising the view that selection acts on genes, not individuals. Where these interests collide (for example, in the social insects), it's the geneotype that gets preserved, not the phenotype. This is the conventional view in evolutionary biology (so much so that you might even call it dogmatic).

quote:
Dawkins himself would argue that the genes almost exclusively determine the phenotype...
No, he wouldn't. Where have you got that idea from? (Serious question, like; it's not meant to sound rude/aggressive.)
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Ginger Yellow
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I think my wording may be letting me down here, although I could just have it backwards. What I mean to say is:
a) While the genotype is what bears the effects of selection, the phenotype bears the selection pressure itself. The genes don't have sex or fail to, but the genotype of the descendants is determined by the selection pressures the ancestors faced.

b) Dawkins would argue that genes interacting with environment almost exclusively determine the phenotype, as opposed to culture or the sort of "random" events in the thought experiment. Is that still wrong? In my limited readings of his work that's the impression I've received.

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boris
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Surely it's all predetermined anyway, as a result of the Big Bang?
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S. aureus
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This is an example of a species entering into a new environment, for example rabbits into Australia, isn't it?
If the species is well suited to this environment it will thrive, if not it won't.
I think that the butterfly example is the same as rabbits in Australia - both species do well in their new environments, but cannot be said to be competing with their fellows that are left behind. Over time the differing evolutionary pressures of the two environments will cause speciation. (So, what GY said further up)

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Wyatt Earp
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It was Dennett, I think, who pointed out, in reply to this guy, that although some form of "habitat selection" may well go on, it can't possibly build complex adaptations (or strictly, it can't probably build complex adaptations). That's because the lineage is constrained to wander in the space of actual environments, but can wander in the space of possible genotypes. The latter is hugely, unimaginably more vast than the former.
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Wyatt Earp
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(So in that sense, gene-level selection is privileged.)
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thom
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Good point. Where did you find Dennett's writings on this stuff?
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Super Sharp Shooter
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Have only skimmed, so apologies if I'm being simplistic, but surely the "without any variation in genetic fitness" caveat is fundamentally flawed. As I understand the theory, genetic variation occurs will-nilly all the time, pretty much at random, whereas environment is a "selector", in that it kills off those random mutations which didn't have the great fortune to be by sheer fluke*, suitable to that environment.

So, unless I'm barking up the wrong tree entirely, it's somewhat flawed to "privilege the role of genes in the selection process". Genes actually have nothing at all to do with the selection process, just the mutation process.


*Obviously, the selection is a triage process, so with the added ingredient of unimaginably long stretches of time, "randomness" becomes order, and fluke mutations become indistinguishable from designed improvements, as long as we are only examining evolutionary successes, rather than the failures which likely greatly outnumber them.

[ 01.12.2005, 10:18: Message edited by: Super Sharp Shooter ]

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