Evolution: time for some change?

Spurious

No that's not quite it.

BBH said "True, but preadaptive change is not an adaptation until the situation arises where it is useful..."

In this case surely preadaptive change is not adaptive at all. It is just happens to seem adaptive later, because of environmental change.

Surely adaptionism requires that adaptions are selected for at the they time that they happen? Otherwise they're not adaptions, they're just non-useful mutations.

Canute

 

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Canute: Adaptations are selected for at the time that they happen, but the mutation that permits the adaptation has to happen before that. Adaptation is a description of the combined process of mutation and selection.

 

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But didn't you you say that expressions of genetic mutations weren't useful until after the environment had changed, (and were thus 'preadaptions' until then) at which time they became adaptions? This seems tautological to me.

 

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That's why Dawkins called his book The BLIND Watchmaker.

 

Maybe, I've read it but forgotten his argument. How would he have answered my point?

It seems to me that pre-adaption means non-adaption, as the metaphor of spandrels suggests. If a feature is not useful it cannot be selected for but is there by pure chance. Even if the environment changes to make it useful I don't see how that alters the fact that it arose by chance.

To put it another way if my son had been born with a beak this mutation would have been not only useless but positively survival-threatening. However if the environment changed and suddenly we all had to live on bird food it would be quite useful. Would we then call it an adaption derived from a preadaption?

 

That's about right. Mutations pretty much can't happen at the time of selection for multicellular organisms, because the mutation has to have an adaptive significance and also be present in the reproductive organs. Mutations generally happen within the gametes and are inherited by the offspring by that mechanism. As such with human beings there can be as much as a 30-40 year lag between mutation and selection, depending on how long you wait to have children.

 

I think I see what you're saying here and it makes sense. But a few more questions to make sure I'm getting this right.

(I assume you mean mutation of phsyical features here).

I agree that mutations can't happen at any particular time, since they can't by definition, but I don't quite understand what you're saying here. Are you saying that a mutation of phenotype has to have an adaptive significance before it can be considered to be an adaption?

If I get you right the mutation is random, the 30-40 years is the preadaption stage, then the mutation becomes an adaption because it suddenly becomes useful in some way.

Wouldn't the time lag have to be shorter than this, otherwise he (let's assume it's a he) does his mating in the pre-adaption period and isn't any better than his competitors at passing on his genes?

Also, how is the mutation selected for here? There has been no change of environment to make it useful.

 

exaptation

This is a pretty interesting and instructive topic. Essentially, we are asking about non-adaptive origins for adaptive characterisitics, wheather we call them spandrels, preadaptations or exaptations. Exaptations do not need to be neutral characterisitics (i.e. nonadaptive). They can be and are adaptations that are shared for a new function. Feathers, e.g., were thought to derive for warmth but also were used for flight.

 

But if feathers were useful for warmth wouldn't they have been an adaption? They were exaptations in terms of wings, but that's just a point of view taken in hindsight.

My problem with all this is that if an adaption only becomes useful with a change of environment than in what sense are they adaptions? Surely they are just useless mutations that by chance become useful.

 

reply

well, the (legitimate) problem is that adaptations do not arrive whole, de novo when needed. These morphological characteristics are thought to derive gradually over time. However, how does natural selection refine partial characters. One answer could be that these 'adaptations', as we see them, had other adaptive functions while evolving. For example, feathers for warmth before being co-opted for flight as well. These questions abound in evolutionary biology, and I'll bet, if you pick up any Evolution issue (the scientific journal) over the last 20 years, you'll find at least one article addressing this, at least in a tangential way. Alternatively, there is the hypothesis that denies that these charactersitics have to evolve gradually from which derives the 'hopeful monster' theory and which is addressed by some 'evo-devo' researchers.

 

paulsamuel, I am not very informed about evolutionary theories, but
what is your take on "Beak of the Finch," the book that won the
1995 Pulitzer Prize by Jonathan Weiner? I have not read the book,
but saw a documentary on TV about Rosemary and Peter Grant's
work a few years ago. I'm sure you are aware of their study of the
quickly changing beak sizes of Darwin's finches to reflect the changing
food supply. Does that represent evolution and natural selection
or something else?
http://www.2think.org/tbotf.shtml

 

Interesting. Darwin seems also to have felt that mutations do not occur entirely at random (although their precise effect might be near random) and that they might be (quantitively at least) an immediate response to environmental change. (Or have I read this wrong).

“Although each modification must have its proper exciting cause, and though each is subjected to law, yet we can so rarely trace the precise relation between cause and effect, that we are tempted to speak of variations as if they spontaneously arose. We may even call them accidental, but this must be only in the sense in which we say that a fragment of rock dropped from a height owes its shape to an accident.” (V ii Darwin’ Notebooks on the Transmutation of Species) 'Darwin' – Jonathon Howard – OUP 1982 p70

 

canute, I don't think anyone considers Darwin an expert on mutation. He was pretty much in the dark as to the basis for the generation of diversity ...

paulsamuel, what do you think about the CH Waddington hypothesis about selection for sets of alleles that alter biological function. The current incarnation of the idea is demonstrated in the Rutherford and Lindquist paper on hsp90:

http://www.ncbi.nlm.nih.gov/entrez/...ve&db=PubMed&list_uids=12101404&dopt=Abstract

The mechanism they suggest involves both cryptic variation and the possibility for selecting sets of alleles rather than single alleles. The basic idea is that hsp90 a chaperone (meaning it helps avoid misfolded proteins) masks certain allelic variants that bias a pathway towards a different behaviour. If you have multiple variants that encourage this behavior hsp90 can no longer mask the effect (possibly because the alleles are in a protein complex and mutually stabilize eachother). Therefore a population can harbor many mutations that alone have no effect and through sex rapidly generate the set that gives the improved phenotype.

I guess as a molecular biologist I feel uncomfortable speaking about things so abstractly that one looses site of mechanism and such ...

 

closer do i get to thee o lamarck

http://www.sqwark.com/Epigenetics.htm

http://en2.wikipedia.org/wiki/Epigenetic_inheritance

 

Most epigenetics says nothing about acquired characters being passed on. In fact most known epigenetic inheritance only involves cycling through states which clearly would not result in contributing information to speed evolution.

The fact that it would be useful if such information could be passed on does not mean it will be. In fact the nervous system is evidence of an alternative approach to passing on acquired characters. It allows you to teach your children. Indeed it is particularly powerful as acquired knowledge can be passed to others than your own progeny.

 

I'm aware of that. But I think he had a point.

 

Canute,
I'm not aware of any mechanism of mutation that could give biased mutations (other than in the immune system and generation of antibody variation). While I would not rule out the possibility that such systems could exist, random mutations should do the trick assuming a large enough population anyhow.

Therefore to me all his statement says is that he's not ignoring causality and physical law. At what height the rock happened to be dropped from has to be random to hold the analogy to mutation.

 

spookz,

epigenetics is not lamarckianism, epigenetics is about the control of genes and protein production, epigenetic factors can turn genes off, on or throttle productions levels depending on signals they receive (internal or external). Epigenetic factors are not or have not been linked to permanent genetic changes from one generation to the next. You can not acquire a epigenetic factor by demand and give it to your progeny.

 

I didn't feel that he was claiming this. Rather he was saying that the rate of mutations (or their clustering) might be affected by environmental factors (climate change or city living perhaps). Whether he also speculated whether to some very general extent the category of mutation that occured could be affected by changes in living conditions he doesn't make clear here.

I don't enough about it to form a strong view, but it seems a reasonable enough idea.

Regards
Canute

 

Fair enough. That more mutations happen under stressfull conditions (ie. when things aren't working out so well), is not that diffidult to imagine as a simple reduction in available energy (as during hunger) could clearly encourage mutation as mutation repair requires energy.

However I do not read any of the nuances you put forward into the passage above. Still you've given rise to the first mechanistic hypothesis as to why mutation may be higher at certain times and it has a natural correlation to when you'd want them. Though in multicellular organisms it isn't clear the timing would work out right (hungry mom has baby with more mutations than normal sounds like it would cause problems as much as evolution).