Jim,
in article [email hidden], Perplexed in
Peoria at [email hidden] wrote on 6/23/04 9:06 AM:
Quoted message said:"Guy Hoelzer" <[email hidden]> wrote in message
"]news:[email hidden]...Quoted message said:Jim,
in article [email hidden], Perplexed
in Peoria at [email hidden] wrote on 6/22/04
1:16 PM:Quoted message said:"Guy Hoelzer" <[email hidden]> wrote in message
"]news:[email hidden]...
> in article [email hidden], Perplexed
> in Peoria at [email hidden] wrote on 6/16/04
> 10:15 PM: Whatever method is used, the result has to be
> an "r" with the following property: Randomly choose one
> of the two genes at any locus in the donor. Suppose
> that the frequency of this allele in the general
> population is "p". Now, randomly choose one of the two
> genes at the same locus in the recipient. It must be
> the case that the probability that the two randomly
> selected genes are identical is (r + (1 - r)p). That
> is, there is a probability r that they are identical
> IBD, but if not, then there is still a probability p
> that they are identical for other reasons - because the
> allele is fairly common in the population.
>
> I think there is a big practical problem with applying
> any models relying on IBD calculations, because our
> data on genealogical history is virtually always very
> shallow and incomplete.No doubt. I have no idea how field workers go about
applying the model. Though I would imagine that the
problems with estimating "b" and "c" are much larger
than the problems of estimating "r".Well, "r" can never be known and can only be estimated
with an underestimation bias. The degree of the bias
depends on the amount of unknown inbreeding in the common
ancestry of two individuals. At least "b" and "c" might
be estimable without bias. Nevertheless, my biggest
problem with the kin selection literature has to do with
the uncritical (even unconscious) acceptance of kin
selection as THE explanation for altruism-like behaviors
observed in nature. I think the difficulty associated
with estimating these parameters has a lot to do with
giving in to this elegant model.I disagree about the effect of unknown inbreeding below.
I fully agree that most altruism-like behaviors in nature
(excluding parental care, and perhaps the social insects)
are based on reciprocity rather than on the unilateral
altruism covered by the Rule.
It's good to know that our estimates of reality are close.
Quoted message said:But this conversation began because, IMO, you made some
false statements about the model - specifically that the
applicability of rb>c depends upon the frequency of the
altruistic allele. Or at least that is what you seemed to
say. I am trying to defend the model against distortion,
not to defend the importance or applicability of the
model. You would have to have that discussion with someone
who knows more "natural history". I am just an amateur OOL
person who happened to get interested in Hamilton's Rule
because it seemed to be controversial in this group. So,
ultimately, I suppose this conversation can be blamed on
Edser :-)Quoted message said:Quoted message said:> For example, all individual organisms (across all
> species) are probably genealogically related, but...
> There is also the problem of genealogical relatedness
> in the face of mutation (common decent without
> identity).If you believe that these issues are "big practical
problems", then I suspect that you don't yet understand
the model. The effect on r of
Quoted message said:Quoted message said:Quoted message said:population.
The problem is not just about pushing the estimate of "r"
back additional generations. If that were the case, then
I would agree that say 3 generations would be sufficient
in most cases. Putting aside the fact that we rarely have
information going back more than one generation, the
greater issue has to do with inbreeding. The parameter
"r" can take on any value in the face of inbreeding. For
example, full sibs actually have 0.5 >= r <= 1. In fact,
it is very common to have levels of inbreeding that make
our naïve estimates of "r" significantly lower than they
actually are. Truth be told, r=1 for every pair of
individuals (even from different species) if you were to
consider all the data (and ignore mutation/divergence).
Do you have a justifiable rule for how ignorant we ought
to be when we try to estimate "r" in order to make the
false estimate useful in understanding kin selection in
nature?It is clear that you still don't understand what "r" is.
The history of the population, and the fact that it may be
inbred in the recent or distant past is totally
irrelevant. The only way inbreeding could be relevant
would be if the population routinely breeds with close
relatives for reasons other than small population.Forget IBD for a moment. Let p be the frequency of the
altruistic allele in the general population. Let P be the
frequency of the allele in the recipients. Define r to be
that value which satisfies the equation P = r + (1-r)p
That is, r represents, in a mathematically wierd way, the
degree to which P exceeds p.
As I said before, I like this way of thinking about the
model, at least in some ways. I think you have internalized
Maynard-Smith's version of Hamilton's model. Those two great
thinkers thought about things in very different ways, IMHO.
I am not sure that Hamilton ever verified that the Maynard-
Smith version was consistent with his thinking. IMHO
Hamilton thought of "r" primarily as a measure of
genealogical relationship, which is not what it means in
your equation above. My comment above about "0.5 >= r <= 1"
was specifically referring to the genealogical "r".
Nevertheless, I will go with the Maynard-Smith version of
things for our dialogue.
Quoted message said:This value r can be calculated (within sampling error) in
the field if you take DNA samples from a random sample of
the donor-recipient pairs. Of course, you have to
normalize against the results for random pairs that are
not donor-recipient. Or, if you have been observing the
population for a few generations and you know genealogies
of individuals, you can estimate r by truncated IBD. Or
use truncated IBD if you are not a fieldworker and you
just want to understand a reason why P>p might be
expected.If you wish to understand why inbreeding is not important,
perform the following thought experiment. Imagine a
population derived from a single breeding pair which has
grown to a population of 64 with the population doubling
each generation. But, to make sure that we have variation
for altruism, make both of the original pair heterozygous.
Assume that mating is random and monogamous. "p" is 1/2.
Assume the altruism is directed to full sibs. I think that
you will find that r is not much larger than .5 and
certainly less than .6. Or, for variety, start with p =
3/4 or 1/4. "r" still will be less than .6 AFAICS
You assumed something like random mating in an exponentially
growing population, which basically the same as assuming
that inbreeding is not occurring. It is certainly not
surprising to conclude that inbreeding does not affect "r"
when you assume that inbreeding does not occur. To
illustrate why inbreeding IS important, consider a
hypothetical population in which sib-sib mating is the norm.
Such a population quickly loses its heterozygosity and
becomes constituted by families filled with altruists and
families lacking altruists. Now P>>p, and your "r" value is
correspondingly much higher. So "r" is sensitive to
inbreeding.
Quoted message said:Quoted message said:Quoted message said:And the chance that a mutation has destroyed an
altruistic allele within those few generations is also
small.In most cases that is probably true. Of course, we
haven't even touched on the most common criticism of
Hamilton's kin selection model, which is that altruistic
behavior is probably not directly caused by a genetic
mutation most of the time.I assume you mean that it is not caused by a single gene.
(If you meant something different by "mutation", you will
have to explain.)
I use the term "mutation" in a more general sense here. For
example, a chromosomal inversion might be involved, which
does not disrupt the function of any particular gene. I am
not trying to argue for the importance of non-genic
mutations. I am merely allowing for effects of a broader
class of mutations that would exhibit Mendelian segregation
as assumed under Hamilton's model.
Quoted message said:Quoted message said:It is probably far more complicated than that, and it is
not clear to what extent the Hamilton's rule would work
outside of the genetic framework. Here again is where my
real problem lies. The vast majority of the empirical
literature claiming to support Hamilton's model never
addresses the issue of genetic control over altruism.Well, it turns out that non-additive epistasis is not a
problem in this model, for the usual reasons. If you doubt
this, just make the penetrance factor "f" below depend on
both "p" and the frequency of some other gene(s). For a
derivation of the rule, I think we are justified in
treating each of these frequencies as a constant. Though
perhaps we are now adding a weak selection assumption.
However, pleiotropy turns out, surprisingly, to be more
problematic. My toy derivation of the Rule, sketched
below, doesn't handle it, for reasons sketched in my reply
to Bill Morse. I don't know whether Hamilton [1964]
handles it.
I don't think Hamilton ever challenged his model with such
mechanistic detail. I wasn't specifically implying anything
about non-additive epistasis in my comment. In fact, I was
thinking more about non-genetic effects altogether. Still,
it is nice to know that the model appears robust to non-
additive epistasis.
Quoted message said:Quoted message said:Quoted message said:>> Or, if like McGinn, you have an intuition that
>> geneological history cannot be causal in this
>> situation, ignore the IBD above. "r" is simply a
>> measure of how much more likely than "p" it is that
>> the two genes are identical for whatever reason. The
>> key thing is that the formula (r + (1-r)p) gives the
>> probability that the alleles are "shared".
>>
> Hmm. There are some things about this formulation that
> I like, and some problems I see. Can you please save me
> a little research time and tell us where you come by
> the formula (r + (1-r)p)? Is this your interpretation
> of Hamilton, or has it been published?It is a straightforward interpretation of the verbal
explanation given in Maynard Smith's "Evolutionary
Genetics" (2nd ed. p169)Now we can picture the genome of the recipient as
consisting of two parts:
1. a fraction r containing genes IBD to genes in the
actor; and
2. a fraction (1-r) consisting of genes that are a
random sample of genes in the population.This is a very familiar modeling trick. The same thing is
done when modeling inbreeding for other purposes. It is,
however, just a trick that makes the math work out
easily. The flaw becomes clear when you recognize that
any random sample of the gene pool will potentially
contain gene copies that are IBD with the target, so the
fractions are not mutually exclusive. Given your
definition of "r", "(1-r)" must be the fraction of the
recipient's genome containing genes that are NOT IBD,
which is different from "a random sample of genes in the
population."I think that my argument above including the phrase
"forget about IBD for a moment" addresses this concern.
I agree that this second way in which you defined your
parameters is more logical. However, that does not validate
the way you defined them at first, which I still argue had a
logical flaw. In fact, I think these two definition sets are
inconsistent with one another, so you should decide which
one you want to use and stick with it.
Quoted message said:Quoted message said:Quoted message said:As a "proof" that this is the correct interpretation,
you can derive "rb>c" from this formula.
1. Assume b and c are the benefits and costs of a single
altruistic action.
2. Assume a "penetrance factor" f (which may depend on
r!) gives the number of times an allele causes
altruism during its organism's lifetime. Assume that
homozygous altruists share the cost between alleles.
The factor f allows us to ignore whether the allele
is dominant or recessive. For example, if the allele
is recessive, then f will be small when p is small.
But if the allele is dominant, then f will start
large and then fall to half the original value, due
to "sharing the credit" between homozygous gene
instances.
3. Calculate the total benefit received by recipients.
Divide this up between carriers and non-carriers,
with the benefit split in heterozygotes. Now
calculate the "per capita" benefit to each haploid
genome of the two types.
4. Calculate the total and per capita costs of altruism.
5. Compare the per capita benefit for non-carriers to
the per capita (benefit - cost) for carriers. You
will notice that the variable factor f cancels out,
as does the allele frequency
p. You are left with the fact that carriers receive
enough extra altruism to compensate for the costs
of acting altruistically when rb>c. Try it. As I
wrote:This is very clever. Given my logical analysis above,
your "proof" would seem to reveal cryptic ambiguities in
Hamilton's original thesis. BTW, have you agreed in the
past that all of this goes out the window for
deterministic reasons when there is only one copy of the
allele around, because then altruism only costs the
allele fitness points?I understand what you are saying. Clearly, the only
altruist in the population cannot also be a recipient,
and hence can't be more fit than the rest of the
population. (I could point out that you are assuming that
the allele is dominant, but that is not my real objection
to your point.)Frankly, I consider this a minor quibble - not much more
forceful than if you had pointed out that a rabbit's speed
is simply a drain on its metabolism if it never encounters
a fox. But, I admit that what you say is true. I'm not
going to try to convince you of the validity of my earlier
response to your point.
So I guess that you concede that the validity of Hamilton's
rule depends "p", at least at this singular point. I suspect
you would even concede that the effect of "p" would be
observed at very low values of "p" when there is more than
one copy of the altruism allele around. So, we only disagree
about how far this effect will reach as you increase the
value of "p", which I have been arguing depends strongly on
population structure (and I would add population size).
Quoted message said:Quoted message said:Quoted message said:>> Why is that particular formula so important? Well,
>> when you do the math, you will see that the average
>> fitness of allele carriers will be greater than non-
>> carriers, as long as the carriers direct their
>> altruism to recipients of relatedness "r". That is,
>> average fitness of carriers will be higher as long as
>> rb>c. And the parameter "p" nicely cancels out of the
>> equations. Hamilton's rule is independent of p. As
>> long as "r" has the meaning above.
>>
> My criticism here is the same as one that I have been
> posting earlier in this thread. Your conclusion is
> sensitive to the implicit assumption that the
> population is large (effectively infinite) and well
> mixed. This combination does not generally exist in
> nature, because the larger a system is the harder it is
> to mix up.Not my conclusion. Hamilton's. But I think you exagerate
the sensitivity.Like you, I would say "just try it." If you do a simple-
minded model of agents interacting in space I think you
will quickly see the sensitivity.Quoted message said:I think that all that is required is that the "well-
mixed" or effectively random mating breeding population
is larger than the local socially-interacting clique of
each individual.Interesting. I would like to see the argument to this
conclusion; or is this your "gut feeling" at this point.
[BTW, I do not intent to impune "gut feelings." I admire
people that are in touch with their guts.]Assume that the population is divided into "states",
within which mating is random (well mixed). Assume that
states are subdivided into "zip-codes", within which
organisms interact. A state can have one zip-code, or
several. Let "p" vary between states, but be constant
within a state. Perform the derivation of the Rule
separately for each state. Since p is constant within a
state, the derivation still works in each case. Note that
this would no longer be valid if a single zip-code covered
two different states with different "p" values.
I'm beginning to lose the thread of our argument, but I
think the assumption of constant "p" values within
subpopulations subverts the problem I was pointing to.
Indeed, that assumption is inconsistent with Hamilton's
model, which is about predicting changes in "p". If you
loosen up your model a little to allow for dynamic "p"
values, then each of your subpopulations can evolve
deterministically in different directions under their own
versions of Hamilton's rule. What would this mean for the
evolution of the whole system? I don't think that Hamilton's
rule would be a very precise guide at the global scale.
Quoted message said:Quoted message said:Quoted message said:And that "r" is pretty much the same everywhere.
As I have indicated before, I think this is a very
unrealistic assumption if you mean that "r" is the same
everywhere for any particular individual. If you mean
that the distribution of "r" is the same in the
neighborhood of every individual, then this may be a
reasonable approximation for some systems.The altruistic gene "directs the altruism" toward kin or
neighbors. I am assuming that the gene produces the same
biased effects everywhere, and, as you say, that
neighborhoods are similar in their kinship structure
everywhere.Quoted message said:Quoted message said:It is not important whether "p" is pretty much the same
everywhere.What if I moved the pieces on the board of life so that
each copy of the altruism allele is surrounded by a sea
of purely selfish individuals? Note that this can only be
done when "p" is small.But this means you have just set "r" to zero (or
actually, to a small negative number)! Everywhere. So the
Rule still works.
Sorry. I forgot about your definition of "r". I was still
using it in the genealogical sense of KIN selection.
Quoted message said:Quoted message said:If "p" is large, then I could still change the balance of
Hamilton's rule when "rb" is only slightly larger than
"c" by isolating many copies of the altruism allele in
local seas of selfishness.Your thought experiments strike me as artificial, and
hence not particularly relevant. Maybe you are trying to
tease out hidden assumptions. I think that the assumption
that you are attacking here is an explicit one - that "r"
is constant. In this case, you are still changing "r".
So your "r" is critically dependent upon population
structure. Right? Didn't you once argue that Hamilton's rule
did not depend upon population structure?
Quoted message said:Quoted message said:Quoted message said:> It also gets harder and harder to effectively mix the
> finite population when "p" is either very small or very
> large, because in either case there are very few quanta
> of the rare allele.True enough, but now you are criticizing how
deterministic Hamilton's rule is, rather than claiming
that it is incorrect (biased) at extreme frequencies.Note my arguments above about bias when "p" is small
and/or unevenly distributed.Noted. I am discounting your small p argument, and
accepting your uneven distribution argument only if you
have interactors that, for some reason, don't interbreed.
Perhaps modern human India, Bosnia, or Northern Ireland
fit your requirements. Hmmm. Now that I think about it,
perhaps my zip-code analysis is extraneous.
What do you think of my "flipside" argument above regarding
systems constrained to sib-sib mating?
Regards,
Guy