Re: Heritability of fitness
- From: William Morse <wdmorse@xxxxxxxxxxxx>
- Date: Sun, 1 Jan 2006 01:19:22 -0500 (EST)
"Anon." <bob.ohara@xxxxxxxxxxxxxxxxx> wrote in
news:dp6luk$1q82$1@xxxxxxxxxxxxxxxxxxx:
> Perplexed in Peoria wrote:
>> In the course of preparing my next posting on my "Breadbox" thread,
>> I came to the conclusion that there should be a relationship between
>> the rate at which information 'about the environment' is accumulated
>> in the genome and the heritability of total fitness. Since the
>> heritability of fitness strikes me as one of the cornerstones of
>> Darwin's theory, I assumed that there must be a host of measurements
>> of this important parameter in a variety of wild and laboratory
>> populations. So I went to the web to see if I could find them.
>>
>> What I found instead is that it is generally known and/or believed
>> that the heritability of fitness in the wild is too small to measure.
>> Some empirical studies report that total fitness heritability was
>> undetectable "as expected". Apparently I don't understand
>> evolutionary theory as well as I thought I did. The following
>> abstract seems to summarize the conventional thinking (and also
>> points out that heritability is probably not EXACTLY zero):
>>
>> Mayo, O., Bürger, R., Leach, C.R.:
>> The heritability of fitness: some single gene models.
>> Theoret. Appl. Genetics 79, 278-284 (1990).
>>
>> Summary Because directional selection exhausts additive-genetic
>> variance, it is frequently claimed that the heritability of fitness
>> should be very close to zero. However, mutation-selection balance
>> generates a certain amount of additive-genetic variance, so that
>> even parent-offspring measures of heritability may be greater than
>> zero at equilibrium. Intra-generation heritability may also be
>> non-zero, providing the potentials for genetic change following
>> environmental change.
>>
>> So my current understanding of this is that the heritability of
>> fitness in the wild is near zero at most times and places for most
>> species. But occasionally, due to environmental change or the freak
>> appearance of a rare beneficial mutation, the heritability of fitness
>> in a population rises to a level high enough to actually DO
>> something.
>>
>> Which leads me to wonder - why was there ANY opposition in the pop
>> gen community to Gould's and Lewontin's ideas regarding punctuated
>> equilibrium?
>>
> Did they notice it? Punk eek was Gould and Eldridge, and was
> paleontology, not population genetics. The time scales are so
> different, it's difficult to link the two.
>
>> Oh, by the way, please ignore my speculations on the "breadbox"
>> thread. It appears I am going to have to do some rethinking.
>>
>> One other comment - I would have thought that there must be enough
>> natural environmental change out in the wild, and enough variation
>> capable of exploiting that change, that fitness heritability would be
>> large enough to measure - at least over timescales in which the new
>> environment is constant. But apparently the 'signal-to-noise' ratio
>> is just too high.
>>
> The problems is that the changes in the environment will lead to
> changes in the selective regime, so you might see variation in what
> fitness actually is. See this paper, for example:
>
> Merilä, J., L.E. Kruuk & B.C. Sheldon (2001) Cryptic evolution in a
> wild bird population Nature 412: 76-79.
>
> Personally, I'm a big fan of the idea that environmental fluctuations
> are a major force, ever since my field trials failed one year because
> of a lack of rainfall....
>
> Bob
>
As you know, I am also a fan of environmental fluctuations as a
determinant. Yes, the models are easier if they assume constant
conditions, and in some environments that may be appropriate, but if you
look at the timescales typically involved in evolution rather than the
timescales involved in human perception of history (about two generations
ahead and back), the only constant is change. On the time scale of
evolution even the geology is not constant.
To get back to Jim's thought about information accumulation and fitness,
perhaps part of the problem is the difference between relative and
"absolute" (whatever that means) fitness. I can understand why the
heritability of relative fitness - which is the only thing that can be
measured - is close to zero. And as you point out, environmental change
doesn't help this because it changes the criteria of what is fit.
"Absolute" fitness - fitness over a very long term - is I would think
heritable. Take flight, for instance. The ability to fly is key to the
fitness of most birds, and is heritable. But because it is key there
isn't any significant difference in the flight abilities of conspecifics,
so there is no relative fitness difference to measure the heritability
of. However the genes that code for flight do accumulate information
about the environment - all those things like air density, wind shear,
adiabatic lapse rate, oxygen content, etc. - that influence the dynamics
of flight. But I'm not sure I see that the _rate_ of information
accumulation should be related to fitness.
Yours,
Bill Morse
.
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