dkomo <
[email protected]> wrote or quoted:
> Tim Tyler wrote:
> > dkomo <
[email protected]> wrote or quoted:
> > > Tim Tyler wrote:
> > > > Drift may have something to say about small populations on islands around speciation events
> > > > - but it's probably a bit player in most other places - as far as the *features* of
> > > > organisms go (as opposed to the makeup of their genes).
> > >
> > > Really? You mean the fact that people have brown, green, or blue eyes, are short or tall, fat
> > > or skinny, beautiful or ugly, have red, black or blonde hair, are prone to get different
> > > diseases in old age, have different reactions to medications, have different physical
> > > abilities like being able to run for long distances, sing very well, be really strong, and so
> > > on and so on -- these are all the result of adaptations and natural selection?
> >
> > Drift and selecion are theories of *evolutionary change*. You are just talking about variation.
> >
> > There can be as much variation in a population you like without the structure of the population
> > changing over time.
>
> That's what I'm getting at. Drift and selection are not the only alternatives. It's possible to
> have *no* selection and *no* drift (if the population is large). The allele frequencies will
> remain constant according to the Hardy-Weinberg Law.
Large populations slow down - but don't eliminate - drift.
You only get no drift in an infinite population - and then it makes no sense to talk about gene
frequencies at all.
> > Also, merely pointing out the existence of variation in a trait does not demonstrate it is
> > neutral (or near enough neutral to be subject to drift) - since:
>
> Neither does pointing out the existence of a trait demonstrate that it is an adaptation that
> occurred in the past.
Of course.
> People who believe that most or all features of organisms have been selected for are known as
> "panadaptationists."
Genetic drift is enough to indicate than not all features of organisms have necessarily been
selected for.
Attributing "most" things to selection seems quite reasonable to me, though.
> I was under the impression that panadaptationism is frowned upon these days by most evolutionists.
Adaptations are certainly all over the place.
"Panadaptationism" is - or ought to mean "adaptations everywhere" - but exactly how common that is
supposed to mean isn't clear.
The word doesn't seem to be in the dictionary - has anyone actually bothered to define it - or is it
just a term of derision?
> I think many of the traits I pointed out are either what Gould called "spandrels":
>
>
http://spandrel.com/about.html
>
> or they are neutral mutations that have persisted within human populations for a very long time.
> They are *not* adaptations to anything.
Most of your traits were statements about the existence of variation within in a trait.
You mentioned: brown, green, or blue eyes, being short or tall, being fat or skinny, being beautiful
or ugly, having red, black or blonde hair - and so on.
I've discussed eye colour already. Height is an adaptive trait - perhaps with some frequency
dependent selection for different heights being caused by the existence of different
lifestyles. Similarly with weight. Also, a fair bit of the observed variation in both traits is
not genetic at all.
Beauty is adaptive - and being ugly is mal-adaptive - and exists through being constantly generated
- and not yet having been weeded out. Hair colour is also adaptive. Black hair protects against UV
radiation - and that's why all African types have black hair (except a few albinos who soon get
ill). Red and blond hair are (I claim) clearly the result
> > * there is such a thing as frequency-dependent selection;
> > * most of the members of the population will not be long-term ancestors - and any variation they
> > exhibit is of pretty low relevance;
> >
> > The second point grows in force when you take a gene's eye view.
> >
> > Of your examples, I would identify the best one as "eye colour".
> >
> > There /may/ be a case that eye colour is the subject of drift.
>
> Why does it have to be the subject of drift? It could simply be an allele which is not under
> selection and whose frequency remains pretty constant in a particular human subpopulation.
If the population is finite, it will drift.
> > ***If*** so, eye colour would be one of the things I classified as a "minor feature".
> >
> > However I am not convinced that eye colour is anywhere remotely *near* neutral.
> >
> > existence of variation in it can be explained adaptively by the hypothesis that those with
> > unusual iris colours tend to be preferred mates.
> >
> > Further, there is geographic variation in the trait - which suggests to me adaptation as a sun-
> > shield.
> >
> > If eye colour is *not* significantly under the influence of selection, how come nature has
> > chosen bright pure pigments for the irises of the eyes of many people's in northern regions -
> > while ensuring a hefty dark melanin pigment in those whose ancestors lived closest to the
> > equator and were most exposed to the sun?
> >
> > In summary, I think there are plenty of signs that iris colouration is strongly adaptive.
>
> It could be. I'm not the diametric opposite of a panadaptationist, as one well known poster to
> this group is, but I do insist that the burden of proof lies with those who claim that particular
> traits are adaptations.
Hmm. It seems to me that whenever I get into a discussion there's always someone who claims that the
burden of proof lies on my side.
Nine times out of ten I can't see why that is the case - and this is one of them.
I suggest "adaptation" should *normally* be the default hypothesis - because adaptation has shown
itself to be the best, most powerful theory for accounting for the forms of organisms.
That's not to say that it's responsible for *everything* - but that - if anything - the burden of
proof ought to normally be on the drift side of the fence - since drift typically hasn't been shown
to be responsible for very much of anything.
> You made an argument about eye color, but you don't have any empirical proof.
...but I could soon find some on request. The dark eyes of african populations is a well known fact
- and has no explanation under the hypothesis that iris colour is subject to drift.
eye colour being adaptive, though ;-)
Frequency-dependent selection favouring rare eye colours has been demonstrated in Drosophila
melanogaster - I believe - in:
``Minority mating advantage of certain eye color mutants of Drosophila
melanogaster. IV. Female discrimination among three genotypes.''
-
http://calorierestriction.org/pmid/?n=3115250
...but this just demonstrates that the idea is plausible - not that it is responsible for the
observed human variation.
--
__________
|im |yler
http://timtyler.org/ [email protected] Remove lock to reply.
