Re: A critique of the BBC aquatic ape programme and the transcript.

On 24 May 2005 09:28:12 -0700, "JAE" <jae@xxxxxxxxxxx> wrote:

[...a long reply to a short and simple question; quite a lot snipped]

>> >[Jason] Evolution and natural selection are not
>> >synonyms.

>> [Pauline]Taking these snips out of context, you are undoubtedly right. But in
>> the context of a discussion about locomotion, what else could it be
>> but natural selection? Random genetic drift? Hardly. Sexual selection?
>> I don't think so. What else but natural selection?
>
>Consider this: Snow is a regular substrate that some animals have to
>move through. What creature moves better through the snow, humans or
>chimps? If it's humans, is it because we've had selection to move
>through the snow? Really?

Snow is an eccentric choice of substrate in the context of humans,
chimps and their relative evolutionary history. Chimps in the wild are
most unlikely ever to encounter it, early hominids ditto and even
modern humans do so only with special footwear. Impossible to make a
judgment on which moves better through it.

> Considering that the human form has
>remained more or less unchanged from a form that evolved in a place
>where the creatures originated [snip]

Whoa! Has it? Interesting throw-away line there.

>If you begin with the assumption that the trait was a direct result of
>postive selection for the trait as expressed, you will reach no other
>conclusion. You've also not demonstrated anything. You've merely
>restated your premise.

Well, firstly neither Algis nor I have talked about *positive*
selection. Clearly if selection is operating, it can be either for or
against a particular trait.

But I would have thought that for any visible, major trait (such as
locomotion), the default assumption is that it is the result of
natural selection; only if that seems to fail would one go on to look
for some other explanation. Is that not so? Is that not the null
hypothesis?

>In context: The argument that Algis is making
>is that selection is responsible for differencesis that we swim better
>than chimps, therefore selection was responsible for this difference.

That's not how I read what he said, rather he proposes that there *is*
a difference in swimming abilities, and he assumes that natural
selection is responsible for that difference.
>
>But there *are* other things that can happen. Is/are the trait(s) (and
>yes, even a locomotive one) a secondary effect of selection elsewhere?
>[Snip] Is
>the difference when compared to another taxon a result of drift? Think
>here not about the difference being one *favoring* a taxon, but one
>where a trait was lost in the other taxon. (e.g. Is our swimming
>ability really much different than the basic mammalian condition and is
>it perhaps the chimps who have experienced diminished abilities
>because, when not needed a trait *can* disappear, though there is no
>expectation that over any given period of time it must?)

Again, a *major* trait like locomotion is (in my view) highly unlikely
to be the result of drift or a secondary effect of selection
elsewhere. Some traits may arise in other ways, sure, but some traits
(reproduction is another example) are so central to a species that
they surely *must* be under selection (positive or negative).

I take your point that chimps may have *lost* their ability in water,
rather than humans gaining it (and I actually think that is more
probable), but nevertheless if they no longer needed to swim or wade
regularly, selective pressure on their remaining forms of locomotion
(terrestrial walking/running, climbing, brachiation) would act very
effectively to reduce their ability in water (where it conflicted).

>[Snip] Are we
>particularly adept swimmers compared to the host of creatures who show
>aquatic adaptations? Are there universal traits that aquatic creatures
>have that we do not? Why? Are the similarities really analogs (e.g.
>is our hairlessness comparable to the hairlessness of a dolphin despite
>the fact that we retain copious amounts of hair over our bodies) or is
>it merely a superficial similarity? Is the suite of traits consistent
>with the level of necessity proposed?

These are interesting questions, which are too broad to address in one
go. I would only say that there are many different ways of approaching
them. For instance, it is often said that human loss of fur cannot be
an aquatic adaptation because many (possibly most) semi-aquatic mammls
have retained their fur. I think this is an invalid comparison. To
properly investigate human loss of fur it is necessary to look at
mammals which have lost their fur; then you can categorise them and
(perhaps) draw up a list of possible reasons. It makes no sense to
start with aquatic or semi-aquatic mammals and say, look, most of them
have fur, because this is also true of terrestrial and burrowing
mammals, a few of whom have also lost their fur. But in almost every
category you can define most mammals are fully furred and only a few
species have lost fur (the exception being the fully aquatic species,
which are universally furless).
>
>Beneath any assumption of adaptation must be some way in which the
>adaptation was or is advantageous. Simply saying that if we swim
>better, we'd be less likely to drown is again overly simplistic since
>it provides no compelling reason why we'd swim.

Yes. But it isn't necessary to calculate the advantage to the nth
degree. *If* human ancestors were regularly exposed to water (to cross
it or get food from it) then selective pressure would inevitably act
to weed out the less adept at wading or swimming or finding food in
water (some might drown, but more would simply do less well). We know
that anatomically modern humans migrated round the coasts and ate fish
and shellfish and had boats, so they, at least, were regularly exposed
to water. For earlier ancestors, it is more speculative, but we know
they generally lived close to water and that their environment was
generally wetter than today, so it's possible that they too were
regularly exposed to water, once they were no longer particularly
arboreal. Nothing more is needed.

You can argue about whether the amount of exposure to water is
sufficient for this or that change, or whether there were
counter-pressures, but barring special circumstances, the general
principle is that regularly doing X leads to improved ability to do X
in the species. And that, surely, is all that Algis is arguing: that
the improved ability of humans in water relative to to chimps is most
probably the result of being in water more often. Or, of course, that
chimps were in water *less*.

> If it were so simple,
>why isn't being an adept swimmer something than every taxa would have
>achieved (and of course, why is it that humans, without training, are
>so damn likely to drown)?

Humans drown for many reasons, and lack of adeptness is only one of
them. Getting caught out by tides and currents is common, so are boat
accidents out at sea, and many children drown because they have not
been given the opportunity to acquire adeptness. Humans are not
unusual in having to learn to swim, just as they have to learn to
walk.

>Hell, even safer is to stay on dry land like
>the bulk of primates do.

Indeed. Why then do you suppose humans persist in taking to the water?

>What compelled a lineage to get in the water
>(often enough to develop specific adaptations)? Simply saying that
>(generic) 'wetlands' are 'food rich' isn't even close to sufficient to
>establish premise. Wetlands are about as variable as any environment
>and the presence of things that might be food is not the same as
>actually turning the plants/animals/fungi into calories and nutrients.

The 'food-rich wetlands' have been discussed many times before, but
without a lot of data. Richard Parker posted some information on this
recently over on the AAT group:

Type of Ecosystem Productivity (@1000 kcal/m2/yr)
----------------------------------------------------------------
desert <0.5

deep ocean <1

grasslands 0.5 to 3.0
deep lakes
mountain forests
some agriculture
continental shelf


moist forests 3 to 10
shallow lakes
moist grasslands
most agriculture


some estuaries 10 to 25
springs
coral reefs
bayous

http://environmentalet.org/env1100/ecosystems.htm

These are fairly broad categories, but the general principle seems to
be: the wetter and shallower, the richer. So perhaps 'food-rich
wetlands' is not so far out.

[Snip more good if unanswerable questions]

--
Pauline Ross
.

Relevant Pages

  • Re: Is the AAH a legitimate hypothesis? Of course it is.
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  • Re: Updated AAH Definition
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  • Re: Is the AAH a legitimate hypothesis? Of course it is.
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