> The claim of "disorientation" is special pleading; there is no evidence that
> the moths were disoriented. Instead this claim is based on an
> anthropomorphic reaction, specifically the "How would you feel if you were in
> the moth's place?" reaction. Even if that were a valid basis for comparison,
> it is not scientific evidence, just a subjective emotional response.
The evidence is: they settled on tree trunks, where they do not
naturally settle! I think Majerus' comments are very important here:
".. ..it has become obvious that much of both the experimental and
theoretical work suffers from artificiality. In many experiments,
moths are placed into artificial situations, which may affect their
own behaviour or that of their predators. For example, in most
predation experiments peppered moths have been positioned on vertical
tree trunks, despite the fact that they rarely chose such surfaces to
rest upon in the wild." (page 116).
> The film itself may have been made under "unnatural" conditions, but it was
> meant to document and illustrate what Kettlewell and Tinbergen were seeing on
> a regular basis. .. ..
No one is denying that birds eat moths. This is not the point I am
arguing.
> Bats are indeed major predators of moths (probably more so than birds, but
> not by much), but because they locate and capture prey using sound rather
> than sight, their "relative importance" with regard to industrial melanism
> would be very slight. .. .. So
> while bats probably eat more moths than birds do, they would eat both color
> phases indescriminantly; with the evidence we have, birds are the most likely
> predator to cause the industrial melanism affect.
There is an element here of reaching concusions via "logical"
argument, rather than via empirical investigation. This makes the PM
even worse as a textbook example: who needs to do experiments? "Using
the tools Darwin has provided, we can reconstruct the melanism story
without going into the field and doing the hard work."
> > 3. We have almost no information about the peppered moth
> > predators in the natural state, nor which is the most important.
>
> We have no *direct* evidence, but the indirect evidence by itself is
> sufficiently convincing to the experts that, until contradictory direct
> evidence is discovered, they will use what they have, plus informed
> speculation, to establish an explanation.
See my comments above. Is this the way you want to present Darwinism
in the textbooks?
> > 4. Majerus's view is that significant bird predation levels are
> > "highly probable" - but this is based on the consensus that bird
> > predation is important in many species. Majerus points to the lack
> > of data relating to the Peppered Moth.
>
> But this lack of specific evidence in the case of the peppered moth is not
> enough to cause Majerus to doubt the overwhelming evidence establishing the
> general trend that birds are the most important predator as far as industrial
> melanism is concerned. That is what we need to keep in mind. The question
> is, what causes industrial melanism in moths? The evidence we have so far
> indicates that it is selective predation by visually-oriented predators. Of
> the three major groups of vertebrate predators of moths -- bats and other
> mammalian insectivores, birds and reptiles/amphibians -- only the latter two
> hunt predominatly by sight, so only the latter two are likely to cause
> industrial melanism. And since birds are far more numerous and far more
> active hunters than reptiles/amphibians, birds are the best candidates to be
> the predator that causes industrial melanism. As such, only if birds took a
> tiny fraction of all moths eaten by all predators would bird selective
> predation have no significant affect, and this is very much not the case;
> they rival bats as far as total intake is concerned.
I repeat: is this how it is to be presented in the textbooks? This
is not the way I want to teach science.
This is not to say you are necessarily mistaken in your comments: but
we do not have the empirical data to even test alternative
hypotheses.
> .. .. In point of fact, the
> evidence we do have paints a very strong picture in favor of selective
> predation by birds as the cause of industrial melanism.
This may be the case, but I shall not be surprised if the story turns
out to be wrong. But this is not the key point: which is whether the
PM story is sufficiently robust to be the classic example of natural
selection changing gene frequencies in a population of organisms.
Best regards,
David J. Tyler.