Re: Heliconius butterfly mimicry - Exquisite adaptation?

Perplexed in Peoria wrote:
On another thread, Bill Morse made that argument that mimicry
in butterflies is one example of near-perfect adaptation in
at least one aspect of organism phenotype. As evidence, he
offered plate 8k of Sean Carroll's 'Endless Forms Most Wonderful'
(opposite p211). For those who don't have the book, this
website offers a similar set of pictures:
http://www.mun.ca/biology/scarr/Mimicry_in_Heliconius.gif

Bill's argument seems to be as follows.
1. We have two distinct species of butterfly (H. melpomene vs
H. erato) which look quite similar. One is mimicking the other
very well.
2. Moreover, the mimicry is carried out in spite of significant
geographical variation in the model species. The mimic species
tracks this geographical variation almost perfectly.
3. Therefore, the mimic is exhibiting near perfect adaptation
and is exhibiting it differently in different geographic locales.

I don't think that the evidence bears this argument out, and
I certainly don't think that Carroll had this argument in mind
when he included plate 8k in his book.

The key fact is that H. melpomene and H. erato are closely related
species, and share the same development pathways for wing patterns.
In fact, they share the same variants of those pathways, and the
same mutations in one species will have the same effect on the
coloration patterns in the other species.

Therefore, we don't necessarily have exquisite adaptation. Instead
we have a small number of developmental variants that are parallel
in the two species.

Not just in those species: there's even a moth in some of the same
mimicry rings:
<http://www.ucl.ac.uk/taxome/jim/Mim/moth.html>
(Jim's pages are worth wandering around)

If one species is mimicking the other, it doesn't have to work very
hard at it. There are only a couple dozen ways of coloring a
Heliconius butterfly, involving differences in a handful of genes,
so the mimic only has to pick the same version of these genes as
did the model.

It is not completely clear that this IS mimicry. It could be that
both species are selecting the coloration pattern that best matches
the local foliage. Not chosing the ideal coloration for that
foliage, but merely the best of the couple dozen patterns available
given the development pathways of the Heliconius genus.

So why are they brightly coloured? Camoflaged species are generally
darker (e.g. Biston). And why is there polymorphism, e.g.

"The pinnacle of Müllerian mimetic polymorphism is found in Heliconius
numata. This species is polymorphic throughout virtually its whole
range, and some populations of the Amazon basin near the slopes of the
Eastern Andes may have up to seven different morphs, each an accurate
mimic of a separate species of Melinaea or Mechaniti (Ithomiinae)."

http://www.ucl.ac.uk/taxome/jim/pap/MalletJoronARES99.pdf
(Mallet & Joron, 1999, Annu. Rev. Ecol. Syst. 30:201?33)

Bob

--
Bob O'Hara
Department of Mathematics and Statistics
P.O. Box 68 (Gustaf Hällströmin katu 2b)
FIN-00014 University of Helsinki
Finland

Telephone: +358-9-191 51479
Mobile: +358 50 599 0540
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Journal of Negative Results - EEB: www.jnr-eeb.org

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