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Dear All,<br><br>
A major paper has come out about hybrid speciation, based on Heliconius,
and I thought you might be interested. <br><br>
Mavárez, J., Salazar, C., Bermingham, E., Salcedo, C., Jiggins, CD.,
Linares, M. Speciation by hybridization in Heliconius butterflies
Nature. 441: 868-871<br>
Available at:<br>
<a href="http://heliconius.cap.ed.ac.uk/butterfly/publications/Mavarez_Nature2006.pdf" eudora="autourl">http://heliconius.cap.ed.ac.uk/butterfly/publications/Mavarez_Nature2006.pdf</a><br><br>
Needless to say, a few critics have argued to the media that Heliconius
heurippa is not a hybrid species at all, but they haven't contacted
anyone who might know about Heliconius to test their doubts. By a
quirk of fate, I was able to hear some of these criticisms which are
normally kept relatively secret among scientific colleagues. You
can read about some of this disagreement from the National Public Radio
website:<br><br>
<a href="http://www.npr.org/templates/story/story.php?storyId=5485880" eudora="autourl">http://www.npr.org/templates/story/story.php?storyId=5485880</a><br><br>
I am a reasonably independent critic, not having been an author on the
paper, and this is what I think:<br><br>
First, the background.<br>
Emsley had included heurippa, with its red and yellow bands, within
melpomene (1964), presuming it to be a hybrid like so many other hybrids
between Amazonian yellow forewing band species and extra-Amazonian
red-banded species. Mauricio Linares has been working for many
years on this problem. His 1989 PhD thesis specifically advocates
that heurippa is a hybrid between Heliconius cydno and H.
melpomene. Larry Gilbert, his (and also my own) PhD supervisor,
also believed this, and it certainly seemed probable to me, given that I
knew of many natural hybrids between the two putative parent
species. <br><br>
I believe the evidence for heurippa being a hybrid is adequately provided
by the Nature paper, for the following reasons.<br><br>
1) Heliconius heurippa is clearly closely related to H. melpomene and H.
cydno, on molecular grounds and morphology. On average, and it is
true for mtDNA, the form seems closer in molecular genetic terms to
cydno. It overlaps with Heliconius melpomene, but the nearest
population of cydno (the subspecies Heliconius cydno cordula) is a few
hundred km to the N. Thus, cydno is genetically distinct from
melpomene, with which it coexists, but not as different, on average from
Heliconius cydno.<br><br>
2) Helionius heurippa is a colour pattern intermediate between the two
species. From (1), if it is not a separate species, it must be
cydno. But it lacks the brown hindwing underside pincer-shaped
markings of that species. In fact, it would be the only subspecies
that lacks these markings. It also has the red basal underside
spots that are typical in H. melpomene. It has the red forweing
band of melpomene, which is pushed to one side, in the mode of the
hybrids in Heliconius melpomene group, by a yellow forewing band.
The yellow band shape is typical of the adjacent cydno race Heliconius
cydno cordula, which occurs in the NE Colombian Andes into
Venezuela. We know the inheritance of these colour patterns due to
crosses performed in Mauricio Linares' lab, by Russ Naisbit et al, and by
a variety of people including Philip Sheppard, John Turner, and others
before them. I think the most convincing thing about this colour
pattern evidence is that it is the two local races Heliconius melpomene
melpomene and Heliconius cydno cordula, rather than any other races, that
could have produced this colour pattern combination. <br><br>
3) Microsatellites show that heurippa is more divergent from melpomene
and cydno than any other race of either species. Thus, when you use
the Bayesian programme STRUCTURE, three groups are identified, cydno,
melpomene, and heurippa.<br><br>
Unfortunately the microsatellite data are not provided in the Nature
paper. We are left in the dark as to whether heurippa differs from
cydno and melpomene in having intermediate gene frequencies, or whether
cydno is closer, or melpomene is closer to heurippa. Even worse, it
could be that cydno and melpomene are closer to each other than
heurippa. Intermediate microsatellite frequencies would provide
good evidence of hybrid origin for heurippa, but because microsatellites
evolve fast, it could be that heurippa diverged more strongly, so finding
that heurippa is the most divergent wouldn't necessarily reject the
hybrid argument.<br><br>
4) There are "pure" divergent haplotype groups in two nuclear
loci (invected and Distal-less) in H. melpomene and H. cydno, but
mixtures of the two groups of haplotypes in the putative hybrid,
heurippa. It is already known that H. heurippa's mtDNA is closer to
cydno than to melpomene.<br><br>
5) Mating behaviour. In Heliconius, the males usually approach
females visally, and mate after a short courtship interaction.
Butterflies are highly visual creatures, and it is perhaps not surprising
that intermediate, heurippa-like patterns in hybrids between cydno and
melpomene are preferred by heurippa males. This (together with
previously published evidence) suggests that reproductive isolation
between Heliconius is partly generated by colour pattern choice.
Probably, once the new heurippa pattern is fixed, the behaviour of males
must coevolve with the new colour pattern environment to maintain maximum
sexual success in males. In any case, heurippa is partially -
strong isolated from both the parent species by mate choice differences,
which depend strongly on colour pattern.<br>
<br>
6) Hybridization is regular in the wild between the two parent species
wherever they co-occur.<br><br>
7) Linares and Mavarez cite a population in San Cristobal Venezuela where
H. cydno cordula and H. melpomene melpomene coexist, a few hundred km N
of the heurippa site, and where colour pattern intermediates are
common. In this unusual population, containing the same subspecies
of the putative parent species, even the mtDNA is flowing between the
species (Mavarez pers. comm.). It is simple to imagine that such a
polymorphic situation might end up with fixation and male choice
adaptation to the new colour pattern.<br><br>
Now the critiques of the Mavárez et al. paper, and some comments.<br>
1) Jerry Coyne at Science Now
<a href="http://sciencenow.sciencemag.org/cgi/content/full/2006/614/2" eudora="autourl">http://sciencenow.sciencemag.org/cgi/content/full/2006/614/2</a><br><br>
"...others remain skeptical. "Maybe this is a hybrid species,
but I'm not convinced from the genetic data that it is," says
evolutionary geneticist Jerry Coyne of the University of Chicago in
Illinois, who says much more sequencing would need to be done to prove H.
heurippa is a hybrid. Coyne believes hybridization can't be common in
nature because current animal family trees would reveal such mixes."
<dl>
<dd>We don't know what his criticism was, but we get the flavour that he
doesn't believe this sort of thing is likely, and therefore tends to ask
for stronger proof. He doesn't understand some of the strong
Heliconius-specific evidence, perhaps, and maybe the Nature paper doesn't
try to head off criticisms enough.
</dl>2) Anonymous commentator to Current Biology (this and the following
are paraphrased to avoid any unnecessary aggression). <br><br>
No evidence for hybrid origin. Author's cite only two genes
supporting this [i.e. invected and Distal-less].
<dl>
<dd>Apart from the approx 20 microsatellites, it is true that only two
sequenced genes were analysed. However, as they were sampled from
from the local populations (this was not mentioned in the paper), the
hybridization hypothesis seem rather convincing in view of the haplotpe
mixing.
</dl>Color pattern similarities might well be due to evolutionary
convergence.
<dl>
<dd>This is certainly sometimes the case. However, it is an
extraordinary coincidence that this parallel evolution has led to the
duplication of patterns expected from transfer between LOCAL populations,
rather than unusual parallel evolution of genes that occur in populations
not in contact.
</dl>"Evidence" cited in an earlier paper is based on
asymmetrical incompatibilities, and is wrong. Michael Turelli
pointed this out and had the story KILLED on National Public Radio.
<dl>
<dd>This refers to the paper: Salazar, C. A. et al. Hybrid
incompatibility is consistent with a hybrid origin of Heliconius heurippa
Hewitson from its close relatives, Heliconius cydno Doubleday and
Heliconius melpomene Linnaeus. J. Evol. Biol. 18, 247–-256 (2005).
However, I still don't understand the criticism.
</dl>There are no phylogenies showing reticulation.
<dl>
<dd>Species-level phylogenies usually show a consensus of binary
branching trees, and therefore almost never show reticulation.
However, genealogies at the two above-mentioned loci do show
reticulation.
</dl>The evidence that the butterflies show wing-color related
assortative mating is not there. No statistics are given in the paper.
<dl>
<dd>I find this mystifying, and believe that the critic has completely
misunderstood the paper and the statistical analysis.
</dl>It is not clear how, according to their scenario, "drift and
frequency-dependent selection" would increase the frequency of a NEW
color pattern in a species that is ENTIRELY SYMPATRIC with one of its
supposed ancestral species. Strong selection for mimicry should
homogenize any variation.
<dl>
<dd>It seems likely that the critic is one of the authors of a well known
diatribe against the shifting balance, which involves genetic drift, and
also believes a priori that sympatric specition is unlikely. The
critic is not interested in the evidence. It seems to me that the
non-mimetic hybridization seen between the parent species at San
Cristobal, Venezuela, disproves this argument.
</dl>The main results are either wrong or buried somewhere else.."
<dl>
<dd>The critic seems not to have carefully read the paper, or checked out
the supplementary data for evidence against his unshakeable belief that
the paper is totally misguided.
</dl>Well, I hope you find this edifying! All the best, Jim<br>
<x-sigsep><p></x-sigsep>
James Mallet<br>
<a href="http://www.ucl.ac.uk/taxome/jim/" eudora="autourl">http://www.ucl.ac.uk/taxome/jim/</a><br>
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