[HELICONIUS] Heliconius hybrids paper online

[Jim Mallet]

Dear Neal,

An interesting paper on Heliconius hybrids is now finally "published".  I 
get the feeling you only search this journal sporadically, but thought you 
might like to know.  By the way, if you were going to choose it for 
"Science Sendings", it'd be great if you didn't circulate the PDF (which is 
anyway not in final nicely formatted form), but instead just circulated 
this website; tbhe paper will always be freely available online.  I think 
if lots of your users clicked on the link it might get it into the highly 
cited category.  Anyway, I'd be keen to see.

See:

Natural hybridization in heliconiine butterflies: the species boundary as a 
continuum
http://www.biomedcentral.com/1471-2148/7/28/
James Mallet, Margarita Beltran, Walter Neukirchen, Mauricio Linares
BMC Evolutionary Biology 2007, 7:28 (23Feb2007)


Background

To understand speciation and the maintenance of taxa as separate entities, 
we need information about natural hybridization and gene flow among species.

Results

Interspecific hybrids occur regularly in Heliconius and Eueides 
(Lepidoptera: Nymphalidae) in the wild: 26-29% of the species of 
Heliconiina are involved, depending on species concept employed. 
Hybridization is, however, rare on a per-individual basis. For one 
well-studied case of species hybridizing in parapatric contact (Heliconius 
erato and H. himera), phenotypically detectable hybrids form around 10% of 
the population, but for species in sympatry hybrids usually form less than 
0.05% of individuals. There is a roughly exponential decline with genetic 
distance in the numbers of natural hybrids in collections, both between and 
within species, suggesting a simple "exponential failure law" of 
compatibility as found in some prokaryotes.

Conclusions

Hybridization between species of Heliconius appears to be a natural 
phenomenon; there is no evidence that it has been enhanced by recent human 
habitat disturbance. In some well-studied cases, backcrossing occurs in the 
field and fertile backcrosses have been verified in insectaries, which 
indicates that introgression is likely, and recent molecular work shows 
that alleles at some but not all loci are exchanged between pairs of 
sympatric, hybridizing species. Molecular clock dating suggests that gene 
exchange may continue for more than 3 million years after speciation. In 
addition, one species, H. heurippa, appears to have formed as a result of 
hybrid speciation. Introgression may often contribute to adaptive evolution 
as well as sometimes to speciation itself, via hybrid speciation. 
Geographic races and species that coexist in sympatry therefore form part 
of a continuum in terms of hybridization rates or probability of gene flow. 
This finding concurs with the view that processes leading to speciation are 
continuous, rather than sudden, and that they are the same as those 
operating within species, rather than requiring special punctuated effects 
or complete allopatry. Although not qualitatively distinct from geographic 
races, nor "real" in terms of phylogenetic species concepts or the 
biological species concept, hybridizing species of Heliconius are stably 
distinct in sympatry, and remain useful groups for predicting 
morphological, ecological, behavioural and genetic characteristics.




All the best, Jim


James Mallet
http://www.ucl.ac.uk/taxome/jim/

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