[Fwd: Rapid Movement of a butterfly (Heliconius) hybrid zone ineastern Panama. EVOLUTION 56(10): 2002]

Michael Gochfeld gochfeld at eohsi.rutgers.edu
Fri Nov 1 20:04:25 EST 2002

I thought this paper might be of general interest. I can't forward the pdf file, but the abstract is interesting enough, along with Neil Smith's historical commentary.


Neal Smith wrote:

> Well this one is timely as but two days ago I inquired about hybrid zones in Panamanian butterflies of one of our butterfly folks and this and the Anartia hybrid  zone were discussed. My informant expressed slight puzzlement that the zones seemed to be going in different directions. That is Anartia vs Heliconius.
>  I won't be able to read this until tonight so I'll hold off saying anything except that 45 km in 17 years does not seem to me to be rapid.
> Anecdote
> John Turner(UK) and I collected both Anartia and these Heliconius in eastern Panama province before the Bayano Dam was constructed and before the massive destruction of the lands between Panama City and Darien. That was in 1971. Individuals of both genera showing introgression were collected much closer to Panama City than can inferred from data spanning but 17 years.
> And perhaps the best of all the butterfly people who has ever worked with Panamanian butterflies, the late Gordon Small, a math teacher in the old Canal Zone pointed out to me the seasonal (DRY)occurrence of H. erato hydra in Panama City!!!!
> No matter, this looks like a good paper and it has a color plate- just the wings but good enough!!!!!!!!
> Evolution: Vol. 56, No. 10, pp. 1992-1998.
> Michael J. Blum,
> Duke University, Department of Biology, P.O. Box 90338, Durham, North Carolina 27708
> Smithsonian Tropical Research Institute, Naos Island Molecular Laboratories, Unit 0948, APO- AA 34002-0948 E-mail: mjblum at duke.edu
> It has been proposed that a moving hybrid zone can be a mechanism for the spread of adaptive traits in phase III of Wright's shifting balance model of evolution. Here I present an example of a moving hybrid zone in warningly colored Heliconius butterflies, a system which is considered to be a possible case of shifting balance evolution. Having moved approximately 47 km in 17 years, the hybrid zone shift has led to the H. erato hydara color pattern rapidly displacing the adjacent H. erato petiverana pattern. The movement is potentially due to dominance drive augmenting a slight selective advantage of H. erato hydara over H. erato petiverana, which is largely consistent with theoretical conditions favoring the success of phase III.
> Keywords: Dominance drive, Heliconius, moving hybrid zone, shifting balance.
> Manuscript Received by the Society September 20, 2001
> Manuscript Accepted July 16, 2002
> Section Editor: J. Mallet!!!!!!
> Understanding of hybrid zone genetics has substantially increased over the last decade, but little attention has been paid to one of the most interesting features of hybrid zones-mobility. Zone movement is theoretically possible, but is difficult to document because it requires long-term study of hybrid zone positions (McDonnell et al. 1978 ; Moore and Buchanan 1985 ; Kohlman and Shaw 1991 ; Urbanelli et al. 1997 ; Hafner et al. 1998 ; Britch et al. 2001 ; Dasmahapatra et al. 2002 ). The relatively few empirical studies documenting positional shifts partly underlies a common assumption that most clines and hybrid zones have reached a migration-selection equilibrium and have been stable for long periods of time (Barton and Hewitt 1985 , 1989 ). Yet such assumptions of long-term stability remain largely untested. Theory suggests that disruption of equilibrium conditions, such as environmental change or genetic asymmetries, may force a hybrid zone to move (Barton 1979 ; Mallet 1986a ;
> Mallet and Barton 1989a ). Hybrid zones acting under exogenous selection are expected to track any underlying ecological changes because differential selection for or against particular genotypes across an ecotone will force clines to settle on the boundary between environments. When under endogenous selection (such as selection against heterozygotes, epistatic selection, or frequency-dependent selection against rare genotypes), the position of the zone becomes arbitrary and may shift in response to any asymmetrical change in migration or selection. Identifying the factors underlying positional shifts may offer key insights into the maintenance and fate of hybrid zones.
> Shifting clines and hybrid zones are potentially important forces of evolutionary change as a form of phase III of Sewall Wright's controversial shifting balance hypothesis (Wright 1932 ; Mallet 1986a , 1993 ; Rouhani and Barton 1987 ). Wright (1932) envisioned evolution as a three-step process. After an initial phase of genetic drift (phase I), alternate states of an adaptive trait become prevalent in separate populations (phase II). The third phase occurs when the global adaptive peak of the fitness-enhancing trait spreads from population to population via interdemic selection. Wright (1932) described phase III as the spread of a fitness-enhancing trait via interdemic selection, and other work showed that cline motion can also result in the spread of such a trait (Rouhani and Barton 1987 ). A moving hybrid zone represents a mechanism for the spread of an adaptive or fitness-enhancing trait because two adaptive peaks may meet and form a cline stabilized by a migration-selection
> balance, which then shifts in favor of the fitter peak (Rouhani and Barton 1987 ). Because there is little evidence for phase III and for all three phases acting in the same system, the shifting balance model of evolution has been largely discounted (Haldane 1959 ; Coyne et al. 1997 , 2000 ). Evidence for cline mobility, especially in a model system that has been widely considered a potential example of the shifting balance process, may encourage more work to clarify the importance of Wright's hypothesis (Wade and Goodnight 1998 , 2000 ).
> In the present study, I demonstrate the existence of cline movement in a system in which the shifting balance has been proposed to be important, warningly colored Heliconius butterflies. Mullerian mimicry in Heliconius butterflies is considered one of the best examples of natural selection maintaining alternative stable equilibria (Turner and Mallet 1996 ). Many of these butterflies have evolved similar aposematic wing color patterns under frequency-dependent predation on individuals expressing rare patterns (Mallet and Barton 1989a ,b ). Evidence for strong selection on wing color pattern helps explain the homogeneous distribution of patterns within species and between Mullerian mimics (Mallet and Barton 1989b ; Kapan 2001 ). However, many Heliconius species are subdivided into wing color pattern races, which participate in multiple mimicry rings. The most thoroughly studied example is H. erato and H. melpomene, which form a mosaic of parapatrically distributed, parallel races that
> are separated by coincident hybrid zones. Mallet (1986a , 1993) as well as Mallet and Joron (1999) have argued that the races formed through a shifting balance process. This argument has been criticized partly because no study has demonstrated the spread of one wing color pattern at the expense of a competitor (Turner and Mallet 1996 ; Coyne et al. 1997 ). By documenting that color pattern cline motion involves the replacement of one pattern over another, this study tacitly supports the shifting balance model of Heliconius evolution (Mallet 1986a , 1993 ). Although it is unclear whether the documented cline motion represents an example of phase III acting in nature, this study nonetheless suggests that wing color patterns are labile traits that may rapidly respond to changes in adaptive conditions.
>   ------------------------------------------------------------------------
>                                        Name: Heliconiushybirdzonemovement.pdf
>    Heliconiushybirdzonemovement.pdf    Type: Portable Document Format (application/pdf)
>                                    Encoding: base64


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