Phylogenetics of mimicry
||This article's introduction may be too long for the overall article length. (February 2010)|
Phylogenetics of mimicry
Mimicry is well understood and heavily studied within specific mimicry groups, individually referred to as mimicry complexes. However the evolutionary and phylogenetic relationships between mimic-model or co-mimicry pairs are less apparent. The difficulty many researchers face in trying to build phylogenies for mimicry complexes is trying to discriminate between analogous traits, shared characteristics developed through convergent evolution, and homologous traits, shared characteristics that are due to a shared common ancestor. In some instances it is clear whether some traits are analogous or homologous, as in mimicry complexes involving completely unrelated organisms or those of different orders. In other cases involving similar or same species mimics with different phenotypes, the explanation for trait evolution becomes less clear.
To build phylogenies for these groups of mimics, scientists would first need to understand which species is the mimic and which is the model, then afterwards determine how evolution will have proceeded to increase the instances of the shared characteristics.
In Müllerian mimicry, defended species have evolved similar appearances as a means to share the cost of predator learning (Müller, 1862). The classic example of Müllerian mimicry is the Heliconius butterfly. There are 54 species of this unpalatable butterfly with over 700 names applied to its various phenotypes (Brower, 1996). It functions as a perfect Müllerian mimic because all species of the Heliconius are inedible and form symbiotic relationships. Extensive research on Heliconius butterflies has even shown not just phenotypic similarities, but also behavioral commonalities within an overlapping territory.
It is commonly accepted that the driving force for mimicry is predator behavior, as can be shown in various computer and mathematical models  and comparative evidence. However, what is lacking is adequate experimentation to specifically study the details of how predatory pressures select for a common phenotype. Mimicry rings, the groups of mimetic species that evolve as a result of an ‘initial condition’ phenomenon, also often overlap and the maintenance of their heterogeneity is also not well understood. The co-existence of multiple mimicry rings may contain individuals who do not completely overlap, which may result in lack of pressure to converge. Therefore, if different mimicry rings contain forms distinct enough, intermediate forms would be disadvantaged and forced to converge and match the phenotypes within the mimicry ring.
In Batesian mimicry, one defended organism is known as the model and the non-defended organism is the mimic (Bates, 1862). Batesian mimicry is often perceived as a parasitic relationship because the mimic benefits from the presence of the model and predatory learning as a result of experience with the model, however the model gains no benefit from the mimic. A classic example of Batesian mimicry is the monarch and viceroy butterfly model-mimic pair. Interestingly, monarchs within their own species also form instances of Batesian mimicry as each butterfly varies in toxicity depending on its diet as a caterpillar, so those with a weaker defense depend on individuals with a stronger one. Polymorphisms are also more prevalent in Batesian mimicry than in Müllerian mimicry, with several important studies focusing on the female Papilio dardanus which mimics several species of defended danaid butterfly.
What is not well understood is how these species evolved through an intermediate stage and what phenotypes they might have displayed prior to successfully mimicking their models. After breeding Papilio dardanus individuals with differing phenotypes, results showed that the cross-breeding yielded variable, non-mimetic offspring. Experiments by Nijout (1991) and Turner (2000) have also suggested the possibility that the main pattern gene is contained within a supergene, a locus containing several tightly linked genes that control wing pattern phenotypes. Their experiments also showed the expression of the genes controlling for wing pattern depend on yet other genes, called modifiers, which co-evolved within a very specific population Kunte et al. (2014) found that the Batesian mimicry exhibited by the butterfly species Papilio polytes is controlled by the doublesex gene.
Understanding the function of the modifiers and the possible role of supergenes, however, does not make drawing up the mimicry ring’s phylogeny any easier, nor do scientists yet know how mimicry is achieved.
- Ruxton, G. D., T. N. Sherrat, and M. P. Steed. 2004. Avoiding attack: the evolutionary ecology of crypsis, warning signals, & mimicry. Oxford Biology.
- Mallet, J. and L. E. Gilbert, Jr. 1995. Why are there so many mimicry rings? Correlations between habitat, behaviour and mimicry in Heliconius butterflies. Biol. J. Linn. Soc. 55: 159-180.
- Franks, D. W. and J. Noble. Batesian mimics influence mimicry ring evolution. P Roy Soc Lond B Bio. 271: 191-196,
- Turner, J. R. G., E. P. Kearney, L. S. Exton. Mimicry and the Monte-Carlo predator – the palatability spectrum and the origins of mimicry. Biol J Linn Soc. 23: 247-268
- Symula, R., R. Schulte, and K. Summers. 2001. Molecular phylogenetic evidence for a mimetic radiation in Peruvian poison frogs supports a Mullerian mimicry hypothesis. P. Roy. Soc. Lond. B. Bio. 268: 2415-2421.
- G. D. Ruxton and M. P. Speed. How can automimicry persist when predators can preferentially consume undefended mimics? P. Roy. Soc. Lond. B. Bio. 273: 373-378
- Clarke, C.A. and P.M. Sheppard. Further studies on genetics of mimetic butterfly papilio-memnon l. Philos. T. Roy. Soc. B. 263: 35-70.
- Nijhout, H. F. Developmental perspectives on evolution of butterfly mimicry. Bioscience. 44: 148-157.
- Kunte, K., Zhang, W., Tenger-Trolander, A., Palmer, D. H., Martin, A., Reed, R. D., ... & Kronforst, M. R. (2014). doublesex is a mimicry supergene. Nature, 507(7491), 229-232.