Heliconius butterflies have been a subject of many studies, due partly to their abundance and the relative ease of breeding them under laboratory conditions, but also because of the extensive mimicry that occurs in this group. From the nineteenth century to the present day, their study has helped scientists to understand how new species are formed and why nature is so diverse. In particular, the genus is suitable for the study of both
Batesian mimicry and
Müllerian mimicry. Because of the type of plant material that
Heliconius caterpillars favor and the resulting poisons they store in their tissues, the adult butterflies are usually unpalatable to predators. Work has been done to understand the genetic changes responsible for the convergent evolution of wing patterns in
comimetic species. Molecular work on two distantly related
Heliconius comimics,
Heliconius melpomene and
Heliconius erato, has revealed that homologous genomic regions in the species are responsible for the convergence in wing patterns. Also, Supple had found evidence of two co-mimics
H. erato and
H. melpomene having no shared
single-nucleotide polymorphisms (SNPs), which would be indicative of introgression, and hypothesized the same regulatory genes for color/pattern had comparably changed in response to the same selective forces. Similarly, molecular evidence indicates that
Heliconius numata shares the same patterning homologues, but that these loci are locked into a wing patterning supergene that results in a lack of recombination and a finite set of wing pattern morphs. More recent studies have found ancient introgressions between H.
erato and H.
melpomene that have suggested greater amounts of introgression than previously believed. One puzzle with Müllerian mimicry/convergence is that it would be predicted the butterflies to all eventually converge on the same color and pattern for the highest predator education. Instead,
Heliconius butterflies are greatly diverse and even form multiple 'mimicry rings' within the same geographical area. Additional evolutionary forces are likely at work. Studies into predator psychology have suggested that traditional models of mimicry do not properly account for the fluctuating nature of predator learning and forgetting mimic patterns.
Speciation Heliconius butterflies are models for the study of
speciation.
Hybrid speciation has been hypothesized to occur in this genus and may contribute to the diverse mimicry found in
Heliconius butterflies. It has been proposed that two closely related species,
H. cydno and
H. melpomene, hybridized to create the species
H. heurippa. In addition, the
clade containing
Heliconius erato radiated before
Heliconius melpomene, establishing the wing pattern diversity found in both species of butterfly. In a DNA sequencing comparison involving species
H. m. aglope,
H. timareta, and
H. m. amaryllis, it was found that gene sequences around mimicry loci were more recently diverged in comparison with the rest of the genome, providing evidence for speciation by hybridization over speciation by ancestral polymorphism. Hybridization is correlated with
introgression. Results from Supple and her team have shown SNP's being polymorphic mostly around hybrid zones of a genome, and they claimed this supported the mechanism of introgression over ancestral variation for genetic material exchange for certain species. Research has shown introgression centering on two known chromosomes that contain mimicry alleles.
Assortive mating reproductively isolates
H. heurippa from its parental species. Melo did a study on the hybrid
H. heurippa to determine its mating habits regarding preference between other hybrids and its parental species. The results showed
H. heurippa chose to reproduce via
backcrossing, while the parental species were highly unlikely to reproduce with the backcrosses. This is significant, because hybrids' mating behavior would relatively quickly isolate itself from its parental species, and eventually form a species itself, as defined by lack of gene flow. His team also hypothesized that along with a mixed inheritance of color and pattern, the hybrids also obtained a mixed preference for mates from their parental species genes. The
H. heurippa likely had a genetic attraction for other hybrids, leading to its reproductive isolation and speciation.
Heliconius butterflies provide examples of a probably rare form of speciation, homoploid hybrid speciation, i.e. hybridization without changing the number of chromosomes. For various reasons, while it remains a good example of introgression of a color trait involved in mating from
H. melpomene,
H. heurippa is no longer regarded as a good example of hybrid speciation ; the problem is that
H. heurippa is today regarded as little more than a local form of the more widespread
H. timareta, which occurs along the eastern slopes of the Andes between Colombia and Peru and whose divergent populations also have many other examples of different color pattern introgression from different geographic forms of
H. melpomene. It is unlikely that
H. heurippa is reproductively isolated enough from its "H. timareta" parent to warrant status as a species in its own right. However, at least two more recent excellent examples of homoploid hybrid speciation have cropped up in
Heliconius. Firstly,
H. hecalesia is almost certainly an ancient hybrid species between
H. telesiphe (+
H. clysonymus +
H. hortense) and the
H. erato +
H. himera lineages. A more recent case is
Heliconius elevatus, which has been shown to be a hybrid species between
H. melpomene and
H. pardalinus.
Sexual selection of aposematic colors Aposematism, using warning colors, has been noted to improve species diversification, which may also contribute to the wide range of
Heliconius butterflies. For aposematism and mimicry to be successful in the butterflies, they must continually evolve their colours to warn predators of their unpalatability. Sexual selection is important in maintaining aposematism, as it helps to select for specific shades of colours rather than general colors. A research team used techniques to determine some the color qualities of a set of butterflies. They found that color was more vivid on the dorsal side of the butterflies than on the ventral. Also, in comparing the sexes, females appeared to have differing brightness in specific spots. It is important to select for specific colors to avoid subtle shades in any of the species involved in the mimicry. Unsuccessful warning colors will reduce the efficiency of the aposematism. To select for specific colours, neural receptors in the butterflies' brains give a disproportionate recognition and selection to those shades. To test the importance of these neural and visual cues in the butterflies, researchers conducted an experiment wherein they eliminated colours from butterflies' wings. When a colour was eliminated, the butterfly was less successful in attracting mates, and therefore did not reproduce as much as its counterparts.
Sexual selection of pheromones In order to attract mates female
Heliconius secrete pheromones from a yellow like sac that they secrete the scent to appear more attractive to the males. They found that typically it is virgin female
Heliconius that secrete these pheromones, The males are able to attach themselves using their denticles to these secretion sacs during mating in order to ensure secretion. Pheromones are vital when it comes to mate choice it determines the more likely chance that there will be a success in mating between the
Heliconius. There is a reproductive isolation between populations so while mates are attracted by pheromones they still will choose to similar patterned winged
Heliconius. Mating and offspring Heliconius has evolved two forms of mating. The main form is standard sexual reproduction. Some species of
Heliconius, however, have converged evolutionarily in regard to pupal mating. One species to exhibit this behavior is
Heliconius charithonia. In this form of mating, the male
Heliconius finds a female pupa and waits until a day before she is moulted to mate with her. With this type of mating there is no sexual selection present.
H. erato has a unique mating ritual, in which males transfer anti-
aphrodisiac pheromones to females after copulation so that no other males will approach the mated females. No other Lepidoptera exhibit this behavior.
Heliconius female butterflies also disperse their eggs much more slowly than other species of butterflies. They obtain their nutrients for egg production through pollen in the adult stage rather than the larval stage. Due to nutrient collection in the adult rather than larval stage, adult females have a much longer life than other species, which allows them to better disperse their eggs for survival and speciation. This form of egg production is helpful because larvae are much more vulnerable than adult stages, although they also utilize aposematism. Because many of the nutrients needed to produce eggs are obtained in the adult stage, the larval stage is much shorter and less susceptible to predation. == Cyanogenic glycosides as a defence ==