Hybridization May Produce New Species

Darwinian evolution asserts that speciation —the formation of new and distinct species— occurs gradually by means of genetic variation and natural selection of the best traits in the struggle for survival and reproduction. Traditionally, it was thought that the mechanism for genetic variation was mutation of the DNA. However, research in the last few decades has shown that genetic recombination, epigenetic modification and hybridization may be more important than mutations in bringing about genetic variation leading to new traits and species (speciation). Genetic recombination in eukaryotes (organisms having cells with a nucleus) is the exchange of genetic material between parental chromosomes during production of gametes (meiosis). This exchange produces new combinations of genetic information that may lead to traits in the offspring that  differ from those occurring in either parent. Epigenetic modifications occur as the result of complex interaction between the genome and the cellular environment, which lead to changes in the development and differentiation of gene expression. Some of those modifications become heritable alterations that are not due to changes in DNA sequence, but to changes in how gene expression is regulated.[1] Hybridization of species is known to occur in plants and some animals and consists of combining the genetic information from two organisms of different genera, species, breeds or varieties through sexual reproduction.

Hybridization between two species or genera is generally unsuccessful, but it may sometimes result in viable offspring. People have long tried to produce new species of animals and plants through hybridization and since the early 20th century scientists have done multiple experiments abundantly documented in the scientific literature.[2] The effects of hybridization in producing new varieties and species of plants has been known for some time, but is being increasingly recognized in animals. However, the role of hybridization in evolutionary diversification of animals remains unclear and whether interspecific hybridization is important as a mechanism that generates biological diversity is a matter of controversy.[3] Hybrid plants are relatively common, but hybrid animal taxa appear to be relatively rare, although hybrid individuals are fairly common.[4] A few animal species are the result of hybridization: some fish,[5] a frog[6] and a few lizards,[7] one marine mammal (the clymene dolphin[8]), and a few birds.[9] The American red wolf could be a hybrid between the coyote and the gray wolf.

A “longwing” butterfly, Heliconius hecale. Image credit: Diego Delso, CC by SA3.0.

Hybridization has been documented in insects, with many instances of diploid, bisexual species of hybrid origin, albeit few have been carefully verified.[10] A recent study shows that hybridization indeed may be the cause of different varieties and species of some insects.[11] A team of researchers led by Nathaniel Edelman of Harvard University (USA) along with other researchers from the USA, Europe, and South America, has shown that hybridization appears to be an important factor in speciation among a group of brightly colored butterflies in the genus Heliconius. There are about 39 species of these “longwing” butterflies, all of which live in the New World tropics and subtropics. Their larvae feed on the vines of passionflowers, and are well-known examples of bad-tasting insects. In this study, genomes from sixteen species of Heliconius were analyzed and compared with genomes from nine species from other genera. Results showed that many of the species have a history of hybridization that has produced new combinations of genes and new varieties of Heliconius butterflies.

This study indicates that natural hybridization may be much more important than realized in producing varieties of plants and animals rapidly, and that it may be added to the list of known mechanisms that can produce new species much faster than can be explained by the neodarwinian proposal of mutation and selection.

Raul Esperante, PhD
Senior Scientist, Geoscience Research Institute


Jim Gibson, PhD
Director, Geoscience Research Institute

[1] Comments on epigenetic inheritance can be found at https://www.grisda.org/caenorhabditis-elegans-role-of-epigenetics-in-microevolution

[2] For example, Grant, V. 1966. The origin of a new species of Gilia in a hybridization experiment. Genetics 54:1189-1199; Bullini, L. 1994. Origin and evolution of animal hybrid species, Trends in Ecology and Evolution 9(11):422-426; Genner, Martin J., Turner, George F. 2012. Ancient hybridization and phenotypic novelty within Lake Malawi’s cichlid fish radiation. Molecular Biology and Evolution 29(1):195–206, https://doi.org/10.1093/molbev/msr183. Whitney, K. D., Ahern, J. R., Campbell, L. G., Albert, L. P., King, M. S. 2010. Patterns of hybridization in plants. Perspectives in Plant Ecology, Evolution and Systematics 12(3):175-182. Abbott, R., D. Albach, S. Ansell, and 36 authors. Hybridization and speciation. Journal of Evolutionary Biology 26(2013):229-246. https://doi.org/10.1111/j.1420-9101.2012.02599.x.

[3] Dowling, T. E., Secor, C. L. 1997. The Role of hybridization and introgression in the diversification of animals Annual Review of Ecology and Systematics 28:593-619, doi.org/10.1146/annurev.ecolsys.28.1.593. Seehausen, O. 2004. Hybridization and adaptive radiation. Trends in Ecology and Evolution 19(4):198-207. Mallet, J. 2007. Hybrid speciation. Nature 446:279-283. https://doi.org/10.1038/nature05706.

[4] Gray, AP. 1954. Mammalian Hybrids: A check-list with bibliography. Commonwealth Agricultural Bureaux; Gray, AP. 1958. Bird Hybrids: A checklist with bibliography. Alva, Scotland: Robert Cunningham and Sons.

[5] Schlupp, I, R Riesch, M Tobler. 2007. Amazon mollies. Current Biology 17(14):R536-537; doi:10.1016/j.cub.2007.05.012.

[6] Christiansen, DG. Gamete types, sex determination and stable equilibria of all-hybrid popultaions of diploid and triploid edible frogs (Pelophylax esculentus). BMC Evolutionary Biology 2009:9:135. doi: 10.1186/1471-2148-9-135.

[7] Cole, CH, HL Taylor, DP Baumann, P Baumann. 2014. Neaves’ whiptail lizard: The first known tetraploid parthenogenetic tetrapod (Reptilia: Squamata: Teiidae). Breviora 39(1):1-20. https://doi.org/10.3099/MCZ171.

[8] Amaral, A. R., Lovewell, G., Coelho, M. M., Amato, G., Rosenbaum, H. C. 2014. Hybrid speciation in a marine mammal: the clymene dolphin (Stenella clymene). PLOS ONE 9(1): e83645. https://doi.org/10.1371/journal.pone.0083645.

[9] Lamichhaney, S., Han, F., Webster, M. T., Andersson, L., Grant, B. R., Grant, P. R. 2018. Rapid hybrid speciation in Darwin's finches. Science 359(6372):224–228. doi:10.1126/science.aao4593PMID 29170277.

[10] Dowling and Secor, 1997.

[11] Edelman, N. B., Frandsen, P. B., Miyagi, M, Mallet. J., and 25 others. 2019. Genomic architecture and introgression shape a butterfly radiation. Science 366:594-599. science.sciencemag.org/cgi/doi … 1126/science.aaw2090.