In 2006 scientists claimed to have recreated the path of evolution by breeding a hybrid butterfly from two South American species, H. Cydno (black wings with white and yellow markings) and H Melpomene, (black wings with red, yellow and orange markings). Hybrids are created by breeding two existing species that can produce live offspring together.

As reported by Ker Than in LiveScience (June 14, 2006), the new lab hybrid </a> has the same markings as a wild South American butterfly,  H. Heurippa, long thought to be a hybrid itself. These three species can meet up in the wild (= they are not reproductively isolated), but H. heurippa butterflies prefer to mate with their own. Therefore, little crossbreeding occurs in natural environments.

Scientists said their laboratory hybrid showed that they had recreated the first steps in the evolution of Heurippa. The original research was reported in Nature, July 15, 2006.

Can hybridization retrace the course of evolution?

The researchers’ idea of trying to determine the course of evolution by breeding back to an earlier, perhaps extinct, species is not new. In the 1920s, German zoologists Lutz and Heinz Heck thought they could do that. Their Heck cattle were an attempt to reconstitute the extinct predecessor of domestic cattle, the aurochs. Their Heck horses were an attempt to retrieve the genetic inheritance of the wild Europeantarpan.

The Hecks used a combination of modern breeds in order to create what they believed to be living tarpans and aurochs. They bred Koniks, Icelandic Ponies, Swedish Gotlands, Polish primitive horses, and wild Przewalski horses to create their tarpans.

Their aurochs were recreated with a combination of crossed Hungarian Grey Cattle, Scottish Highland, Brown, Murnau-Werdenfels, Angeln, German Friesian, Podolic and Corsican cattle.

So backbreeders try to resurrect genetic traits of ancient species that are latent (= present but not expressed) in modern ones. Back breeding experiments create hybrids that may resemble an extinct ancestor. Heck cattle and Heck horses do look like their ancient ancestors in many respects. However, it is not clear that the breeders actually recreate the extinct animal, as opposed to creating an animal that looks that way.

What’s much more interesting today is the question of how frequently hybrids create new species.

At one time, hybrids were thought to be common among plants but rare among animals. But as more animal hybrids are found, some scientists ask whether hybrids are not a more common means of creating new species than previously thought. For example, George Turner, Professor in the Department of Biological Sciences at the University of Hull in England, believes that DNA analysis may uncover more examples of speciation through hybridization.

An intense focus on Darwin’s theory that natural selection is the main cause of new species has often meant that other possibilities are neglected.

Species of cattle, dogs, and cats (bovoids, canines, and felines) are all known to interbreed with closely related species. Most interbreeding occurs at the species level, but some interbreeding can occur between genera (the next level up from species) - such as the beefalo, a hybrid of bison and domestic cattle.

Wild hybrids may have advantages over their "purebred" parents. For example, the beefalo can withstand the harsh North American prairies better than many breeds of domestic cattle. While some first generation hybrids are infertile, infertility is often overcome by backbreeding, which can occur naturally (and is believed to have occurred with the butterflies*). For example, some first generation beefalo males are infertile, but second generation beefalo can breed with either cattle or bison. )

*In the case of the butterflies, the first generation hybrid females rather than the males were sterile. It is assumed that in nature the males bred at first with non-hybrid females until some non-sterile females hatched (a natural form of backbreeding).

The idea that hybridization creates species is not new. Carolus Linnaeus (1707-1778), creator of the binomial classification system used by biologists today, predicted  hybrid speciation as a way that new species are created.

Linnaeus thought species could multiply through hybridization and even suggested that hybridization could occur at the genus level, as in the case of the beefalo.

Darwin’s theory of natural selection holds that descendants of a common ancestor diverge into separate species and genera over time because natural selection acts to conserve those random changes in the genes which favour survival of the fittest individuals.

Hybridization, by contrast, unites the genomes of two species into one, thus possibly conferring new advantages on the successful hybrid – for example, the beefalo’s sturdiness in the face of harsh winters, from its North American bison ancestor.

Hybridization experiments may have something to teach us about ways that speciation might occur, even if they do not retrace the path of evolution or resurrect extinct species, as the Heck brothers hoped.

Further Reading

Hybrid animals

Butterfly hybrids

The Linnaean Society of London

Extinct horses and cattle (Equus ferus Boddaert, 1785 = E. przewalskii Poljakov, 1881) and Aurochs,(Bos primigenius Bojanus, 1827 from the WWF Large Herbivore Initiative by Margret Bunzel-Drüke )