Interspecies Sex: Evolution's Hidden Secret?

James Owen
for National Geographic News
March 14, 2007
The act of mating with a species other than your own may not be as ill
advised or peculiar as it seems.

Recent research indicates that hybridization is not only widespread in nature but it might also spawn many more new species than previously thought.

A growing number of studies has been presented as evidence that two animal species can combine to produce a third, sexually viable species in a process known as hybrid speciation. Newly identified examples include both insects and fish.

This evolutionary process, while known to be common in plants, has long been considered extremely rare among animals.

Animals are generally thought to evolve the opposite way, when a single species gradually splits into two over many generations.

But some scientists now believe that the behavior that has been called animals' sexual blunders could be an important force in their evolution.

"Given the fact there have been several reported cases of hybrid speciation in animals, I think it's possible that's just the tip of the iceberg," said biologist James Mallet of University College London in the United Kingdom.

Mallet said that advances in technologies for decoding genes are only now giving scientists the opportunity to make such discoveries.

Hybrid-formed species are usually extremely difficult to detect because of their close physical resemblance to their parent species, he said.

But today scientists are able to collect the detailed molecular data needed to identify previously unrecognized hybrids.

Fast Evolving Groups

Plenty of opportunities exist for hybrid species to emerge, especially among diverse and fast-evolving groups of animals, Mallet said.

In a review of the field appearing in the current issue of the journal Nature, the scientist notes that, on average, 10 percent of animal species and 25 percent of plant species are now known to hybridize.

"In the past people have often viewed hybridization as a mistake," Mallet said. "But this is probably not an unnatural phenomenon."

And, he said, "sex with another species may be very occasionally quite a good idea."

Hybridization can increase genetic variability within a population, perhaps offering adaptations particularly suited to new or altered environments.

"It might be worth throwing the dice every now and then to try for something really weird and see if it works out," he said.

Occasionally the act produces sexually fertile hybrids that may have the opportunity to evolve into separate species.

This process in animals involves so-called homoploid speciation, in which the hybrid offspring's DNA is packaged into the same number of chromosomes as the parents' (get a genetics overview).

Homoploid speciation has traditionally been seen as unlikely, because the hybrid could easily breed with its parent species and thus not evolve into its own genetically distinct creature.

But recently identified examples suggest how hybrid species might be able to give themselves room to develop separately.

Last year a team led by Jesus Mavarez of the Smithsonian Tropical Research Institute published details about a hybrid butterfly species from Venezuela and Colombia that appears to use several tactics to isolate itself.

The hybrid butterfly, Heliconius heurippa, inherited yellow wing markings from one parent species and red from the other.

The study team found that both wing colors where needed to attract a mate, so the butterfly tended to breed only with its own kind.

The hybrid insect was also found to live at a slightly higher altitude than either of its two parent species. And the butterfly's caterpillar appears to prefer different plants as food.

Another study reported in 2005 indicated that a hybrid fruit fly from the northeastern United States had made a distinct niche for itself by basing its lifecycle around a non-native plant, the honeysuckle.

Likewise, a hybrid sculpin fish discovered in Germany appears to have put evolutionary distance between itself and its forebears by inhabiting muddy canal waters that don't suit its parent species.

Mallet said such examples suggest that "the weight of evidence is in favor of hybrid speciation being reasonably common."

But as Mallet himself admits other scientists are far more cautious.

Critics say that the likelihood of a hybrid establishing in reproductive isolation from its parents is very low, and that hybrids form less than 0.1 percent of animal populations.

Given this low number, animal hybrid species are likely to always be rare no matter how sophisticated or exhaustive the genetic analysis is.

More examples may emerge, skeptics add, but these are likely to be the exceptions and not the rule.

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