Toads will choose to mate with a different species if it gives their progeny a better chance of survival, showing for the first time how females from one species can apply sexual selection pressure on another.
Researchers writing in Science found that Plains spadefoot toads actively opted to breed with Mexican spadefoot males when the climate was right in order to birth fitter offspring. This stands in contrast to conventional wisdom that views hybridization as a maladaptive behavior producing less viable offspring. Think of donkey-horse hybrids (or mules), which are typically infertile.
"Basic biology classes teach that different species usually cannot interbreed successfully and that rarely produced crossbred offspring (hybrids) are often infertile or of lower fitness," Professor Marlene Zuk, American evolutionary biologist and behavioral ecologist, said in an accompanying editorial.
However, this assumption is being challenged by new research presenting cases where "adaptive hybridization" could boost the fitness of the offspring in a similar way to selecting the best mate among members of their own species.
In this instance, mating with Mexican spadefoots can lead to hybrids that develop faster than typical tadpoles — a trait that may be a deciding factor in their chances of reaching adulthood. Particularly if the desert ponds these animals inhabit are shallow and may dry before the tadpoles mature.
"The fact that this study reports an instance of hybridization, in and of itself, is not groundbreaking," James Hanken, Professor of Biology and Director of the Museum of Comparative Zoology at Harvard University, told Newsweek.
"What's particularly novel about this study is its demonstration that one species (Spea bombifrons) can mediate sexual selection within a second species (Spea multiplicata) because of species-one females' ability to select males of species two who will enhance the viability of her offspring."
As Zuk points out, it is more typically males that initiate this type of interspecies interactions.
"Historically, some 25 percent of plant and 10 percent of animal species are thought to hybridize, but advances in genomic technologies are revealing that hybridization is more common than once thought," Catherine Chen, a PhD student in the Department of Biology at the University of North Carolina at Chapel Hill, told Newsweek. "It seems likely that at least some species hybridize because it is adaptive."
Chen and others came to their conclusions after breeding 20 female Plains spadefoots with 20 male Mexican spadefoots, each displaying different mating call characteristics, and tracking the tadpoles' development.
They found the mothers' body size was a positive predictor of their offsprings' fitness. However, the correlation between the fathers' calls and the offsprings' fitness was even stronger. Males with slower pulse rates fathered hybrid tadpoles that were larger, heavier, and faster to develop.
With the genetic advantages of a slow pulse rate established, the researchers wanted to find out how mating calls affected females' preference of mate. And so, they offered female Plains spadefoots a selection of mating calls with varying pulse rates. The females were placed in different conditions — shallow water and deep water — to see how the environment influenced their choice.
The depth of the water was a deciding factor in the females' selections. Those in shallow water preferred Mexican spadefoots with a slower pulse rate. Those in the deeper water did not seem to mind.
It is not know how often this interspecies mating takes place in the wild, said Chen. Observations suggest as many as 20 percent of females could be hybridizing with Mexican spadefoot males but it can be hard to work out year-on-year estimates. However, she says it taking place often enough to refine the behavior so that they aren't simply mating with another species but they are mating with the "best" of another species.
This type of adaptive behavior is unlikely to be unique to Plains spadefoots, Chen and the study's other authors say.
Is is also likely to be affected by human-mediated changes in the environment — such as, the development of cattle ranches — and climate change, which has the potential to bring closely related species that don't typically meet but theoretically could hybridize together.
"Both of these factors are increasing in scope and frequency, so we might expect that this phenomenon might become more common going forward," said Hanken.
Chen said it is important to note that adaptive hybridization can happen in other ways than mating with high-quality individuals of another species — as demonstrated in the paper. It may take place out of necessity when, for example, there are no members of an animal's own species.
"Hybridization might be bad, but it is better than not mating at all," she said.
As to the pros and cons of the type of adaptive hybridization described in the paper, it is a mixed bag.
"On the one hand, hybridization can bring in new genetic variation to a species, which can be a good thing and potentially allow for adaptation—as is famously the case for ancient humans mating with Denisovans," said Chen. This is particularly true in rapidly changing environments and stressful conditions. "Essentially it can rescue populations from extinction."
"On the other hand, hybridization can depress fitness and become an "evolutionary trap": where for any individual hybridization might be better than the alternative, but the long-term consequences could be population decline and extinction," she added.
In short, there is a lot of uncertainty over the benefits and disadvantages of adaptive hybridization, and how it positively and negatively impacts the gene pool.
"In a world where many species are facing rapid global change and being moved around, answering this question is becoming critically important to understanding what species might adapt and what species are at risk of extinction," said Chen.
