Whenputting organisms into categories (taxonomy), we go from bigger, more general categories to smaller more specific ones. The more similar two organisms are, evolutionarily and genetically, the more levels of their taxonomic classification they will have in common.
Question of the day:
A tigon is the result of a cross between male tiger and a female lion, while a liger is the offspring of a male lion and a tigress. But are these new species? And just how far can you go when you cross-breed?
There are instances when different species mate, but the outcomes, while interesting, may or may not be new species. It helps to know the classification levels.
Kingdom (domain) - Insects, dogs and people are all very different, but they are all animals.
Phylum (Division for plants) – usually these group things by a common body plan or some other morphologic character, or a certain degree of genetic relatedness. Arthropods are all related by a chitin exoskeleton, so flies and lobsters are both arthropods, but flowering plants have fruits and conifers have cones, so they are in different divisions.
Class – these groups have more in common, either physiologically or genetically. Cows and dogs both have hair and give birth to live young, so they are both in the class Mammalia. However, flowering plants are divided into two classes, monocots and dicots, based on seed and vascular tissue differences.
Family – Now we are getting down to smaller differences, but still just as important. All the big-eared bats and all the thick thumbed bats are both included in vespertine family, because they come out to feed in the evenings. On the other hand, maples and mahogany trees are both in the order of Sapindalae, but they belong to different families based on their leaf and flower anatomies.
Genus –comes from the Greek for “kin,” so these organisms in the same genus are closely related. In animals, both moths and butterflies are in the same order, but they are broken into 124 different families, and the family Nyphalidae, which contains the Monarch butterfly, has over 600 genera (the plural of genus).
Species – These are the individual distinct group of organisms. Usually, the distinction is made based on whether their breeding can produce fertile offspring. So bulldogs and St. Bernards are both species of dog, since they can make mutts.
Now that we have that information – let’s rephrase our question of the day. Can you breed (hybridize) different species and create a new species?
Cross-breeds are common within species (intraspecies hybridization), like with cats or dogs – not cats with dogs, you’d never want to do that! And we know they are fertile, so you end up with some dogs that are ¼ this, 1/8 that, and ¼ the other. What about between species?
Interspecies hybridsusually don’t give you fertile offspring. Since the definition of a species is a group of animals that can mate to give fertile offspring, then you would be hard pressed to create a new species by breeding different species together.
For example, the liger and tigon males are always sterile, so even thought the females are sometimes fertile, they still can’t mate a tigon to a tigon. This would be necessary to make a stable species. So the chances are low on the interspecies level.
That would mean that new species coming from breeding of animals from different genera would be even less likely to produce new species. However, individuals can be hybridized. Intergeneric hybridization is easier to do in plants; orchid growers have made many different intergeneric crosses, like little Dr. Frankensteins with green thumbs.
Butthey can occur in animals. A wolphinwas born at Hawaii Sea Park in 1985, the result of a mating between a bottlenose dolphin and a false killer whale. These species are in the same family (Delphinidae) but different genera. Named Kekaimalu, this female is fertile and has mated with male bottlenose dolphins. The first two offspring did not live very long, but her third calf is still alive and well, at ¾ dolphin and ¼ false killer whale. However, this wouldn’t be a new species unless wolphins mated with wolphins and produced fertile wolphins.
The rarest hybridization is the interfamilial hybrid. Most examples have occurred in birds, where game fowl are housed together. The Pea-guinea is a hybrid between a peacock and a guinea fowl hen. They look weird and don’t survive beyond a year or two, so there is no way that these could form a stable species.
It would take a bunch of posts to talk about why certain hybrids will work and others won’t, and why new species are not generally produced in this way. But for now - how about two exceptions?
The creation of this new fruit fly species did have a little help from humans. For about 250 years, honeysuckle plants have been imported to the North America from Europe. In the 1990’s scientists found the Lonicera fly and tried to see what other flies it was related to. Low and behold it was a hybrid of the snowberry maggot and the blueberry maggot. But why didn’t the hybrids breed with the parent species and dilute the hybrid genome back into the two stable species? How did the hybrid become a new, stable species?
These hybrid flies preferred to feed on the honeysuckle, so they lived on the imported plants, while the parent species lived on their favorites (snoberry or blueberry). This was a kind of geographic isolation; the Lonicera hybrids found only Lonicera hybrids when it came time to mate and they ended up mating hybrid to hybrid for many generations. This resulted in a stable species, the process is called hybrid speciation.
The hybrids preferentially mate with other butterflies with the bold stripes, so they are mating hybrid to hybrid and are stabilizing the new species. Darwin would blow his top – or maybe not. He never said this couldn’t happen, just that it was less likely.
Stay tuned, molecular techniques are beginning to show us that this may not be such an exception – a 2011 study identified another butterfly species created by hybrid speciation and it happens all the time in plants, like sunflowers. Three younger species, the desert, the puzzle, and the sand – seem to live where their parent species cannot, so they tend to pollinate with their similar hybrid brethren and make new species.
Next week we will ask why some birds migrate while others stay put year round.
Jesús Mavárez1, Camilo A. Salazar, Eldredge Bermingham1, Christian Salcedo, Chris D. Jiggins & Mauricio Linares (2006). Speciation by hybridization in Heliconius butterflies Nature, 41, 868-871 DOI: 10.1038/nature04738