Lions And Tigers and Ligers, Oh My!

A full grown liger is a biiggg cat! A male lion and

a female tiger get to know each well, and their

love child is huge. It doesn’t happen in the wild

because the ranges of lions and tigers don’t

overlap, and they don’t have computer dating

services. The liger is bigger than either parent,

and this is a problem during birthing. They have

many birth defects and often die young.

When putting 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.

Here is the classification scheme for several familiar

mammals. Cats are all in the same family, but there are

several genera, while all dogs fit in one genus and all

wolves in another. Black bears and Kodiak bears are a

different genus than polar bears, but hikers and bigfoot

monsters have reported sightings of polar and Kodiak

bear hybrids…. well hikers have been reporting them.

Order – Even more specific, this level of classification has been altered greatly by molecular biology. Armadilloes and anteaters are both in the Order Edentata – as toothless mammals. However, roses and magnolia trees are both dicots, but they belong to different orders (Magnoliidae and Rosidae).

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.

The africanized honeybee is more likely to swarm and

migrate when food supplies are low, so they can be seen

in masses like the one above. For hives, they usually

invade an existing hive, quit out the queen and install their

own. The danger in these bees is that they are more likely

to swarm when agitated, and they will chase the agitator

for a much longer distance (a mile or more) than regular

honeybees (100 yards or so).

Sub-Species - this is like the different breeds of cats we keep or that unfortunate incident where African bees were crossed with South American honeybees and created killer bees!

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 hybrids usually 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 though 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.

Compare the two marine mammals that are jumping. One

is bigger, darker colored, and apparently can jump

higher. That one is the wolphin. Her name is Kekaimalu,

the offspring of a bottlenose dolphin and a false killer

whale. Her offspring, Kawili Kai, is bigger than a dolphin

as well, but is lighter colored than mama.

But they can occur in animals. A wolphin was 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?

What do you get when you cross a blueberry with a

snowberry (maggots, that is)? You get a Lornicera fly –

O.K. not a funny joke, but a pretty cool twist in

evolution. As fruitflies go, this is a pretty cool looking

one; you don’t have the ghoulish red eyes to deal with

and three stripes make it look a little like a lightbulb!

The Lonicera fly is a new species produced by natural interspecies hybridization! Just when you think you understand nature, there it goes again, kicking you in the seat of your pants.

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 (snowberry bush or blueberry bush).  This is a kind of geographic isolation; the Lonicera hybrids find only Lonicera hybrids when it comes time to mate and they end up mating hybrid to hybrid for many generations. This resulted in a stable species, the process is called hybrid speciation.

The Heliconius heurippa butterfly has an unusually

large black bar that crosses its body. This must be

fairly obvious to other butterflies of the same

hybridization and it must also be pretty attractive.

Both male and female hybrids search out the wide

black bands. I would love to know the molecular

biology of that specificity of attraction.

The second exception is the Heliconius heurippa butterfly in South America. An interesting 2006 study in which hybridization was repeated in the laboratory showed that H. heurippa in nature is the result of breeding of two other species of butterflies. The hybrid does produce fertile offspring, both male and female, but that isn’t the end of the story. In this case, there isn’t any geographic isolation forcing hybrid-hybrid mating - they choose to mate together. Their choice is related to the fact that the hybrids have bold black stripes on its wings, while neither parent species does.

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