In 2006, a trophy hunter shot what he thought was a polar bear in the Northwest Territories
of the Canadian Arctic. But what he killed instead was something never before documented
in the wild: a bear with the white fur of a polar bear but the long claws, humped back,
shallow face and brown patches of fur like a grizzly. This was definitely not a typical
bear.
Officials seized the creature, its species unknown and under question. A DNA test was
conducted, and led them to discover it was neither a grizzly or polar bear, but a hybrid,
with a polar bear mother and a grizzly bear father, the first ever discovered in the wild.
And while this hybrid, now called a pizzly or a grolar, was a rare discovery, it may
be just the beginning of an era of these hybrid bears.
Since then, sightings of the hybrids have been increasing, with a 2017 study showing
eight hybrids springing from a single female polar bear who mated with two grizzly bears.
But it's not just bears. In 1990, a scientist noticed a very strange skull hanging on an
Inuit hunter’s wall. It looked like neither a beluga whale skull, nor a narwhal skull,
but something in between. Later genetic analysis would show that the skull belonged to a narwhal-beluga
hybrid, the first ever confirmed case.
Hybrids are commonly thought of as an interesting anomaly, but besides that, evolutionarily
useless. The thinking has always been: even if a hybrid can exist, that’s the end of
line, since hybrids themselves are thought to be incapable of producing offspring. But
now, scientists are understanding that there is more to the story. And in the current state
of the world, experts think we might be entering a new era of hybrid animals.
But what benefit can mating across species lines possibly give? What is pushing animals
to hybridize - and how is it even possible for animals of different species to mate?
Everyone has heard of mules, a mashup of a horse and donkey, and maybe even ligers, a
mix of tiger and a lion. Hybrids are made when two different species interbreed and
produce offspring
In vertebrates, it has always been thought to be a rare occurrence. Logically, it seems
like different species should not be able to mate, because of the very definition of
the word “species.”
A widely accepted concept in biology is called the Biological Species Concept. It says that
organisms belong to the same species if they can interbreed and produce viable and fertile
offspring.
Speciation is a gradual process that depends on random mutations to introduce new traits
that create a subpopulation better suited for an environment or finding a mate.
Eventually, they become isolated both physically and genetically due to a diverging genome,
resulting in new species that are unable to mate with their ancestors or each other due
to a number of barriers called “mechanisms of reproductive isolation.” These work in
two ways.
The first is by making it impossible for two different species to physically mate. These
are prezygotic barriers, meaning they take place before the formation of a zygote, which
is the result of a sperm and egg fusing.
These include mechanisms like living in different habitats (habitat isolation), or having different
courtship behaviors (behavioral isolation), or not having reproductive organs that fit
together (mechanical isolation).
The other way reproductive isolation works is by making it impossible for embryos to
develop into healthy, fertile adults. These are called postzygotic barriers and are typically
the result of a mismatch of chromosomes.
Normally when two animals mate, they each contribute the same number of chromosomes
to their offspring.
Human cells contain 46 chromosomes for example, with 23 coming from our mothers, and 23 from
our fathers. But now let’s look at horses and donkeys, which diverged from their common
ancestor around 4 million years ago. Donkeys and horses have a major problem if they want
to mate. Horses have 64 chromosomes, while donkeys only have 62. When they mate, the
mule offspring gets 32 chromosomes from its horse mother and 31 chromosomes from its donkey
dad. This is close enough to create a living animal, but not close enough for it to ever
have its own offspring.
For a male and female mule to produce their own offspring, they would have to produce
gametes. To form gametes, the chromosomes need to pair up, which requires genetic similarity
between the chromosomes, and the same number of them. Donkey and horse DNA is similar enough
for some pairing of the chromosomes, as is the case with pairs 1-13, and horse chromosomes
14-19 and donkey 25-30. But the rest have no direct homologs in the other species, making
the cell division that creates haploid gametes impossible.
Because of reasons like this, for a long time researchers considered the hybridization of
animals in the wild a fluke, having little significance in the big picture of evolution.
But it turns out, as environments change, the barriers separating species from one another
can become less rigid or disappear altogether, opening the door for animals to mate across
species lines .
Genetically, post zygotic barriers can be broken if two species diverged recently enough
in the tree of life.
Horses and donkeys diverged around 4 million years ago, and in that time, their genetics
drifted to a point where they could mate, but their chromosomes no longer aligned. For
most animal combinations, even getting that far is impossible.
But grizzly bears and polar bears only diverged around 150,000 years ago. This means their
genetics have not had a chance to drift as far. They each still have 74 chromosomes,
and enough genetic similarity that the two species can mate and produce viable, fertile
offspring.
As for the narluga, we don’t know if it would be able to produce viable offspring.
But both narwhals and belugas have 44 chromosomes, so it may indeed be possible.
These post-zygotic barriers change on a timescale of hundreds of thousands, or millions of years.
Pre-zygotic barriers on the other hand can change in a relative blink of an eye.
The pre-zygotic barriers separating polar bears and brown bears are behavioral isolation
and habitat isolation - grizzlies live on land and polar bears live on sea ice.
Some ice remains year-round in the Arctic, providing vital habitat for wildlife such
as seals and polar bears. But sea ice that persists in the Arctic for more than a year
has been disappearing rapidly. Since the 70s, when satellite record began, we know that
it has been declining at a rate of around 13% per decade.
And this is bad news for the polar bears, because the sea ice is a polar bear’s hunting
ground. When polar bears diverged from brown bears 150,000 years ago, the last ice age
was coming to a close. The speciation occurred when a population of brown bears followed
retreating ice northward, and adapted specifically to their new arctic home. Their home is disappearing,
and they are not well suited to anything else. But with no choice, they are forced south
onto land, where they can forage sea bird eggs or caribou, albeit not always with much
success
And as the polar bears are forced south, grizzlies move north, pushed by increasing human construction
and development.
These two types of bear are increasingly crossing paths, and the pre-zygotic barriers are beginning
to dissolve.
For other animals, like narwhals and belugas, changing sea ice patterns may be affecting
their mating behaviors too. However, both species of whale breed at a time of year when
thick sea ice prevents research boats from getting in, so we don’t actually know very
much about what exact pressures may be spurring the existence of narlugas.
We can see how circumstances may push certain animals together, but besides just an increase
in chance encounters, there is a deeper reason hybrids are popping up in the world more and
more: With new species, come new survival tactics.
Some, such as narlugas, carve out their own ecological niche, one that is different from
both of their parents.
Belugas have 40 teeth, narwhals have none. Narlugas split the difference, with 18. And
like a beluga, narlugas have small teeth. But, like a narwhal’s tusk, these teeth
are twisted. [11] And this is thought to be the reason why narluga don’t feed on the
same stuff as their parents, like fish, octopuses, and shrimp found in the middle of the water
column - but instead, feed off the bottom of the ocean, like walruses do. As environments
change, this new ability could unlock an underutilized food resource in that area.
Pizzlies too, have advantages that their parents do not. Polar bears are struggling to survive
without their usual hunting grounds and their population is expected to decline by 30% over
the next 30 years. Their skulls and teeth are built for a very specific food source
that is hard to find in their new home: blubber.
Pizzlies, on the other hand, may have inherited some useful traits from grizzly bears, which
are able to eat pretty much anything. Grizzlies are formidable scavengers, with diets that
consist of insects, a variety of plants, roots, tubers, grasses, berries, rodents, fish, carrion,
along with other meat sources. Generalist animals like grizzlies are the best survivors
of rapid change to their environment, not highly specialized apex predators like polar
bears. And inheriting these generalist traits could make pizzlies the new king in the north.
Thus, a polar bear lineage may only be able to live on within the hybridized pizzlies,
even as pure polar bears are pushed closer to extinction. And in a yet another sad blow
to polar bears, their hybrid versions may accelerate the decline of their purebred relatives,
outcompeting them even further, and with hybrid offspring replacing purebred offspring.
Losing polar bears is undoubtedly a terrible tragedy, but pizzlies may be a necessary compromise
given the current warming trends.
But as depressing as the plight of the polar bear is, hybridization is not always a sad,
last resort.
It can also increase biodiversity by creating more species, like in the case of the African
cichlids. These little fish hybridize like crazy. And when two different species get
together, their gene pool gets much larger, allowing for genetic variation among offspring
and soon leading to many new species.
The classic method of speciation, where a genetic mutation gradually leads to one or
two new species over many generations, takes a long long time. But hybridization can create
new species much faster. In just the 15,000 years since Lake Victoria formed, around 500
new species of cichlids have been created.
And hybridization is a part of our past too, afterall.
Around 60,000 years ago, our ancestors mated with two other, now extinct, species of humans:
Neaderthals and Denisovans. As their offspring continued to mate with our ancestors, their
genes have been passed down through each generation to us. Now, in some populations, as much as
2% of DNA comes from Neanderthals and up to 6% from Denisovans. This translates to hundreds
of genes.
As scientists continue their investigation into hybrids through fossil records and genetics,
the “tree of life” that we often associate with the evolution of species is looking less
and less accurate. Instead, we should picture a “web of life” with interbreeding and
hybridization occurring throughout the evolutionary process, bringing about new and, at times,
improved species.
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