Why Hybrid Animals May Take Over the North

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 where it had the white fur of a polar bear but the long claws humped back shallow face and brown patches of fur like a grizzly where officials seized the creature with species unknown and a DNA test was conducted leading them to discover it was neither a grizzly nor a polar bear but a hybrid with a polar bear mother and a grizzly bear father the first ever discovered in the wild now called a pizzly or grolar bear.

Since the first pizzly discovery sightings of the hybrids have been increasing where a 2017 study showed eight hybrids springing from a single female polar bear who mated with two grizzly bears where this demonstrates that pizzlies are not only viable but also fertile capable of producing their own offspring where the increasing frequency suggests that polar bear and grizzly contact is becoming more common creating opportunities for hybridization as climate change alters Arctic habitats.

In 1990 a scientist noticed a very strange skull hanging on an Inuit hunter’s wall where it looked like neither a beluga whale skull nor a narwhal skull but something in between where later genetic analysis would show that the skull belonged to a narwhal-beluga hybrid the first ever confirmed case where this discovery revealed that hybridization occurs not just in bears but also in marine mammals of the Arctic.

A widely accepted concept in biology is called the biological species concept where it says that organisms belong to the same species if they can interbreed and produce viable and fertile offspring where this definition emphasizes reproductive compatibility as the fundamental criterion for species membership where viability means the offspring can survive and fertility means it can reproduce creating a clear genetic boundary between species.

Speciation is a gradual process that depends on random mutations to introduce new traits that create a sub-population better suited for an environment or finding a mate where 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 barriers called mechanisms of reproductive isolation where this slow accumulation of differences over time creates the diversity of life.

Species that are unable to mate with their ancestors or each other face barriers called mechanisms of reproductive isolation where these work in two ways where the first is by making it impossible for two different species to physically mate these are pre-zygotic barriers meaning they take place before the formation of a zygote which is the result of a sperm and egg fusing where the other way reproductive isolation works is by making it impossible for embryos to develop into healthy fertile adults these are called post-zygotic barriers and are typically the result of a mismatch of chromosomes.

Pre-zygotic barriers making it impossible for two different species to physically mate include mechanisms like living in different habitats or having different courtship behaviors or not having reproductive organs that fit together where these barriers can change on a time scale of hundreds of thousands or millions of years for post-zygotic barriers but pre-zygotic barriers on the other hand can change in a relative blink of an eye where environmental changes can rapidly eliminate habitat separation or alter behaviors creating opportunities for hybridization.

Post-zygotic barriers are typically the result of a mismatch of chromosomes where normally when two animals mate they each contribute the same number of chromosomes to their offspring where human cells contain 46 chromosomes for example with 23 coming from our mothers and 23 from our fathers where when species with different chromosome numbers mate the offspring may survive but to produce gametes the chromosomes need to pair up which requires genetic similarity between the chromosomes and the same number of them where mismatch makes this impossible.

Horses and donkeys which diverged from their common ancestor around 4 million years ago have a major problem if they want to mate where horses have 64 chromosomes while donkeys only have 62 where when they mate the mule offspring gets 32 chromosomes from its horse mother and 31 chromosomes from its donkey dad where this is close enough to create a living animal but not close enough for it to ever have its own offspring where donkey and horse DNA is similar enough for some pairing of the chromosomes but the rest have no direct homologues making gamete formation impossible.

Grizzly bears and polar bears only diverged around 150,000 years ago where this means their genetics have not had a chance to drift as far as horses and donkeys where they each still have 74 chromosomes and enough genetic similarity that the two species can mate and produce viable fertile offspring where when polar bears diverged from brown bears the last ice age was coming to a close where the speciation occurred when a population of brown bears followed retreating ice northward and adapted specifically to their new Arctic home.

Both narwhals and belugas have 44 chromosomes so the narluga hybrid may indeed be able to produce viable offspring but this remains unknown where both species of whale breed at a time of year when thick sea ice prevents research boats from getting in so scientists don’t actually know very much about what exact pressures may be spurring the existence of narlugas where the matching chromosome number suggests fertility is possible but requires confirmation.

Some sea 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 1970s when satellite records began where we know that it’s been declining at a rate of around 13 percent per decade and this is bad news for the polar bears because the sea ice is a polar bear’s hunting ground where their Arctic home is disappearing forcing them to adapt or perish.

With the disappearing sea ice polar bears have no choice and are forced south onto land where they can forage seabird eggs and caribou albeit not always with much success where as the polar bears are forced south grizzlies move north pushed by increasing human construction and development where these two types of bear are increasingly crossing paths and the pre-zygotic barriers of habitat isolation and behavioral isolation are beginning to dissolve where climate change and human activity create a perfect storm for hybridization.

Narlugas carve out their own ecological niche different from both parents where belugas have 40 teeth and narwhals have none where narlugas split the difference with 18 teeth and like a beluga narlugas have small teeth but like a narwhal’s tusk these teeth are twisted where this unique morphology means 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 unlocking an underutilized food resource.

Polar bear population is expected to decline by over 30 percent in the next 30 years where this means that 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 where in yet another sad blow to polar bears their hybrid versions may accelerate the decline of their purebred relatives outcompeting them even further where 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.

Polar bears are struggling to survive without their usual hunting grounds and their population is expected to decline where their skulls and teeth are built for a very specific food source that’s hard to find in their new home blubber where pizzlies on the other hand may have inherited some useful traits from grizzly bears which are able to eat pretty much anything where grizzlies are formidable scavengers with diets that consist of insects variety of plants roots tubers grasses berries rodents fish carrion along with other meat sources where generalist animals like grizzlies are the best survivors of rapid change not highly specialized apex predators like polar bears.

Hybridization is not always a sad last resort but can also increase biodiversity by creating more species like in the case of the African cichlids where 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 where the classic method of speciation where 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 where in just the 15,000 years since Lake Victoria formed around 500 new species of cichlids have been created and hybridization is a major part of this explosive radiation.

Hybridization is a part of our past too where around 60,000 years ago our ancestors mated with two other now-extinct species of humans Neanderthals and Denisovans where as their offspring continued to mate with our ancestors their genes have been passed down through each generation to us now where in some populations as much as 2 percent of DNA comes from Neanderthals and up to 6 percent from Denisovans where this translates to hundreds of genes demonstrating that hybridization has shaped human evolution.