Why Hybrid Animals May Take Over the North

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 a 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, which is now called a pisley or a growler, 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 8 hybrids springing from a single female polar bear who mated with two grizzly bears. But it's not just the 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 the line, since hybrids themselves are thought to be incapable of offspring. But now, scientists are understanding that there's 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 a donkey, and maybe even ligers, a mix of a tiger and a lion. Hybrids are made when two different species interbreed and produce offspring. In vertebrates, it's 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 sub-population 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, or having different courtship behaviors, or not having reproductive organs that fit together. 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. 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 through 13 and horse chromosomes 14 to 19 and donkey 25 to 30. But the rest have no direct homologues 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 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 records began, we know that it's 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 seabird eggs and 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 may be 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, for example, and 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. 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 over 30% in the next 30 years. Their skulls and teeth are built for a very specific food source that's 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. So inheriting these generalist traits could make Pizzlies the new king in the north. And this may mean 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. And in 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. Around 60,000 years ago, our ancestors mated with two other, now extinct species of humans, Neanderthals 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 could picture a web of life, with interbreeding and hybridization occurring throughout the evolutionary process, bringing about new, and at times, improved species. There's always so much happening in the world, with climate change, economics, and technology. But in the last few years, I stopped watching or reading the news. It was stressing me out too much to wake up and have people arguing and yelling at me immediately. So to stay in touch with the world, I was waking up and browsing the usual social media platforms to see the latest news. But this too comes with a dose of terribleness, and it's not the best way to start the day. But I still wanted to be able to join in on conversations about what Bitcoin is up to, or how Jeff Bezos and company are floating around in space. 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