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We often think of categories as fixed, with a species being a species no matter what. The thing is, life doesn’t have to follow our rules.

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Organizing living things into kingdoms and families and species and stuff helps us understand the vastness of life on Earth. It gives it order.

And we tend to think of these categories as fixed, with a species being a species no matter what. Except… life doesn't have to follow our rules. Species interbreed to produce hybrids more often than we tend to think.

And understanding how and why they break the rules can help us understand them even better. One of biologists' most commonly used methods for differentiating one species from another is called the biological species concept. It defines a species as a population of organisms that regularly interbreed and produce fertile offspring.

So by that definition, if two different species breed, they can't have fertile offspring. Which checks out if you think about hybrids intentionally created by humans -- like mules or ligers. They're almost always sterile and can't produce offspring.

But what about when humans aren't monkeying around with other species' gene pools? Actually, interbreeding happens a lot. Scientists used to think hybridization was a genetic dead end.

But we now understand that it's a major driving force for speciation, or the creation of new species. We can see evidence for it happening all over the place, especially in plants. In fact, according to one study published in 2005, natural hybridization occurs in 25 percent of plant species and about 10 percent of animal species alive today.

And this seems to be because plants are more likely than animals to be polyploid -- meaning they can have extra sets of chromosomes. This can happen if the parent species accidentally duplicates its DNA, producing a complete extra set of chromosomes. Long story short, this is terrible news for being able to reproduce, because cells tend to freak out if the number of chromosomes doesn't match up.

These polyploid individuals can now only mate with other polyploids -- barring a genetic trick here or there. Now, it's reproductively isolated from its parent species, but can mate with similar individuals. Meaning -- it's a new species!

And this process is why we have pizza dough, cookies, and parker house rolls -- that is, wheat. Most wheat is actually a polyploid hybrid. The group of wheat known as Triticum naturally has multiple sets of chromosomes -- which makes it easier for them to hybridize.

So when humans started to domesticate various wild wheats to make them better for harvesting, the domestic and wild strains were still able to interbreed. The gene flow from wild and domestic strains of wheat over time lead to the creation of new hybrid wheat species, like durum wheat, which is used to make pasta. But animals are much less tolerant of polyploidy than plants – it's usually fatal in animals because our cells don't handle the extra DNA so well.

So with animals, we more often see homoploid hybrid speciation, where there's no change in chromosome number. And while scientists aren't always 100% sure how homoploid hybrid speciation works, some think it has to do with the mixing of genes. One way to achieve this gene mixing is through backcrossing.

Backcrossing is when two species mate and produce a hybrid, then that hybrid mates with one of the parent species and manages to produce fertile offspring. When this happens repeatedly over generations, it's called introgression. Genes transfer back and forth like two different decks of cards being shuffled and dealt for a poker game.

Sometimes you get more cards from one deck than the other, so the hybrid might look more like one parent than the other. And much like in a card game, you can be dealt a losing hand -- where the hybrid dies out and the two distinct species remain. Or you can get a winning combination, where the hybrid inherits a lucky combination of genes that make it fitter than either of its parents.

Over time, the hybrid's combo of genes wins out and a new species forms. Such is the case with the hybrid Italian sparrow. When researchers looked at its genes in a 2018 study, they found that some individuals had more genes in common with one parent species, house sparrows -- while others got more genes from their other parent, the Spanish sparrow.

In fact, different combinations of genes seemed to be more successful in different environments. Even Darwin's famous finches -- the birds he used to demonstrate incremental adaptations to different environments -- seem to be able to hybridize with each other. And that genetic mixing may help drive their ability to rapidly adapt to changing conditions on the Galapagos islands they call home.

So try as we might to create order in the universe by carefully categorizing and defining things, nature doesn't care about our definitions! And while the biological species concept can be useful, evolution is a bit of a rebel. And hybrids are material for evolution to work with.

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