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  • {♫Intro♫}

  • 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.

  • Exceptlife 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 plantsit'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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  • {♫Outro♫}

{♫Intro♫}

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B2 hybrid wheat parent speciation offspring fertile

When Two Species Mix

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    林宜悉 posted on 2020/04/05
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