When Two Species Mix


{♫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. 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. Thanks for watching this episode of SciShow,
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