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  • So if you've seen our videos before you may know that one of us is a cat person.

  • Well the other one of us is a dog person, and when we were younger, we used to wonderwouldn't

  • it be cool to have one animal that is half dog and half cat.

  • A dogcat.

  • There was a cartoon we used to watch as a kid about that, although because of our tv

  • reception issues, it never came in very clear.

  • Hence, our childhood of instead watching nature shows.

  • As cool as a half cat, half dog animal would likely be---we need to understand that cats

  • and dogs are different species.

  • What is a species?

  • Well organisms that are in the same species can interbreed AND their offspring can reproduce.

  • That means---the huge variety of domesticated dog breeds---they are all the same species.

  • Even though they can look very different.

  • Same with domesticated cats.

  • All the different cat breeds are STILL the same species.

  • You can watch our classification video to learn about the hierarchy levels in taxonomy,

  • but in this video, we're just going to focus on the level of species itself.

  • Now can two different species breed and have offspring?

  • Yes---for example, let's consider the magnificent Zonkey.

  • Yes, it's a thing.

  • It's a hybrid actually---a cross between two different species---a donkey and a zebra.

  • Pretty rare but it can happen.

  • They typically are going to be sterile though---so even though the donkey and the zebra had the

  • baby zonkey---that Zonkey will not be fertile.

  • Donkeys and zebras are different species so this fits into that species rule.

  • Speciation, which means the development of a new species---can occur when populations

  • are reproductively isolated in some form.

  • Why?

  • Well first, check out our natural selection video which talks about natural selection

  • as a mechanism of evolution and how change over time can lead to a new species.

  • Our focus right now is to see how isolation can happen in the first place which can give

  • rise to speciation.

  • If we're going to get a little more fancy, we can talk about two main types of speciation:

  • allopatric speciation and sympatric speciation.

  • And here comes our disclaimer---in our short video clip, we are only giving some examples

  • of isolation and how speciation can occur.

  • In allopatric speciation, there is a geographic barrier that separates the populations.

  • So while natural selection is acting on these populations, there is also some big geographical

  • barrier---like a river, a mountain---that keeps them from being able to interbreed and

  • share the same gene pool.

  • Eventually over a period of time there can be change over time in the separated populations

  • in their separated areas from mechanisms like natural selection or genetic drift.

  • Over time, these populations can have significant genetic differences that may not allow them

  • to interbreed even if they were brought together.

  • Populations separated geographically can form different species.

  • You might think you would always need a geographic boundary to separate populations, but in sympatric

  • speciation, the speciation happens in the same area.

  • Yet, there's something else isolating them.

  • What is it?

  • Well it can be a lot of things.

  • We'll talk about just a few of them now.

  • Let's start with prezygotic barriersthat means---barriers that occur before you can

  • even make a zygote.

  • A zygote is a fertilized egg so a prezygotic barrier is not even allowing fertilization

  • to happen.

  • So of these prezygotic barriers, let's start with this first one here: behavioral isolation.

  • This is when species can have different behaviors, even very slight differences, that can isolate

  • them.

  • For example, birds having different songs---some only having very slight differences---can

  • prevent the males from attracting females of other populations.

  • And these birds can look very similar.

  • Appearances are deceiving.

  • One of the common biology examples is the Eastern and Western meadowlark.

  • Surprisingly, you can find them in the same area.

  • They look SO similarbut they are isolated by their behavior.

  • Mates are attracted with a different song.

  • Temporal isolation---species could breed at different seasons, years, even different times

  • of the day.

  • They may look very similar but if they don't have the same breeding season, then you're

  • going to have an isolation.

  • Habitat isolation.

  • See even assuming that organisms live in the same area---that doesn't mean their habitats

  • are exactly the same.

  • You could have two species of amphibians living in the general same area but if one prefers

  • an aquatic environment and one prefers a terrestrial environment, that will be a habitat isolation.

  • Now you can have postzygotic barriers too.

  • That means that mating and fertilization actually occurred because you have the zygote---a fertilized

  • egg---but there is some barrier that separates the species even still.

  • Remember our zonkey example?

  • Perfect example: the offspring is not fertile.

  • That's one barrier that separates donkeys and zebras.

  • Sometimes offspring that are produced between two different species are very weak and do

  • not survive long.

  • And sometimes if species interbreed, the offspring is NOT able to develop in even very early

  • embryonic stages because there is a genetic incompatibility.

  • All of these are postzygotic barriers that can happen in the same environment.

  • Now in our short video, we want to point out three things.

  • First, this is just a very few set of examples of isolations that can lead to speciation.

  • We encourage you to explore the huge list of other isolation types that can lead to

  • speciation.

  • Second, species can be impacted by more than one type of isolation.

  • Third, please understand that isolation itself is not the mechanism for the actual change

  • over time.

  • Change over time, which can occur in gene pools of populations, can be due to mechanisms

  • like genetic drift or natural selection.

  • For example, let's take the case of natural selection.

  • Remember that variety naturally exists in the populations.

  • However, genes in a gene pool that result in high fitness---meaning more offspring---can

  • INCREASE in the frequency of the population which can cause the population to change over

  • time.

  • Isolation is what separates the gene pools of species so the mechanism ACTING on the

  • populations is acting on themseparately.

  • Well that's it for the Amoeba Sisters and we remind you to stay curious!

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B1 中級 美國腔

物種(Speciation)

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    Fang Lu 發佈於 2023 年 11 月 18 日
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