Placeholder Image

字幕表 動画を再生する

  • Hi, I'm Hank. And I'm a human, but let's pretend for a moment

  • that I'm a moth. And not just any moth, a peppered moth.

  • Now let's pretend that I'm living in London in the early 1800s,

  • right as the industrial revolution is starting. Life's swell.

  • My light-colored body lets me blend in with the light-colored

  • lichens and tree bark, which means birds have a hard time seeing me,

  • which means I get to live.

  • But it's getting noticeably darker around here with all these

  • coal-powered factories spewing soot into the air, and suddenly

  • all the trees have gone from looking like this to looking

  • like this.

  • So thanks to the soot-covered everything, I've got problems.

  • But you know who doesn't have problems? My brother.

  • He looks like this Yeah, he has a different form

  • of the gene that affects pigmentation.

  • Moths like him represent about 2 percent of all the peppered

  • moths at the start of the industrial revolution.

  • But by 1895 it'll be 95 percent!

  • Why? Well, you're probably already guessing, as the environment

  • gets dirtier, darker moths will be eaten less often, and therefore

  • have more opportunities to make baby moths.

  • The white ones will get eaten more, so over time,

  • the black-colored trait will become more common.

  • As for me? [Eaten.]

  • This, my friends, is a wonderful example of

  • natural selection. The process by which certain inherited traits

  • make it easier for some individuals to thrive and multiply,

  • changing the genetic makeup of populations over time.

  • For this revelation, which remains one of the most important

  • revelations in biology, we have to thank Charles Darwin, who first

  • identified this process in his revolutionary 1859 book,

  • On the Origin of Species by Natural Selection.

  • Now lots of factors play a role in how species change over time

  • including mutation, migration and random changes in how frequently

  • some alleles show up, a process known as genetic drift.

  • But natural selection is the most powerful and most important cause

  • of evolutionary change, which is why today we're going to talk

  • about the principles behind it, and the different ways

  • in which it works.

  • Darwin came to understand the process of selection because he

  • spent his adult life, even most of his childhood, obsessed with

  • observing nature.

  • He studied barnacles, earthworms, birds, rocks, tortoises, fossils,

  • fish, insects and to some extent, even his own family.

  • I'll get back to that in a bit.

  • But it was during Darwin's famous voyage on the H.M.S. Beagle

  • in the 1830s, a surveying expedition around the world,

  • that he began to formulate this theory. Darwin was able to

  • study all kinds of organisms, and he kept amazing journals.

  • Looking back on his notes, he hit upon a couple of

  • particularly important factors in species' survival.

  • One of them was the many examples of adaptations he noticed on

  • his journey. The ways in which organisms seemed to be nearly

  • ideally shaped to enhance their survival and reproduction in

  • specific environments.

  • Maybe the most famous example of these were the variations of beaks

  • Darwin observed among the finches in the remote Galapagos Islands

  • off the coast of South America. He observed more than a dozen

  • closely-related finch species, all of which were quite similar

  • to mainland finch species, but each island species had

  • different shaped and sized beaks that were adapted to the food

  • available specifically on each island.

  • If there were hard seeds, the beaks were thick.

  • If there were insects, the beaks were skinny and pointed.

  • If there were cactus fruit, the beaks were sharp

  • to puncture the fruit's skin.

  • These superior inherited traits led Darwin to another idea,

  • the finches' increased fitness for their environment, that is, their

  • relative ability to survive and create offspring.

  • Explaining the effects of adaptation and relative fitness

  • would become central to Darwin's idea of natural selection.

  • And today we often define natural selection, and describe how it

  • drives evolutionary change, by four basic principles,

  • based on Darwin's observations.

  • The first principle is that different members a population

  • have all kinds of individual variations.

  • These characteristics, whether their body size,

  • hair color, blood type, facial markings, metabolisms

  • or reflexes, are called phenotypes.

  • The second is that many variations are heritable and can be passed

  • on to offspring. If a trait happens to be favorable,

  • it does future generations no good if it can't be passed on.

  • Third: this one tends to get glossed over a lot, even though

  • it's probably the most interesting, is Darwin's observation that

  • populations can often have way more offspring than resources,

  • like food and water, can support.

  • This leads to what Darwin called "the struggle for existence."

  • He was inspired here by the work of economist Thomas Malthus,

  • who wrote that when human populations get too big,

  • we get things like plague and famine and wars,

  • and then only some of us survive and continue to reproduce.

  • If you missed the SciShow Infusion that we did on human overpopulation

  • today and Malthus's predictions, you should check it out now.

  • This finally leads to the last principle of natural selection,

  • which is that, given all of this competition for resources,

  • heritable traits that affect individuals' fitness can lead to

  • variations in their survival and reproductive rates.

  • This is just another way of saying that those with favorable traits

  • are more likely to come out on top and will be more successful

  • with their baby-making.

  • So to wrap all these principles together, in order for natural

  • selection to take place, a population has to have

  • variations, some of which are heritable, and when a variation

  • makes an organism more competitive, that variation will

  • tend to be selected.

  • Like with the peppered moth. It survived because there was

  • variation within the species, the dark coloration,

  • which was heritable, and in turn allowed every moth

  • that inherited that trait to

  • better survive the hungry birds of London.

  • But notice how this works. A single variation in a single

  • organism is only the very beginning of the process.

  • The key is that individuals don't evolve.

  • Instead, natural selection produces evolutionary change

  • because it changes the genetic composition of entire populations,

  • and that occurs through interactions between individuals

  • and their environment.

  • Let's get back to Darwin for a minute.

  • In 1870, Darwin wrote to his neighbor and parliamentarian

  • John Lubbock requesting that a question be added to England's

  • census regarding the frequency of cousins marrying and the

  • health of their offspring.

  • His request was denied, but the question was something

  • that weighed heavily on Darwin's mind,

  • because he was married to Emma Wedgwood, who happened to be

  • his first cousin.

  • Her grandfather was Josiah Wedgwood,

  • founder for the company that remains famous for its

  • pottery and china.

  • Oh, and he was also Darwin's grandfather.

  • In fact, much of Darwin's family tree was...complicated.

  • His marriage to Emma was far from the first Wedgewood-Darwin pairing.

  • Darwin's maternal grandparents and mother were also Wedgwoods,

  • and there were several other marriages between cousins

  • in the family, though not always between those two families.

  • So Darwin, and to a greater extent his children, carried more genetic

  • material of Wedgwood origin than Darwininan. And this caused

  • some problems, the likes of which Darwin was all too aware of,

  • thanks to his own scientific research.

  • Darwin of course spent time studying the effects of

  • crossbreeding and inbreeding in plants and animals,

  • noting that consanguineous pairs often resulted in weaker

  • and sickly descendants. And the same was true of his family.

  • Emma and Charles had 10 children, three of whom died in childhood

  • from infectious disease, which is more likely to be

  • contracted by those with high levels of inbreeding.

  • And while none of Darwin's seven other children had any deformities,

  • he noted that they were "not very robust"

  • and three of them were unable to have children of their own,

  • likely another effect of inbreeding.

  • Now, so far we've been talking about natural selection in terms

  • of physical characteristics, like beak shape or coloration.

  • But it's important to understand that it's not just organism's

  • physical form, or its phenotype, that's changing but its

  • essential genetic form, or genotype.

  • The heritable variations we've been talking about are a function

  • of the alleles that organisms are carrying around. And as organisms

  • become more successful, evolutionarily speaking,

  • by surviving in larger numbers for longer and having more kids,

  • that means that the alleles that mark their variation

  • become more frequent.

  • But these changes can come about in different ways.

  • To understand how, let's walk through the different

  • modes of selection.

  • The mode we've been talking about for much of this episode is an

  • example of directional selection, which is when a favored trait is

  • at one extreme end of the range of traits, like from short to tall,

  • or white to black, or blind to having super-night-goggle vision.

  • Over time this leads to distinct changes in the frequency of that

  • expressed trait in a population, when a single phenotype is favored.

  • So our peppered moth is an example of a population's trait

  • distribution shifting toward one extreme, almost all whitish moths,

  • to the other extreme, almost all blackish.

  • Another awesome example is giraffe necks. They've gotten

  • really long over time because there was selection pressure

  • against short necks, which couldn't reach all of those

  • delicious leaves.

  • But there's also stabilizing selection, which selects against

  • extreme phenotypes and instead favors the majority that are well

  • adapted to an environment. An example that's often used is a

  • human's birth weight: Very small babies have a harder time defending

  • themselves from infections and staying warm, but very large

  • babies are too large to deliver naturally. Because of this, the

  • survival rate for babies has historically been higher for those

  • in the middle weight range, which helped stabilize

  • average birth weight. At least, until Cesarian sections

  • became as common as bad tattoos.

  • So what happens when the environment favors extreme traits

  • at both ends of the spectrum, while selecting against

  • the common traits? That's disruptive selection.

  • Now examples of this are rare, but scientists think they found

  • an instance of it in 2008, in a lake full of tiny crustaceans

  • called Daphnia. The population was hit with an

  • epidemic of yeast parasite, and after about a half-dozen

  • generations, a variance had emerged in how the Daphnia

  • responded to the parasite. Some became less susceptible to

  • the yeast, but were smaller and had fewer offspring. The others

  • actually became more susceptible to the parasite, but were bigger

  • and able to reproduce more, at least while they were

  • still alive. So there were two traits that were

  • being selected for, both in extremes and both to the exclusion

  • of each other: susceptibility and fecundity.

  • If you got one, you didn't get the other.

  • An interesting example, of selection being

  • driven by a parasite.

  • Now while these are the main ways that selective pressures can

  • affect populations, those pressures can also come from

  • factors other than environmental ones like food supply or predators

  • or parasites. There's also sexual selection,

  • another concept introduced by

  • Darwin and described in The Origin of Species as depending

  • "not on a struggle for existence, but a struggle between individuals

  • of the same sex, generally the males, for the possession

  • of the other sex."

  • Basically, for individuals to maximize their fitness,

  • they not only need to survive but they also need to reproduce more,

  • and they can do that one or two ways:

  • One, they can make themselves attractive to the opposite sex.

  • Or two, they can go for the upper hand by intimidating, deterring

  • or defeating the same-sex rivals.

  • The first of these strategies is how we ended up with this:

  • I mean, the peacock tail isn't exactly camouflage. But the more

  • impressive the tail, the better chances a male will find a mate

  • and pass its genes to the next generation.

  • Sad-looking peacock tails will diminish over generations,

  • making it a good example of directional sexual selection.

  • The other strategy involves fighting, or at least looking like=

  • you want to fight, for the privilege of mating,

  • which tends to select for bigger or stronger

  • or meaner-looking mates.

  • And finally, thanks to us humans there are also un-natural forms

  • of selection, and we call that artificial selection.

  • People have been artificially selecting plants and animals

  • for thousands of years, and Darwin spent a lot of time in Origin of

  • Species talking about the breeding of pigeons and cattle and plants

  • to demonstrate the principles of selection.

  • We encourage the selection of some traits and discourage others.

  • It's how we got grains that produce all those nutrients.

  • Which is how we managed to turn the gray wolf into domesticated

  • dogs that can look like this

  • or like that, two of my favorite examples of artificial selection.

  • Now these are different breeds of dogs-

  • Oh, where you goin'? No. No.

  • But they're both still dogs. They're the same species.

  • Technically, a corgi and a greyhound could get together and

  • have a baby dog, though it would be a weird looking dog.

  • But, what happens when selection makes populations so different

  • that they can't even be the same species any more?

  • Well, that's what we're going to talk about next episode on

  • Crash Course Biology: how one species can turn

  • into another species.

  • In the meantime, feel free to review what we've gone over today,

  • ask us questions down in the comments below,

  • or on Facebook or Twitter,

  • We'll see you next time. [WOOF]

Hi, I'm Hank. And I'm a human, but let's pretend for a moment

字幕と単語

ワンタップで英和辞典検索 単語をクリックすると、意味が表示されます

B2 中上級

自然選択 - クラッシュコース生物学 #14 (Natural Selection - Crash Course Biology #14)

  • 60 11
    Chi-feng Liu に公開 2021 年 01 月 14 日
動画の中の単語