Variation and selection
IGCSE Biology Topic 18 7:35 English narration · English + 中文 subtitles burned in
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In the year eighteen hundred, almost every peppered moth in England was pale.
在一八零零年，英国几乎每一只桦尺蛾都是浅色的。
Fifty years later, in the smoky cities, almost every one was black.
五十年后，在烟雾弥漫的城市里，几乎每一只都是黑色的。
No moth changed colour.
没有一只蛾子改变了颜色。
So what happened?
那么发生了什么？
The answer is the most important idea in biology, and by the end of this lesson you will be able to explain it in five clear steps.
答案是生物学里最重要的一个想法， 学完这节课，你就能用五个清楚的步骤把它解释出来。
Welcome to variation and selection.
欢迎来到变异与选择。
Two kinds of variation, where new alleles come from, and how nature and humans both choose who breeds.
两种变异、新的等位基因从哪里来， 以及自然和人类如何各自选择谁来繁殖。
Let's begin.
让我们开始吧。
Variation means the differences between individuals of the same species.
变异指的是同一物种内个体之间的差异。
There are two kinds, and the exam always wants you to tell them apart.
它分成两种，考试总是要你把它们区分开。
Continuous variation gives a smooth range of phenotypes between two extremes.
连续变异给出两个极端之间平滑的一系列表现型。
Body length and body mass are the standard examples: people are not tall or short, they are every height in between.
身长和体重是标准例子：人不是只有高和矮，而是中间的每一个高度都有。
Discontinuous variation gives a small number of separate groups with nothing in between.
不连续变异给出少数几个分开的组，中间没有过渡。
Blood group is the standard example: you are A, or B, or AB, or O, and never halfway between two of them.
血型是标准例子：你是 A 型、B 型、AB 型或 O 型，绝不会介于两种之间。
Now the cause, and this is a favourite exam question.
现在讲原因，这是考试很爱考的一题。
Discontinuous variation is usually caused by genes only.
不连续变异通常只由基因引起。
Nothing you eat or do will move you from blood group A to blood group B — the ABO blood groups are fixed by your genes.
你吃什么、做什么都不会把你从 A 型血变成 B 型血。
Continuous variation is caused by both genes and the environment.
连续变异由基因和环境共同引起。
Your genes set a range for your height, and your diet decides where inside that range you finish.
你的基因给身高设定一个范围，你的饮食决定你最后落在这个范围里的哪个位置。
That mixing of two causes is exactly why the result is a smooth range and not separate groups.
正是这两个原因的混合，才让结果是平滑的一系列，而不是分开的几组。
Where does variation come from in the first place?
那么变异最初是从哪里来的？
Mutation.
突变。
A mutation is a genetic change, and it is the way new alleles are made.
突变是遗传上的改变，它是产生新的等位基因的方式。
That one sentence matters more than it looks: every allele that has ever existed started as a mutation, so mutation is the original source of all variation.
这一句话比看起来更重要：曾经存在过的每一个等位基因，最初都是一次突变， 所以突变是所有变异的最初来源。
For the supplement, be precise. A gene mutation is a random change in the base sequence of DNA.
补充内容要说得精确：基因突变是 DNA 碱基序列中随机的改变。
Change one base and the protein may change, and a new version of the gene now exists.
改变一个碱基，蛋白质就可能改变，这个基因就有了一个新版本。
Mutations happen naturally all the time, but two things raise the rate of mutation: ionising radiation, such as X-rays, and some chemicals.
突变一直都在自然发生，但有两样东西会提高突变的速率： 电离辐射，例如 X 射线，还有某些化学物质。
Notice the wording.
注意用词。
They do not decide which mutation happens.
它们并不决定发生哪一种突变。
Mutation is random, and the radiation only makes random changes more frequent.
突变是随机的，辐射只是让随机的改变更频繁。
For the supplement, you should be able to list four sources of genetic variation in a population: mutation, meiosis, random mating, and random fertilisation.
补充内容要求你能列出种群中遗传变异的四个来源： 突变、减数分裂、随机交配和随机受精。
Mutation makes the new alleles; the other three keep shuffling them into new combinations.
突变制造新的等位基因，另外三个不断把它们洗成新的组合。
An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.
适应特征是一种遗传的特征，帮助生物在它的环境中生存和繁殖。
Three parts, and the mark scheme wants all three: inherited, helps it survive, helps it reproduce.
三个部分，评分标准三个都要：遗传的、帮助生存、帮助繁殖。
The camel is the classic example.
骆驼是经典例子。
It stores fat in its humps rather than all over its body, so heat escapes more easily.
它把脂肪储存在驼峰里，而不是遍布全身，所以热量更容易散出去。
It can go a long time without water, and its long eyelashes keep sand out.
它能很长时间不喝水，长长的睫毛把沙子挡在外面。
Every one of those is inherited, not something the animal learned.
这些每一样都是遗传来的，而不是这只动物学来的。
The supplement asks for two plant groups.
补充内容要求两类植物。
Hydrophytes are water plants.
水生植物生活在水里。
They have large air spaces to help them float, and their stomata are on the upper leaf surface, because that is the side in contact with the air.
它们有很大的气腔帮助漂浮，气孔长在叶的上表面，因为那一面才接触空气。
Xerophytes are desert plants.
旱生植物生活在沙漠。
They have a thick waxy cuticle, few small stomata, often sunk into pits, and many can store water in thick stems or leaves.
它们有厚厚的蜡质角质层、少而小的气孔，常常凹陷在小坑里， 很多还能把水储存在粗壮的茎或叶里。
Every feature cuts water loss.
每一个特征都在减少水分流失。
Two groups solving the same problem from opposite sides.
两类植物从相反的两边解决同一个问题。
Here is natural selection, and I want you to learn it as five steps in order.
这就是自然选择，我要你按顺序把它记成五个步骤。
One: there is genetic variation in the population, made by mutation.
第一：种群中存在遗传变异，由突变产生。
Two: organisms produce many more offspring than can possibly survive.
第二：生物产生的后代远远多于能够存活下来的数量。
Three: so there is a struggle to survive, with competition for resources like food and space.
第三：因此出现生存斗争，为食物和空间等资源展开竞争。
Four: the individuals that are better adapted have a greater chance of surviving and reproducing.
第四：适应得更好的个体有更大的机会存活并繁殖。
Five: they pass their alleles to the next generation, so the helpful allele becomes more common.
第五：它们把等位基因传给下一代，于是有利的等位基因变得更常见。
Write those five and you have the marks.
把这五点写出来，分数就到手了。
Watch it happen.
来看它发生的过程。
The pale moths rest on pale bark and the birds cannot see them.
浅色的蛾停在浅色的树皮上，鸟看不见它们。
Then soot from the factories darkens the trunks.
然后工厂的煤烟把树干熏黑。
Now the pale moths show up and the few dark ones are hidden, so birds eat more pale moths.
现在浅色的蛾很显眼，而少数深色的被藏了起来， 所以鸟吃掉更多浅色的蛾。
The survivors breed, and each generation carries more of the dark allele, until almost the whole population is dark.
存活下来的繁殖， 每一代携带的深色等位基因都更多，直到几乎整个种群都是深色的。
Now write the answer.
现在把答案写出来。
The dark moths were already there, a few of them, made by chance mutation.
深色的蛾本来就有，只有少数几只，是偶然的突变造出来的。
On sooty bark they are better camouflaged, so birds eat fewer of them and more of them survive.
在被煤烟熏黑的树皮上，它们伪装得更好，所以鸟吃掉的更少，存活下来的更多。
Those survivors reproduce and pass on the dark allele, so it becomes more common in every generation.
这些幸存者繁殖并传下深色的等位基因，于是它每一代都变得更常见。
Follow that chain: variation, survival, reproduction, the allele spreads.
跟着这条链走：变异、生存、繁殖、等位基因扩散。
And avoid the trap.
还要避开那个陷阱。
The moths did not turn dark in order to hide.
蛾子不是为了躲藏才变黑的。
The dark ones simply survived more often.
只是深色的那些活下来的次数更多。
The same five steps explain antibiotic resistance, and this is the example examiners love.
同样的五个步骤也解释了抗生素耐药性，这是考官最喜欢的例子。
Before any treatment, a few bacteria already carry a resistant allele, made by mutation.
在任何治疗之前，就已经有少数细菌携带耐药的等位基因，由突变产生。
Give the antibiotic, and it kills the ones that are not resistant.
用了抗生素，它把不耐药的那些杀死。
The resistant ones survive, and bacteria divide very fast, so within days the whole population is resistant.
耐药的存活下来，而细菌分裂非常快， 所以几天之内整个种群都耐药了。
Over many generations like this, the population becomes more suited to its environment.
经过很多这样的世代，种群变得更适合它的环境。
That process is called adaptation.
这个过程叫做适应。
Now the human version.
现在讲人类的版本。
In selective breeding, also called artificial selection, three steps.
在选择育种，也叫人工选择，一共三步。
One: choose the individuals that have the feature you want.
第一：选出具有你想要的特征的个体。
Two: cross them to produce the next generation.
第二：让它们杂交，产生下一代。
Three: choose the offspring that show the feature, and repeat.
第三：选出表现出这个特征的后代，然后重复。
That last word is the one students forget.
最后那个词是学生最容易忘的。
It only works over many generations.
它只有经过很多世代才有效。
This is how we get crops with a higher yield and cows that give more milk.
我们就是这样得到产量更高的农作物和产奶更多的奶牛。
So what is the difference?
那么区别是什么？
In natural selection, the environment decides which individuals survive.
在自然选择中，是环境决定哪些个体存活。
In artificial selection, humans decide which individuals breed.
在人工选择中，是人类决定哪些个体繁殖。
Everything else is the same machinery: variation must already exist, selection acts on it, and the chosen alleles become more common over many generations.
其余的机制完全一样：变异必须已经存在，选择作用在它上面， 被选中的等位基因经过很多世代变得更常见。
One sentence, one clear difference.
一句话，一个清楚的区别。
That is the mark.
那就是得分点。
Four things to take away.
有四点要带走。
First, range means continuous and genes plus environment; separate groups means discontinuous and genes only.
第一，一系列连续的值就是连续变异，由基因加环境引起； 分开的几组就是不连续变异，只由基因引起。
Second, mutation makes new alleles, so it is the source of all variation.
第二，突变产生新的等位基因，所以它是所有变异的来源。
Third, write natural selection as those five steps in order, and never say an organism changed because it needed to.
第三，把自然选择按顺序写成那五个步骤， 绝不要说一个生物因为需要才发生了改变。
Fourth, in natural selection the environment chooses; in selective breeding, humans choose.
第四，自然选择由环境来选，选择育种由人类来选。