Variation and Evolution
A-Level Biology Topic 17 8:23 English narration · English + 中文 subtitles burned in
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In the early eighteen hundreds, almost every peppered moth in England was pale and speckled, which hid it beautifully on lichen-covered bark.
在十九世纪初，英格兰几乎每一只桦尺蛾都是浅色带斑点的， 这让它们在长满地衣的树皮上藏得极好。
Then the factories came, the soot blackened the trees, and within about fifty years the dark form had taken over the industrial areas.
后来工厂来了，煤烟把树熏黑， 大约五十年之内，深色型就在工业区占了上风。
Nothing designed that.
这一切并没有谁去设计。
The birds simply ate whichever moth they could see, and the bark decided which allele spread.
鸟只是把它们能看见的蛾吃掉，而树皮决定了哪个等位基因扩散开来。
In this lesson we take that process apart and learn to calculate with it.
这节课我们把这个过程拆开，并学会用它来计算。
Variation just means the differences between individuals, and it has three possible causes.
变异就是指个体之间的差异，它有三种可能的原因。
Some variation is genetic only, set purely by the alleles you inherit — human blood group is the standard example.
有些变异纯粹是遗传的，完全由你继承到的等位基因决定——人的血型是标准例子。
Some is down to environmental factors only, set by your surroundings, like a scar or the language you speak.
有些纯粹是环境造成的，由你的周围环境决定，比如一道疤，或者你说的语言。
And most variation is a combination of both: height and body mass depend on your genes and on your diet and lifestyle.
而大多数变异是两者结合：身高和体重既取决于基因，也取决于饮食和生活方式。
Variation also comes in two patterns.
变异还有两种模式。
Discontinuous variation falls into clear separate groups with nothing in between, like blood group A, B, AB or O, and it is usually controlled by one or a few genes.
不连续变异落在界限分明的几个组里，中间没有过渡， 比如血型 A、B、AB 或 O，通常由一个或少数几个基因控制。
Continuous variation is a smooth range from one extreme to the other, like height, and it is controlled by many genes together plus the environment.
连续变异是从一个极端到另一个极端的平滑范围，比如身高， 由许多基因共同控制，再加上环境。
To compare the means of two samples, you use a t-test.
要比较两个样本的平均值，就用 t 检验。
Watch the population change.
看着这个种群发生变化。
At the start the pale form is common and the dark form is rare.
一开始浅色型很常见，深色型很少。
Change the background, and the moths that stand out get eaten while the ones that blend in survive to breed.
改变背景之后，显眼的蛾被吃掉，而与背景融为一体的活下来繁殖。
Nothing about any individual moth changes.
没有任何一只蛾本身发生了改变。
What changes is how common each form is in the population.
改变的是每种类型在种群中所占的比例。
Write natural selection out as a sequence and you will not lose marks.
把自然选择写成一个顺序，你就不会丢分。
First, variation already exists in the population.
第一，种群中本来就存在变异。
Second, a population produces far more offspring than can possibly survive, so they must compete for food and space — the struggle for existence.
第二，种群产生的后代远远多于能够存活的数量，所以它们必须争夺食物和空间——生存斗争。
Third, a selection pressure acts.
第三，选择压力起作用。
The individuals that are best adapted to the conditions are the most likely to survive, reproduce, and pass their alleles on.
最适应当前条件的个体最有可能存活、繁殖并把等位基因传下去。
And fourth, over many generations the helpful alleles become more common.
第四，经过许多世代，有利的等位基因变得越来越常见。
That last step is the one students leave out, and it is the one that says what evolution actually is.
最后这一步是学生最常漏掉的，而它恰恰说明了进化到底是什么。
Antibiotic resistance is exactly this sequence: a chance mutation makes a few bacteria resistant, the drug kills the rest, and the survivors reproduce.
抗生素耐药性正是这个顺序：一个偶然的突变让少数细菌产生耐药性， 药物把其余的杀死，幸存者繁殖起来。
Selection can push a population in three different directions, and you should be able to tell them apart from a graph.
选择可以把一个种群推向三个不同的方向，你应该能从图上把它们区分开。
Stabilising selection favours the average and removes both extremes, so the distribution gets narrower but the mean stays where it was.
稳定选择偏好平均值、去掉两个极端，所以分布变窄，但平均值留在原处。
Directional selection favours one extreme, so the whole distribution shifts that way and the mean moves — the peppered moth is the classic case.
定向选择偏好某一个极端，于是整个分布朝那边移动，平均值也跟着移动—— 桦尺蛾就是经典例子。
Disruptive selection favours both extremes and removes the average, so a single peak splits into two.
分裂选择偏好两个极端、去掉平均值，于是单峰分裂成双峰。
When you describe one, say what happens to the mean and what happens to the range.
描述其中一种时，要说清平均值发生了什么、范围发生了什么。
Naming the type alone is only half the answer.
只说出名称，只答对了一半。
Allele frequencies can also change without any selection at all, and there are three named cases.
等位基因频率也可能在完全没有选择的情况下改变，有三种有名字的情况。
The founder effect: a few individuals leave and start a new population, so they carry only some of the alleles that were in the original group, purely by which individuals happened to go.
奠基者效应：少数个体离开去建立一个新种群，所以它们只带走了原来群体中的一部分等位基因， 纯粹取决于碰巧是哪些个体走了。
Genetic drift: in a small population, allele frequencies wander from generation to generation by chance alone, because only a sample of the alleles gets passed on.
遗传漂变：在小种群里， 等位基因频率仅仅因为偶然就在世代之间游走，因为每次只有一部分等位基因被传下去。
And the bottleneck effect: a sudden crash in population size leaves very few survivors, so most of the variety is lost, and the population that grows back is far less varied than the one before it.
还有瓶颈效应：种群数量突然崩塌，只剩下极少数幸存者， 于是大部分多样性丧失了，重新长回来的种群比原来的单调得多。
The Hardy-Weinberg principle lets you calculate allele and genotype frequencies in a population.
哈迪-温伯格原理让你能计算一个种群里的等位基因频率和基因型频率。
Call the frequency of the dominant allele p, and the frequency of the recessive allele q.
把显性等位基因的频率记作 p，隐性等位基因的频率记作 q。
If there are only two alleles, then p plus q must equal one.
如果只有两个等位基因，那么 p 加 q 必然等于一。
The genotype frequencies also add up to one, which gives the second equation: p squared plus two p q plus q squared equals one.
基因型频率加起来也等于一，于是得到第二个方程：p 平方加二 p q 加 q 平方等于一。
Learn what each term means.
要记住每一项的含义。
P squared is the homozygous dominant, q squared is the homozygous recessive, and two p q is the heterozygotes — the carriers, which is usually what a question asks for.
p 平方是显性纯合子，q 平方是隐性纯合子， 而二 p q 是杂合子——也就是携带者，题目通常问的就是这个。
One warning: this only holds for a large population with random mating, and no mutation, no migration and no natural selection.
有一点要注意：这只在大种群、随机交配、且没有突变、没有迁移、没有自然选择时成立。
Here is the standard calculation.
这是标准的计算题。
A recessive condition affects one person in every hundred.
一种隐性疾病每一百人中有一人患病。
How many are carriers?
有多少人是携带者？
Always start from q squared, because only the homozygous recessive genotype actually shows the condition.
永远从 q 平方开始，因为只有隐性纯合的基因型才真正表现出这种病。
So q squared is one over a hundred, which is nought point nought one, and q is the square root of that, which is nought point one.
所以 q 平方是一百分之一，也就是零点零一，而 q 是它的平方根，也就是零点一。
Subtract from one to get p, which is nought point nine.
用一减去它得到 p，是零点九。
The carriers are the heterozygotes, two p q, so that is two times nought point nine times nought point one, which is nought point one eight.
携带者是杂合子，也就是二 p q， 于是是二乘零点九乘零点一，等于零点一八。
So about eighteen per cent of the population are carriers — far more than the one per cent who actually show the condition.
所以大约百分之十八的人口是携带者——远远多于真正表现出这种病的百分之一。
That gap is the whole point of the calculation.
这个差距正是这道计算题的意义所在。
Selective breeding, also called artificial selection, is the same machinery with a different driver.
选择育种，也叫人工选择，是同一套机制，只是换了驱动者。
Instead of the environment deciding who breeds, we choose.
不是由环境来决定谁繁殖，而是我们来选。
Each generation, only the individuals with the feature we want are allowed to reproduce, so that feature becomes more and more common.
每一代只让具有我们想要的特征的个体繁殖， 于是那个特征变得越来越普遍。
Three examples worth knowing: breeding disease resistance into varieties of wheat and rice; using inbreeding and hybridisation to make vigorous, uniform maize; and breeding dairy cattle for higher milk yield.
有三个值得记住的例子： 把抗病性育入小麦和水稻的品种；用近交和杂交培育健壮而整齐的玉米； 以及为提高产奶量而选育奶牛。
The mechanism is identical to natural selection — only the selector is different.
机制与自然选择完全相同——只是选择者不同。
Evolution is the slow formation of new species from earlier ones, as the gene pool — all the alleles in a population — changes from generation to generation.
进化是新物种从早先的物种缓慢形成的过程，随着基因库—— 也就是一个种群里的全部等位基因——一代一代地改变。
DNA sequence data can show how closely two species are related: the more similar their sequences, the more recently they shared a common ancestor.
DNA 序列数据能显示两个物种的亲缘关系有多近：序列越相似， 它们共有祖先的时间就越近。
Speciation happens when two populations become genetically separated, so they can no longer breed together. That genetic isolation comes about in two ways.
当两个种群在遗传上被隔开、无法再一起繁殖时，就发生物种形成， 而这有两条途径。
Allopatric speciation needs a geographical separation — a sea, a river, a mountain range — which physically keeps the populations apart while they evolve differently.
异域物种形成需要地理屏障——海、河或山脉—— 在物理上把种群隔开，让它们各自演化。
Sympatric speciation happens in the same area, where the populations are separated by differences in behaviour or way of life instead.
同域物种形成发生在同一个区域，种群是被行为或生活方式的差异隔开的。
Either way the result is the same: the two populations become isolated and can no longer interbreed.
无论哪一种，结果都一样：两个种群被隔离，不能再相互交配。
Four things the examiner keeps asking for.
考官反复要的四点。
First, write natural selection as a sequence: variation, then a selection pressure, then the better adapted survive and reproduce, then the allele frequency changes.
第一，把自然选择写成一个顺序：变异，然后是选择压力， 然后是更适应的个体存活并繁殖，然后是等位基因频率改变。
Second, in a Hardy-Weinberg question, start at q squared — that is the frequency of the recessive phenotype, and it is the only thing you can read straight off.
第二，做哈迪-温伯格的题目，要从 q 平方开始—— 那是隐性表现型的频率，也是唯一一个你能直接读出来的量。
Third, when you describe a type of selection, say what happens to the mean and range, not just its name.
第三，描述某种选择类型时，要说清平均值和范围发生了什么，而不只是说出名称。
Fourth, be clear which barrier you mean: allopatric is geographical, sympatric is behavioural.
第四，要讲清楚你指的是哪种屏障：异域是地理上的，同域是行为上的。
So there it is.
就是这样了。
Differences that already exist, a pressure that sorts them, arithmetic that predicts them, and enough time for two populations to stop being one.
本来就存在的差异，一种把它们分选出来的压力， 一套能预测它们的算术，以及足够让两个种群不再是一个种群的时间。
Learn these eight terms and the whole topic holds together.
记住这八个术语，整个主题就串起来了。
See you in the next lesson.
下节课见。