Natural Selection
AP Biology Topic 7 8:10 English narration · English + 中文 subtitles burned in
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Transcript
This is a dish of bacteria.
这是一皿细菌。
Each white disc holds a different antibiotic — a drug made to kill them.
每个白色圆片上都放着一种不同的抗生素——一种用来杀死它们的药。
Look at the clear rings.
看这些透明的圆环。
A wide ring means the drug still works.
环越宽，说明这种药仍然有效。
Where there is no ring, the bacteria grew right up to the disc.
哪里没有环， 细菌就一直长到了圆片边上。
Nobody designed them.
没有人设计它们。
Evolution did, in a few years.
是进化做到的，只用了几年。
Natural selection explains it, in five steps.
自然选择可以解释这件事，一共五步。
First, individuals are not identical. There is variation.
第一，个体并不完全相同，存在变异。
Second, more offspring are born than can survive.
第二，出生的后代比能活下来的多。
Third, they compete for food, space and mates.
第三，它们要为食物、空间和配偶而竞争。
Fourth, the best-adapted survive and reproduce more.
第四，适应得最好的存活并留下更多后代。
Fifth, they pass their alleles on.
第五，它们把自己的等位基因传下去。
Over many generations the population becomes better adapted — the population, not one individual. The chain to write is: variation, then differential survival and reproduction, then helpful traits become common. Well-suited traits are adaptations.
经过许多世代，整个种群变得更适应环境——是种群，而不是某一个个体。
Here is the classic case.
这是最经典的例子。
Pale and dark moths rest on pale bark.
浅色和深色的桦尺蛾停在浅色的树皮上。
The dark ones are easy to see, so birds eat them first.
深色的很显眼， 所以鸟先吃掉它们。
The pale moths survive and breed, so the pale form spreads.
浅色的蛾活下来并繁殖，于是浅色型扩散开来。
Then soot blackens the bark, and the pressure flips.
后来煤烟把树皮熏黑，压力就反过来了。
Fitness is not strength. It is how many surviving offspring you leave.
适合度不是强壮， 而是你留下多少能存活的后代。
Selection can push a trait three ways.
选择可以把一个性状推向三个方向。
The dashed curve is before, the solid curve is after.
虚线是之前，实线是之后。
In stabilizing selection the middle wins, so the curve narrows.
在稳定选择中，中间取胜，曲线变得更窄。
In directional selection one extreme wins, and the whole curve slides across.
在定向选择中，一个极端取胜，整条曲线向一侧移动。
In disruptive selection both extremes win, and the middle is lost.
在分裂选择中，两个极端都取胜，中间被淘汰。
Humans do the same on purpose.
人类也有意做同样的事。
In artificial selection we choose which individuals breed.
在人工选择中，由我们决定谁来繁殖。
Start with a mixed generation.
先从一个混杂的世代开始。
Keep the ones with the feature you want, and breed them.
只留下具有你想要性状的个体，让它们繁殖。
In the next generation that feature is more common.
到下一代，这个性状就更常见了。
Repeat, and you get an improved variety.
不断重复，就得到改良品种。
Same mechanism — we replace the environment.
机制完全一样——只是我们取代了环境。
Now zoom out.
现在把视野放大。
The gene pool is every allele in the population, and evolution means those frequencies change.
基因库是种群中所有的等位基因，进化就是这些频率发生改变。
Selection is only one cause.
选择只是原因之一。
Mutation makes brand-new alleles.
突变产生全新的等位基因。
Genetic drift changes frequencies by chance, strongest in a small population.
遗传漂变靠随机改变频率，在小种群中最明显。
Gene flow is migration: individuals move in or out with their alleles.
基因流就是迁移：个体带着自己的等位基因迁入或迁出。
Drift is worth watching, because it is pure luck.
漂变值得看一看，因为它完全是运气。
Here is one allele, with no selection at all.
这里是一个等位基因，没有任何选择。
In a large population the frequency barely moves — thousands of individuals cancel out each other's luck.
在大种群中，频率几乎不动——成千上万个体的运气互相抵消。
In a small population there is no such safety.
在小种群中就没有这种保障。
The line wanders, and the allele is lost.
曲线随意游走，等位基因被丢失了。
That is chance, not selection.
这是偶然，不是选择。
Two special cases matter for the exam.
有两种特殊情况在考试中很重要。
A bottleneck: a large, varied population is cut down by a disaster, and only a few survive.
瓶颈效应：一个大而多样的种群被灾难压缩， 只有少数存活下来。
Those few carry only some of the alleles.
这少数个体只带着一部分等位基因。
When numbers grow again, the population is large but the variety is gone.
当数量重新增长时，种群变大了，但多样性回不来了。
The founder effect is the same idea: a few individuals start a new population.
奠基者效应是同一个道理：少数个体去建立一个新种群。
How do we tell whether a population is evolving at all?
我们怎么判断一个种群到底有没有在进化？
First we need a picture of one that is not: the Hardy Weinberg model.
先要有一个没有进化的参照， 也就是哈迪-温伯格模型。
Let the generations pass.
让世代一代代过去。
Nothing moves, so nothing is evolving.
什么都没动，所以没有在进化。
The first equation says the two allele frequencies add up to one.
第一个式子说，两个等位基因频率加起来等于一。
The second gives the genotypes.
第二个给出基因型的比例。
Read the three terms: homozygous dominant, heterozygous carriers, homozygous recessive.
看这三项：显性纯合、杂合携带者、隐性纯合。
It holds under five conditions.
它只在五个条件下成立。
A large population.
种群足够大。
No migration.
没有迁移。
No new mutations.
没有新的突变。
Random mating.
随机交配。
And no natural selection.
以及没有自然选择。
No real population meets all five — that is the point: it is a null hypothesis.
真实种群从来不能同时满足这五条——这正是关键：它是一个零假设。
Numbers that do not match mean the population is changing.
数据对不上，就说明这个种群正在改变。
Now the exam's favourite question.
现在是考试最爱考的题。
Sixteen percent of a population show the recessive phenotype.
一个种群中有百分之十六表现出隐性性状。
What fraction are carriers?
携带者占多少？
Pause here and try it yourself.
先暂停，自己试一试。
Ready?
好了吗？
Start from the recessive phenotype: only the recessive homozygote shows it, so q squared is zero point one six.
从隐性性状入手：只有隐性纯合体才会表现出来， 所以 q 的平方是零点一六。
Take the square root: q is zero point four, so p is zero point six.
开平方：q 等于零点四，于是 p 等于零点六。
Carriers are the heterozygotes: two times zero point six times zero point four — forty-eight percent.
携带者就是杂合子：二乘零点六乘零点四——百分之四十八。
Check it: the three add to one.
检查一下：三项加起来等于一。
Separate kinds of evidence all agree.
不同来源的证据彼此一致。
The fossil record shows organisms changing over time, and the rocks can be dated.
化石显示生物随时间改变，而且岩石可以定年。
Homologous structures — the same bones in a whale flipper and your arm — point to a shared ancestor, and vestigial structures are the leftovers of organs a lineage no longer uses.
同源结构——鲸的鳍和你的手臂里是同样的骨头——指向共同祖先； 而痕迹器官则是某个类群不再使用的器官留下的残余。
Shared embryology says the same thing earlier: related animals look alike as embryos.
共同的胚胎发育在更早的阶段说了同样的话：亲缘关系近的动物在胚胎期长得很像。
Molecular evidence is strongest: related species share DNA sequences.
分子证据最有力：亲缘关系近的物种共享 DNA 序列。
And biogeography explains where species live.
生物地理学则解释物种为什么分布在那里。
Common ancestry goes deeper than bones.
共同祖先比骨头更深一层。
Look at this fossil.
看这块化石。
It has feathers, but also teeth and a long bony tail.
它有羽毛，但也有牙齿和一条长长的骨质尾巴。
It sits between two groups we now keep far apart.
它正好位于我们今天分得很开的两个类群之间。
Now look at the tree of all life.
现在看这棵生命之树。
Every branch uses DNA, the same genetic code, ribosomes and ATP.
每一个分支都使用 DNA、同一套遗传密码、核糖体和 ATP。
Those shared basics point to one ancestor.
这些共同的基本特征指向同一个祖先。
This is a phylogenetic tree, or cladogram.
这是一棵分支图。
Here are four animals.
这里有四种动物。
The lancelet is the out-group: least related, and it anchors the tree.
文昌鱼是外类群：它亲缘关系最远， 用来给整棵树定位。
Everything above it shares jaws.
在它上面的都有颌。
A smaller group also shares four limbs.
更小的一组还共同有四肢。
Only the lizard adds an amniotic egg.
只有蜥蜴多了羊膜卵。
Each mark is a shared derived character, and each branch point is the most recent common ancestor.
每一个标记都是一个共同衍生特征， 每一个分支点都是最近共同祖先。
A tree is a hypothesis — new data can redraw it.
一棵树是一个假说——新数据可以把它重画。
A new species forms when populations become reproductively isolated.
新物种的形成需要生殖隔离。
In allopatric speciation a physical barrier splits one population: a river, a mountain, a new sea.
在异域物种形成中，一道物理屏障把一个种群分开： 一条河、一座山、一片新的海。
Kept apart, the two halves collect different mutations, until they can no longer breed together.
被隔开之后，两半各自积累不同的突变， 直到再也不能一起繁殖。
In sympatric speciation there is no barrier — the split happens in the same place, through behaviour or timing.
在同域物种形成中没有屏障—— 分化在同一个地方发生，靠行为或时间上的差异。
Now we can answer the dish from the start.
现在我们可以回答开头那个培养皿了。
In any large population of bacteria, a few already carry a resistance mutation — by chance, before any drug arrives.
在任何一个大的细菌种群里， 都会有少数个体本来就带着抗性突变——这是偶然的，在药物出现之前就有了。
Give the antibiotic, and it kills the rest.
用上抗生素，它会杀死其余的细菌。
The resistant few are left, and they multiply fast.
剩下的就是那些抗性个体，它们迅速繁殖。
A population with plenty of diversity survives a shock; one with little can be wiped out.
变异丰富的种群能挺过冲击；变异很少的可能被整个消灭。
One last question: where did life begin?
最后一个问题：生命从哪里开始？
Earth formed four point six billion years ago, too hot for life until three point nine.
地球形成于约四十六亿年前， 直到三十九亿年前都太热，无法孕育生命。
Layered rocks like these are built by mats of microbes, going back three point five billion years.
像这样的层状岩石由微生物席堆积而成，可以追溯到三十五亿年前。
The first cells were prokaryotes; eukaryotes came later, by endosymbiosis — one cell taking another inside it.
像这样的海底热液喷口可能提供了所需的能量和化学物质。
Hot vents like this one could have supplied the energy and the chemicals. In the RNA world idea, RNA came first, because it can both store information and act as a catalyst.
按照 RNA 世界的设想，RNA 最先出现，因为它既能储存信息，又能起催化作用。
The first cells were simple prokaryotes.
最早的细胞是简单的原核生物。
Five marks students lose.
五个最容易丢分的地方。
One: define selection in full — variation, heritable, and more offspring than can survive.
第一，把自然选择说完整——变异、可遗传、 后代多于能存活的数量。
Two: fitness means reproductive success, not strength.
第二，适合度指繁殖成功，不是强壮。
Three: populations evolve, individuals do not.
第三，进化的是种群，个体不会进化。
Four: in Hardy Weinberg, always start from the recessive phenotype — the only genotype you can see.
第四，做哈迪-温伯格题一定从隐性性状入手—— 那是唯一你能直接看到的基因型。
Five: name your evidence — fossils, homology, DNA.
第五，把证据说出名字——化石、同源结构、DNA。
Get those right, and this unit is yours.
做到这几点，这一单元就拿下了。