Topic 6: Inheritance, variation and evolution
AQA · GCSE · Biology Tema 6 20:51 Narración en inglés · Subtítulos en inglés + 中文 quemados en pantalla
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Transcripción
The lesson links chromosome number, DNA and inheritance probabilities, then compares natural selection with selective breeding, genetic engineering and cloning.
本课把染色体数目、DNA和遗传概率联系起来，然后把自然选择与选择育种、基因工程和克隆进行比较。
These processes use different mechanisms.
这些过程使用不同的机制。
A strawberry plant makes seeds after fertilisation.
草莓植株在受精后产生种子，也会长出匍匐茎，不经配子融合就形成新植株。
It also grows runners that form new plants without any gametes fusing.
解释每一种途径：哪种是有性生殖，哪种是无性生殖？
Explain each route: which is sexual and which asexual, and which cell division makes the gametes?
产生配子的是哪种细胞分裂？
Why are runner plants genetically identical?
为什么匍匐茎长出的植株遗传上完全相同？
And if a new disease arrives, what advantage could the seed-grown plants have?
如果出现一种新的疾病，由种子长成的植株可能有什么优势？
Pause and try it.
暂停并试着回答。
Seeds: gametes made by meiosis fuse at fertilisation, and mixing the genetic information produces variation among the offspring.
种子：由减数分裂产生的配子在受精时融合，遗传信息的混合使后代产生变异。
Runners: one parent makes new plants by mitosis, with no gametes fusing, so they are clones, apart from mutation.
匍匐茎：由一个亲本通过有丝分裂产生新植株，没有配子融合，所以除突变外，它们都是克隆。
With a new disease, variation means some seed-grown plants may resist it and survive to reproduce.
出现新疾病时，变异意味着有些由种子长成的植株可能抵抗疾病，存活下来并繁殖。
In stable, favourable conditions, asexual reproduction is fast and needs no mate.
在稳定而适宜的条件下，无性生殖速度快，而且不需要配偶。
Neither route guarantees survival.
两种途径都不能保证存活。
A human body cell has forty-six chromosomes, in two sets, and its DNA is copied before meiosis.
人的体细胞有四十六条染色体，分为两套，在减数分裂之前DNA先复制。
After the two divisions, how many gametes form, and how many chromosomes does each have?
经过两次分裂，形成多少个配子？ 每个配子有多少条染色体？
How does fertilisation restore the number?
受精如何恢复染色体数目？
And which division increases the number of cells as the embryo develops?
胚胎发育时，哪种细胞分裂使细胞数目增加？
Pause and try it.
暂停并试着回答。
Meiosis gives four gametes, each with one set of twenty-three chromosomes.
减数分裂产生四个配子，每个配子有一套染色体，即二十三条。
Fertilisation fuses two gametes, restoring two sets, forty-six chromosomes.
受精使两个配子融合，恢复为两套染色体，即四十六条。
Then mitosis grows the embryo.
然后有丝分裂使胚胎生长。
A scientist finds a section of DNA whose base sequence codes for the amino acid sequence of a protein.
一位科学家发现一段DNA，它的碱基序列编码一种蛋白质的氨基酸序列。
Name that section, and say how it relates to a chromosome.
说出这段DNA的名称，以及它与染色体的关系。
How is it different from the organism's genome?
它与生物的基因组有什么区别？
And why could comparing genomes help investigate an inherited disorder?
为什么比较基因组有助于研究遗传病？
Pause and try it.
暂停并试着回答。
DNA is a polymer of two strands, forming a double helix.
DNA是由两条链组成的聚合物，形成双螺旋。
A gene is a section of DNA that codes for the amino acid sequence of a protein.
基因是DNA上编码一种蛋白质氨基酸序列的片段。
A chromosome contains one long DNA molecule and many genes.
一条染色体含有一个很长的DNA分子和许多基因。
And the genome is all of the organism's genetic material.
基因组是生物全部的遗传物质。
Comparing genomes can identify genes linked to an inherited disorder, but the comparison alone does not prove they cause it.
比较基因组可以找出与遗传病有关的基因，但仅凭比较并不能证明这些基因导致了疾病。
A DNA strand contains many nucleotides.
一条DNA链含有许多核苷酸。
Name the three parts of one nucleotide, and the four possible bases.
说出一个核苷酸的三个组成部分，以及四种可能的碱基。
Explain what the order of the bases controls in a protein.
解释碱基的顺序控制蛋白质的什么。
For higher tier: write the complementary bases for the sequence A, C, G, T, keeping the same positions.
高级卷：写出序列A、C、G、T的互补碱基，位置保持对应。
Pause and try it.
暂停并试着回答。
Each nucleotide contains a sugar, a phosphate group and one base: A, C, G or T.
每个核苷酸含有一个糖、一个磷酸基团和一个碱基：A、C、G或T。
Nucleotides join into strands with sugar-phosphate backbones, and two strands form the double helix.
核苷酸连接成链，糖和磷酸构成骨架，两条链形成双螺旋。
A sequence of three bases codes for one amino acid, so the order of the bases sets the order of the amino acids.
三个碱基组成的序列编码一个氨基酸，所以碱基的顺序决定氨基酸的顺序。
For higher tier: A pairs with T, and C pairs with G, so opposite A, C, G, T, write T, G, C, A.
高级卷：A与T配对，C与G配对，所以与A、C、G、T相对的是T、G、C、A。
For higher tier.
高级卷内容。
A mutation changes the base sequence of a gene, and a student says the enzyme must stop working.
一次突变改变了某个基因的碱基序列，一位学生说这种酶一定会失去作用。
Explain the route from the gene to a folded protein, explain why the student is too certain, and suggest how a change in non-coding DNA could affect how much protein is made.
解释从基因到折叠好的蛋白质的过程，说明这位学生的结论为什么过于肯定，并推测非编码DNA的变化如何影响蛋白质的合成量。
Pause and try it.
暂停并试着回答。
In protein synthesis, a template is used at the ribosomes, and carrier molecules bring the amino acids in the coded order.
在蛋白质合成中，模板在核糖体上被使用，载体分子按编码的顺序运来氨基酸。
The chain folds into a specific shape, and an enzyme's active site must fit its substrate.
氨基酸链折叠成特定的形状，酶的活性部位必须与底物吻合。
So a mutation may leave the protein unchanged, alter it slightly, or change its shape enough to affect its function.
所以突变可能使蛋白质不变、稍有改变，或者使形状改变到足以影响功能的程度。
A variant in non-coding DNA can affect whether a gene is switched on or off, changing its expression.
非编码DNA中的变异可以影响基因是开启还是关闭，从而改变基因的表达。
In a simple single-gene model, the dominant allele D causes polydactyly, extra fingers or toes; d does not.
在一个简单的单基因模型中，显性等位基因D导致多指（趾），d则不会。
A person has the genotype D d.
某人的基因型为Dd。
Name the two alleles and say whether the genotype is homozygous or heterozygous.
说出这两个等位基因，并判断这个基因型是纯合子还是杂合子。
Predict the phenotype, and explain 'dominant' without saying 'stronger'.
预测表现型，并在不用“更强”一词的情况下解释“显性”。
Then list the alleles the person's gametes can carry.
然后列出此人配子可能携带的等位基因。
Pause and try it.
暂停并试着回答。
D d contains two different alleles, so the person is heterozygous; D D and d d are homozygous.
Dd含有两个不同的等位基因，所以此人是杂合子；DD和dd是纯合子。
The person has polydactyly, because D is expressed when even one copy is present.
此人有多指（趾），因为只要有一个D，它就会表达。
Each gamete receives one allele, either D or d, never both.
每个配子只得到一个等位基因，D或d，不会两个都有。
Genotype means the alleles present; phenotype means the characteristics expressed.
基因型是指存在的等位基因；表现型是指表达出来的性状。
Most characteristics involve several genes.
大多数性状涉及多个基因。
An affected father and an unaffected mother have an unaffected son.
一位患病的父亲和一位未患病的母亲生了一个未患病的儿子。
D is dominant and d recessive.
D是显性，d是隐性。
Give the genotypes of the mother and son, say which allele the father passed on, and use both his phenotype and his son's genotype to deduce the father's genotype.
写出母亲和儿子的基因型，说出父亲传给儿子的是哪个等位基因，并根据父亲的表现型和儿子的基因型推断父亲的基因型。
Pause and try it.
暂停并试着回答。
The mother and son are both d d, because a D would cause the dominant phenotype.
母亲和儿子都是dd，因为如果有D，就会表现出显性性状。
The son received one d from each parent, so the father carries d.
儿子从父母双方各得到一个d，所以父亲携带d。
But the father is affected, so he also has D.
但父亲患病，所以他也有D。
Together, that makes him D d.
综合起来，父亲是Dd。
This is an inference from one family, not a rule that every affected person is heterozygous.
这是根据一个具体家庭作出的推断，并不是说每个患者都是杂合子。
For higher tier: cystic fibrosis is recessive.
高级卷：囊性纤维化是隐性遗传病。
Let capital C be the normal allele and small c the cystic fibrosis allele.
用大写C表示正常等位基因，小写c表示致病等位基因。
Two unaffected carriers have a child.
两位未患病的携带者生育孩子。
Write the parents' genotypes and their gametes, construct the Punnett square, and find the probability of cystic fibrosis and of being a carrier.
写出双亲的基因型和配子，画出庞纳特方格，求孩子患囊性纤维化的概率和成为携带者的概率。
For comparison, complete the X X by X Y sex cross.
作为比较，再完成XX与XY的性别遗传图解。
Pause and try it.
暂停并试着回答。
For the carrier cross, one in four children is affected, and one in two is a carrier.
携带者杂交中，四分之一的孩子患病，二分之一是携带者。
For the sex cross, half are X X and half X Y.
性别遗传中，一半是XX，一半是XY。
Each child is a new, independent event.
每个孩子都是一次新的、独立的事件。
An affected mother is D d and an unaffected father is d d.
患病的母亲是Dd，未患病的父亲是dd。
They already have three unaffected sons.
他们已经有三个未患病的儿子。
Construct the cross, find the probability that their fourth child has polydactyly, and explain whether the three earlier births change that probability.
画出遗传图解，求第四个孩子患多指（趾）的概率，并解释前三次生育是否会改变这一概率。
Pause and try it.
暂停并试着回答。
Every sperm carries d.
每个精子都携带d。
An egg with D gives D d, affected; an egg with d gives d d, unaffected.
卵细胞带D时，后代为Dd，患病；带d时，后代为dd，不患病。
Two of the four equally likely combinations are D d, so the probability is two quarters, one half.
四种可能性相同的组合中有两种是Dd，所以概率是四分之二，即二分之一。
Each fertilisation is independent: three unaffected children do not make an affected child due.
每次受精都是独立的：已有三个未患病的孩子，并不意味着下一个孩子就“该”患病了。
A carrier couple is offered embryo screening.
一对携带者夫妇可以选择进行胚胎筛查。
It can identify embryos with the cystic fibrosis genotype, but the treatment is costly, and some embryos may not be used.
筛查能识别出具有囊性纤维化基因型的胚胎，但费用高昂，而且有些胚胎可能不会被使用。
Give one potential benefit, explain an economic issue and an ethical concern, and write a balanced conclusion that recognises families may value these differently.
给出一项可能的好处，解释一个经济问题和一个伦理问题，并写出一个平衡的结论，承认不同家庭对这些问题的看法可能不同。
Pause and try it.
暂停并试着回答。
A potential benefit: identifying embryos that are c c helps the couple decide whether to use an embryo predicted to develop cystic fibrosis.
可能的好处：识别出cc胚胎，有助于夫妇决定是否使用预计会患囊性纤维化的胚胎。
An economic issue: treatment costs may limit access, and funding decisions involve competing uses of resources.
经济问题：治疗费用可能限制人们获得这项服务，资金分配涉及资源的不同用途之间的取舍。
An ethical issue: some people object to selecting embryos or not using them, while others put reducing inherited disease first.
伦理问题：有些人反对挑选胚胎或弃用胚胎，另一些人则把减少遗传病放在首位。
A reasoned judgement uses the information given; the cross alone cannot settle the ethical decision.
合理的判断要依据所给的信息；仅凭遗传图解无法解决伦理问题。
Some seedlings differ in inherited disease resistance.
一些幼苗在遗传的抗病能力上有差异。
Plants of the same genotype grow to different heights when given different amounts of light.
基因型相同的植株在光照量不同时，长到的高度也不同。
Identify a genetic and an environmental source of variation, explain why a phenotype can depend on both, and explain how resistance could become more common over generations during a disease outbreak.
指出一个遗传变异来源和一个环境变异来源，解释为什么表现型可以同时取决于二者，并解释在疾病暴发期间，抗病性如何在几代中变得更普遍。
Pause and try it.
暂停并试着回答。
The seedlings already differ in the alleles that affect resistance, and their heights can also differ because of light.
这些幼苗在影响抗病性的等位基因上本来就有差异，它们的高度也会因光照不同而不同。
Resistant plants are more likely to survive the disease and reproduce.
抗病植株更可能在疾病中存活并繁殖。
Survivors pass their resistance alleles to their offspring, so over generations those alleles can become more common.
存活者把抗病等位基因传给后代，所以几代之后，这些等位基因会变得更普遍。
A mutation is a change in DNA, not something produced because an organism needs it: selection acts on variation that already exists.
突变是DNA的改变，不是因为生物需要才产生的：选择作用于已经存在的变异。
Describe natural selection, using a population of insects that already vary in inherited resistance to an insecticide.
用一个在遗传上对杀虫剂抗性本来就有差异的昆虫种群，描述自然选择。
Identify the variation and the selection pressure, connect survival to reproduction and the passing on of alleles, and explain why one insect does not evolve just by being sprayed.
指出变异和选择压力，把存活与繁殖以及等位基因的传递联系起来，并解释为什么一只昆虫不会仅仅因为被喷洒了杀虫剂就发生进化。
Pause and try it.
暂停并试着回答。
Some insects carry alleles that give greater resistance; others are susceptible.
有些昆虫携带使抗性更强的等位基因，另一些则对杀虫剂敏感。
The insecticide kills more of the susceptible insects, so resistant survivors are more likely to reproduce.
杀虫剂杀死更多敏感的昆虫，所以有抗性的存活者更可能繁殖。
The survivors pass the favourable alleles to their offspring, so the population's inherited characteristics change over generations.
存活者把有利的等位基因传给后代，于是种群的遗传性状在几代中发生改变。
Evolution is a change in a population over time; a new species forms only when populations can no longer interbreed to produce fertile offspring.
进化是种群随时间的变化；只有当种群之间不能再交配产生可育后代时，才形成新物种。
A grower has a mixed population of crop plants, and some show inherited disease resistance.
一位种植者有一个混合的作物种群，其中一些植株表现出遗传的抗病性。
The grower wants resistant offspring over many generations.
种植者希望在许多代中得到抗病的后代。
Describe which parents and offspring to choose and what to repeat, explain why choosing one plant once is not enough, and give one risk of repeatedly breeding closely related plants.
描述应选择哪些亲本和后代、需要重复什么，解释为什么只选一株、只选一次是不够的，并说出反复让近亲植株杂交的一个风险。
Pause and try it.
暂停并试着回答。
Choose plants from the mixed population that show the inherited resistance, and breed them.
从混合种群中选出表现出遗传抗病性的植株，让它们杂交。
Then choose resistant offspring and breed them together, repeating the selection and breeding over many generations.
再选出抗病的后代，让它们相互杂交，在许多代中重复选择和育种。
The risk: breeding closely related organisms can increase inherited defects and reduce genetic variation.
风险在于：近亲繁殖会增加遗传缺陷，并减少遗传变异。
In selective breeding, humans choose which organisms reproduce; natural selection favours survival in the environment, and genetic engineering puts a gene into a genome.
选择育种是由人类决定哪些生物繁殖；自然选择是环境有利于某些个体存活和繁殖；基因工程则是把一个基因导入基因组。
Genetically modified soya plants resist the weedkiller glyphosate.
转基因大豆能抵抗除草剂草甘膦。
A farmer grows them and sprays glyphosate, which kills the weeds.
农民种植这种大豆并喷洒草甘膦，杂草被杀死。
Explain why the crop survives while the weeds are removed, connect less competition to photosynthesis and biomass, and add a role for nitrate ions.
解释为什么作物能存活而杂草被除掉，把竞争减少与光合作用和生物量联系起来，并说明硝酸根离子的作用。
Why is 'GM means more yield' not a complete answer?
为什么“转基因就意味着高产”不是完整的答案？
Pause and try it.
暂停并试着回答。
Glyphosate kills the weeds, while the resistant soya plants survive.
草甘膦杀死杂草，而有抗性的大豆存活下来。
Fewer weeds means less competition for light, water and mineral ions.
杂草减少，对光、水和矿质离子的竞争就减少。
More light and water can support more photosynthesis, producing glucose for respiration and for building biomass.
更多的光和水能支持更多的光合作用，产生用于呼吸作用和构建生物量的葡萄糖。
And more nitrate ions can support making amino acids and proteins, adding to growth and yield.
更多的硝酸根离子能支持合成氨基酸和蛋白质，促进生长、提高产量。
Connect the modification to weed removal, resources, biomass and the harvest.
要把这种改造与除草、资源、生物量和收成一步步联系起来。
For higher tier, the steps of genetic engineering.
高级卷：基因工程的步骤。
Enzymes isolate the required gene, such as the gene for human insulin.
用酶分离出所需的基因，例如人胰岛素基因。
The gene is inserted into a vector, such as a bacterial plasmid.
把这个基因插入载体，例如细菌的质粒。
The vector introduces the gene into the required cells.
用载体把基因导入所需的细胞。
The modified cells then express the gene and produce the desired substance.
被改造的细胞表达这个基因，产生所需的物质。
An insect-resistant crop reduces damage by a target pest.
一种抗虫作物减少了目标害虫造成的损害。
A field study also finds fewer non-target insects nearby, but does not show why.
一项田间研究还发现附近的非目标昆虫减少了，但没有查明原因。
Explain a potential benefit for farming, identify an ecological concern and the evidence still needed, and separate a reported association from proof that the inserted gene caused the decline.
解释这种改造对农业的一项可能好处，指出一个生态方面的担忧以及还需要的证据，并把观察到的相关关系与“插入的基因导致昆虫减少”的证明区分开。
Pause and try it.
暂停并试着回答。
The benefit: less pest damage can leave more crop to harvest; compare yields under similar growing conditions.
好处：害虫损害减少，可收获的作物更多；要在相似的种植条件下比较产量。
The concern: fewer non-target insects could affect food webs; compare modified and unmodified fields, accounting for pesticide use and habitat.
担忧：非目标昆虫减少可能影响食物网；要比较转基因和非转基因田地，同时考虑农药使用和栖息地的差异。
The judgement: the observation justifies investigation, but on its own it does not identify the cause.
判断：这一观察值得进一步调查，但仅凭它不能确定原因。
Evaluate the particular modification; not all GM crops share the same benefits or risks.
要评价具体的改造；并非所有转基因作物的好处和风险都相同。
Three aims.
三个目的。
A nursery wants many identical plants from a rare variety.
一家苗圃想从一个稀有品种得到许多相同的植株。
A farmer wants genetically identical embryos from one early embryo.
一位农民想从一个早期胚胎得到多个遗传上相同的胚胎。
And a researcher wants to copy the nuclear genes of an adult animal.
一位研究者想复制一只成年动物的核基因。
Choose tissue culture, embryo transplants, or adult cell cloning for each.
为每个目的选择组织培养、胚胎移植或成体细胞克隆。
When must the embryo cells be split, and which animal provides the nucleus in adult cell cloning?
胚胎细胞必须在什么时候分开？ 在成体细胞克隆中，细胞核来自哪只动物？
Pause and try it.
暂停并试着回答。
Tissue culture grows small groups of plant cells into new plants.
组织培养是把一小团植物细胞培养成新植株。
Cuttings grow a piece of a parent plant into a new plant.
扦插是让亲本植物的一段长成新植株。
Embryo transplants split unspecialised embryo cells, and transfer the identical embryos into host mothers.
胚胎移植是把未分化的胚胎细胞分开，再把这些相同的胚胎移入代孕母体。
Adult cell cloning puts the nucleus of an adult body cell into an egg whose own nucleus was removed.
成体细胞克隆是把成年体细胞的细胞核放入一个已去核的卵细胞中。
Cloning copies genetic information, but the environment can still affect the phenotype, and cloning is not selective breeding.
克隆复制的是遗传信息，但环境仍会影响表现型；克隆也不是选择育种。
Adult cell cloning, in order.
成体细胞克隆的步骤。
Remove the nucleus from an unfertilised egg cell.
从一个未受精的卵细胞中取出细胞核。
Insert the nucleus from an adult body cell.
放入一个成年体细胞的细胞核。
An electric shock stimulates the cell to divide and form an embryo.
用电击刺激细胞分裂，形成胚胎。
Then transfer the ball of embryo cells into a womb to develop.
再把这团胚胎细胞移入子宫中发育。
Darwin published his theory of natural selection in eighteen fifty-nine, after joint writings with Wallace in eighteen fifty-eight, when the mechanism of inheritance was still unknown.
达尔文在一八五九年发表了自然选择学说，此前在一八五八年与华莱士联合发表过论文，当时遗传的机制还不清楚。
Give two distinct reasons the theory was not accepted at once.
给出两个不同的理由，说明这一学说为什么没有立即被接受。
Distinguish Darwin and Wallace's contribution from Mendel's plant-breeding work, and explain why later evidence about genes strengthens the account.
把达尔文和华莱士的贡献与孟德尔的植物育种研究区分开，并解释为什么后来关于基因的证据使这一解释更有力。
Pause and try it.
暂停并试着回答。
Darwin and Wallace each proposed natural selection independently; their joint writings appeared in eighteen fifty-eight, and Darwin's book in eighteen fifty-nine.
达尔文和华莱士各自独立地提出了自然选择；他们的联合论文发表于一八五八年，达尔文的著作发表于一八五九年。
The theory challenged the belief in creation, some scientists found the evidence insufficient, and the mechanism of inheritance was unknown.
这一学说挑战了神创论，一些科学家认为证据不足，而且遗传的机制还不清楚。
Mendel's plant-breeding work suggested inherited units passed to offspring, but its importance was recognised only after his death.
孟德尔的植物育种实验表明，有某种遗传单位传给后代，但它的重要性直到他去世后才被认识到。
Later chromosome and DNA work gave those units a physical mechanism.
后来对染色体和DNA的研究为这些遗传单位找到了物质基础。
In the mid nineteenth century, Mendel studied inheritance through plant-breeding experiments.
十九世纪中期，孟德尔通过植物育种实验研究遗传。
In the late nineteenth century, chromosomes were seen during cell division.
十九世纪后期，人们在细胞分裂中观察到了染色体。
In the early twentieth century, their similar behaviour linked Mendel's units to genes on chromosomes.
二十世纪初，由于二者行为相似，孟德尔的遗传单位与染色体上的基因联系了起来。
And in the mid twentieth century, the structure of DNA and how genes work gave a molecular explanation.
二十世纪中期，DNA的结构和基因的功能提供了分子水平的解释。
A barrier splits a population into two groups living in different conditions.
一道屏障把一个种群分成两群，它们生活在不同的环境条件下。
After many generations, the groups cannot interbreed to produce fertile offspring.
经过许多代之后，两群之间不能交配产生可育后代。
Explain how different conditions lead to different inherited characteristics, connecting mutation, natural selection and inheritance, and identify the evidence that they are now separate species.
解释不同的环境如何导致不同的遗传性状，把突变、自然选择和遗传联系起来，并指出说明它们已成为不同物种的证据。
Pause and try it.
暂停并试着回答。
Different selection pressures act in each habitat: the variants suited to each are more likely to survive and reproduce there.
每个栖息地中的选择压力不同：适应各自环境的变异个体更可能在那里存活和繁殖。
Mutations supply new variants, and selection and the passing on of favourable alleles make the populations increasingly different.
突变提供新的变异，选择和有利等位基因的传递使两个种群差异越来越大。
The evidence for two species is that they can no longer interbreed to produce fertile offspring.
说明它们是两个物种的证据，是它们不能再交配产生可育后代。
The barrier starts the separation; it does not instantly turn one species into two.
屏障只是分隔的开始；它不会立刻把一个物种变成两个。
Three examples: a footprint preserved in rock, a buried shell replaced by minerals, and a soft-bodied organism that decays without a trace.
三个例子：保存在岩石中的脚印，被矿物质替代的埋藏贝壳，以及一个腐烂后没有留下任何痕迹的软体生物。
Identify two ways fossils form here.
指出这里显示的两种化石形成方式。
Explain why an incomplete fossil record still provides evidence.
解释为什么不完整的化石记录仍然能提供证据。
And if a species disappears from one site, is that enough to show it is extinct?
如果一个物种从某个地点消失，这是否足以说明它已经灭绝？
Pause and try it.
暂停并试着回答。
The footprint is a preserved trace, and the shell shows mineral replacement; fossils can also form where conditions prevent decay.
脚印是保存下来的痕迹，贝壳显示的是矿物质替代；在阻止腐烂的条件下也能形成化石。
Soft tissues often decay, and geological activity destroys traces, so the record is incomplete, but the fossils that survive still show past organisms and how they changed.
软组织常常腐烂，地质活动也会破坏痕迹，所以化石记录不完整，但留存下来的化石仍能显示过去的生物以及它们的变化。
Extinction means no individuals of the species remain alive anywhere, so absence from one site is not enough.
灭绝是指这个物种在任何地方都没有活着的个体了，所以仅仅从一个地点消失是不够的。
An evolutionary tree of four species.
一棵四个物种的进化树。
Which pair is more closely related, A and B, or A and C?
哪一对亲缘关系更近，A和B，还是A和C？
Trace back to where their branches fork.
沿分支往回找到它们的分叉点。
And can the lengths of the branches here date a split?
这里分支的长度能否确定分化的时间？
Pause and try it.
暂停并试着回答。
Trace A and B back to their first shared fork: that is their most recent common ancestor.
把A和B沿分支往回追溯到第一个共同分叉点：那就是它们最近的共同祖先。
A and C meet further back, at the ancestor shared by all four species.
A和C在更早的地方汇合，即四个物种共同的祖先。
So A and B share a more recent ancestor, and are more closely related.
所以A和B的共同祖先更近，亲缘关系更近。
This diagram has no time scale, so the drawn lengths cannot date a split or measure genetic difference: the branching pattern is what you interpret.
这幅图没有时间刻度，画出的长度不能确定分化时间，也不能衡量遗传差异：要解读的是分支的方式。
Before exposure, a population of bacteria contains both susceptible and resistant variants.
在接触抗生素之前，一个细菌种群中既有敏感的变异个体，也有耐药的变异个体。
The antibiotic kills the susceptible bacteria, and the resistant survivors reproduce.
抗生素杀死敏感细菌，耐药的存活者继续繁殖。
Explain where the variants come from and why the antibiotic does not create a needed mutation, explain how the proportion of resistant bacteria increases, and correct the claim that resistance makes every antibiotic useless.
解释这些变异来自哪里，为什么抗生素不会制造出所需的突变；解释耐药细菌所占的比例如何增加；并纠正“耐药意味着所有抗生素都没用了”这一说法。
Pause and try it.
暂停并试着回答。
Mutations produce genetic variants, and some are resistant to a particular antibiotic.
突变产生遗传变异，其中一些对某种抗生素具有耐药性。
The susceptible bacteria are killed, while resistant ones survive and reproduce rapidly, passing on the resistance genes.
敏感细菌被杀死，耐药细菌存活下来并迅速繁殖，把耐药基因传下去。
So the resistant strain becomes more common and can spread: this is evidence of evolution.
于是耐药菌株变得更普遍，并可能传播：这是进化的证据。
But resistance concerns one bacterium and a particular medicine.
但耐药性是针对某种细菌和某种特定药物而言的。
It can make treatment harder; it does not mean every treatment fails, or that people have no immune defence.
它会使治疗更困难，但并不意味着所有治疗都会失败，也不意味着人没有免疫防御。
The lion is Panthera leo and the tiger is Panthera tigris: each name has two words.
狮子的学名是Panthera leo，老虎是Panthera tigris：每个学名都由两个词组成。
What do the first and second parts represent?
第一部分和第二部分分别代表什么？
Write the Linnaean ranks from kingdom to species.
写出林奈分类系统从界到种的各个等级。
And explain why chemical and microscopic evidence can lead scientists to revise a classification.
并解释为什么化学和显微镜证据会使科学家修订分类。
Pause and try it.
暂停并试着回答。
Panthera is the genus, and the second word names the species within it, so lions and tigers share a genus, not a species.
Panthera是属名，第二个词是该属中的种名，所以狮子和老虎同属而不同种。
The ranks run kingdom, phylum, class, order, family, genus, species.
分类等级依次是：界、门、纲、目、科、属、种。
Woese's three-domain system uses chemical evidence to separate archaea, bacteria and eukaryota, which includes protists, fungi, plants and animals.
乌斯的三域系统利用化学证据，把生物分为古菌、细菌和真核生物，真核生物包括原生生物、真菌、植物和动物。
Better microscopes and biochemistry can reveal relationships that appearance alone does not show.
更好的显微镜和生物化学证据，能揭示单凭外形看不出的亲缘关系。
Three cases, without your notes.
不看笔记，完成三个问题。
A human gamete has twenty-three chromosomes: how can a new body cell have forty-six?
人的配子有二十三条染色体：新的体细胞怎么会有四十六条？
A D d parent and a d d parent have three unaffected children: what is the next child's probability of polydactyly?
一个Dd亲本和一个dd亲本已有三个未患病的孩子：下一个孩子患多指（趾）的概率是多少？
And resistant bacteria become more common during antibiotic use: explain the mechanism, and how it differs from cloning.
使用抗生素期间，耐药细菌变得更普遍：解释其中的机制，以及它与克隆的区别。
Pause and try them.
暂停并试着回答。
Fertilisation fuses two gametes: twenty-three plus twenty-three is forty-six.
受精使两个配子融合：二十三加二十三等于四十六。
Meiosis made the gametes, and mitosis then increases the number of body cells.
配子由减数分裂产生，之后有丝分裂使体细胞数目增加。
D d crossed with d d gives D d, D d, d d, d d, so the probability of polydactyly is one half at every birth, whatever happened before.
Dd与dd杂交得到Dd、Dd、dd、dd，所以每次生育患多指（趾）的概率都是二分之一，与之前的结果无关。
Resistant variants survive, reproduce and pass on resistance genes; cloning copies genetic information, while selection changes which variants leave more offspring.
耐药变异个体存活、繁殖并传递耐药基因；克隆复制遗传信息，而选择改变的是哪些变异个体留下更多后代。