6.4 微进
Section outline
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School Days
::学校日数Except for their plastic lunch coolers, you might think that this picture of children on their way to school came from the 1800s. In fact, the photograph was taken in 2006. The children are part of a religious community called the Amish, who are descended from a small group of founders that first came to the U.S. in the 1700s. Amish people shun modern conveniences, such as electricity and automobiles. Their lives are more similar to the lives of their founders than to those of most other people in the U.S. today. As you will learn when you read this concept, the Amish are an example of how having a small number of founders can affect the evolution of a .
::除了塑料式午餐冷却器之外,你可能会认为,儿童上学途中的照片来自1800年代。事实上,照片是2006年拍摄的。孩子们是名为阿米什的宗教社区的一部分,他们来自1700年代首次来到美国的一小群创始人。阿米什人回避电和汽车等现代便利。他们的生活与其创始人的生活相比,比今天美国大多数其他人的生活更为相似。当你读到这个概念时,你就会知道,阿米什人就是这样一个例子:拥有少量的创始人可以如何影响一个概念的演变。Genes in Populations
::人口中的基因Individuals do not evolve because their genes do not change over time. Instead, evolution occurs at the population level . A population consists of organisms of the same that live in the same area. In terms of evolution, the population is assumed to be a relatively closed group, meaning that most mating takes place within the population. Evolutionary change that occurs over relatively short periods of time within populations is called microevolution . The science that focuses on evolution within populations is population genetics , and it is a combination of evolutionary theory and Mendelian genetics .
::个人不会因为其基因不会随时间而变化而演变,而是在人口水平上演变,人口是由同一地区生活的相同生物构成的。在演变方面,人口被假定为一个相对封闭的群体,这意味着大多数交配是在人口内部发生的。在人口内部相对较短的时间内发生的演化变化被称为微变。侧重于人口内部演化的科学是人口遗传学,是进化理论和门德利基因学的结合。The Gene Pool
::基因池The genetic makeup of an individual is the individual’s genotype . A population consists of many individuals — and therefore many genotypes. All of the genotypes together make up the population’s gene pool . The gene pool consists of all the genes of all the members of the population. For each gene, the gene pool includes all the different for the gene that exist in the population. For a given gene, the population is characterized by the frequency of the different alleles in the gene pool.
::个人的基因构成是个人的基因型,人口由许多个人组成,因此包括许多基因型,所有基因型合在一起构成人口的基因库。基因库由人口所有成员的所有基因组成。对于每一种基因来说,基因库包括人口中存在的所有不同基因。对于一种特定基因来说,人口的特点就是基因库中不同基因的频率。Allele Frequencies
::千秋千秋千金Allele frequency is how often an allele occurs in a gene pool relative to the other alleles for the same gene. Look at the simple hypothetical example in the table below. The population in the table has 100 members. In a sexually reproducing species, each member of the population has two copies of each gene. Therefore, the total number of copies of each gene in the gene pool is 200. The gene in the example exists in the gene pool in two forms, alleles A and a . Knowing the genotypes of all the population members, we can count the number of alleles of each type in the gene pool. The table shows how this is done.
::相较于其他异系基因的基因库中, 异频频率是指同种基因在基因库中发生异变的频率。 看看下表的简单假设例子。 表中的人口有100个成员。 在性繁殖的物种中, 人口中的每个成员都有每个基因的两份副本。 因此, 基因库中每个基因的复制件总数是200个。 这个例子中的基因以两种形式存在于基因库中, 以A类和a种形式存在。 了解所有人口成员的基因类型, 我们可以计算基因库中每种类型的异变的数目。 表格显示了如何做到这一点 。Genotype Number of Individuals in the Population with that Genotype Number of Allele A Contributed to the Gene Pool by that Genotype Number of Allele a Contributed to the Gene Pool by that Genotype AA 50 50 × 2 = 100 50 × 0 = 0 Aa 40 40 × 1 = 40 40 × 1 = 40 aa 10 10 × 0 = 0 10 × 2 = 20 Totals 100 140 60 By convention, allele frequencies are usually represented by the letters p and q . If we let p represent the frequency of allele A and q represent the frequency of allele a, we can calculate p and q as follows:
::按惯例,Alele频率通常由字母p和q表示。如果我们让 p 表示 Alele A 和 q 表示 Ale A 的频率,我们可以计算p 和 q 如下:-
p
= number of
A
alleles/total number of alleles = 140/200 = 0.7
::p = alleles 数/ alleles 总数= 140/200 = 0.7 -
q
= number of
a
alleles/total number of alleles = 60/200 = 0.3
::q = 排量数/排量总数= 60/200 = 0.3
Notice that p + q = 1.0. There are only two alleles for the gene in this population. Therefore, the sum of their frequencies must equal 1.0.
::注意 p + q = 1. 0。 在这个组中, 基因只有两个相位数。 因此, 它们频率的总和必须等于 1. 0 。Hardy-Weinberg Theorem
::哈迪温贝格理论Evolution occurs in a population when allele frequencies change over time. What causes allele frequencies to change? That question was answered by Godfrey Hardy and Wilhelm Weinberg in 1908. Hardy was an English mathematician, and Weinberg was a German doctor. Each worked independently to derive the founding principle of population genetics. Today, that principle is called the Hardy-Weinberg theorem . It shows that allele frequencies do not change in a population if certain conditions are met. A population like this is said to be in Hardy-Weinberg equilibrium . The conditions for equilibrium are:
::当Alele频率随时间变化时,进化就发生在人口之中。是什么原因导致Alele频率变化?这个问题在1908年由Godfrey Hardy和Wilhelm Weinberg回答。Hardy是英国数学家,Weinberg是德国医生。每个人都独立工作,以获得人口基因学的创始原则。今天,这一原则被称为Hardy-Weinberg理论。它表明如果满足某些条件,Aleen频率在人口当中不会变化。这样的人口据说处于Hardy-Weinberg的平衡状态。平衡的条件是:-
does not occur.
Therefore, no new alleles are created.
::没有发生。 因此, 没有创造新的 alleles 。 -
There is no
.
N
o one is moving into or out of the population.
::没有,没有人进出人口。 -
The population is very large.
::人口非常多。 -
Mating is at random in the population.
I
ndividuals do not choose mates based on genotype.
::在人口中,配对是随机的,个人不选择基于基因型的伴侣。 -
There is no
.
A
ll members of the population have an equal chance of reproducing and passing their genes on to the next generation.
::人口所有成员都有平等的机会再生基因并传给下一代。
When all of these conditions are met, allele frequencies stay the same, so evolution does not occur. Genotype frequencies also remain constant.
::当所有这些条件都得到满足时,Allele频率保持不变,所以进化不会发生。热亚型频率也保持不变。G enotype frequencies can be expressed in terms of allele frequencies, as shown in the following table:
::如下表所示,热亚型频率可以用阿列频率表示:Genotype Frequencies in a Hardy-Weinberg Equilibrium Population Genotype Genotype Frequency AA p 2 Aa 2 pq aa q 2 Forces of Evolution
::演变力量的演变The Hardy-Weinberg theorem describes populations in which allele frequencies are not changing. By definition, such populations are not evolving. The conditions for Hardy-Weinberg equilibrium are unlikely to be met in real populations, so h ow does the theorem help us understand evolution in the real world? From the theorem, we can infer the factors that do cause allele frequencies to change. These factors are the . There are four such forces: mutation, , , and natural selection.
::Hardy- Weinberg 论者描述的人群中所有频率没有变化。 顾名思义, 这些人群没有变化。 Hardy- Weinberg 论者不可能在真实人群中满足Hardy- Weinberg 平衡的条件, 所以理论如何帮助我们理解现实世界的演变? 从理论中, 我们可以推断出导致所有频率变化的因素。 这些因素就是 。 有四种这种力量: 突变, 和自然选择 。Mutation
::变异Mutation creates new in a gene pool. It is how all new alleles first arise. In sexually reproducing species, the mutations that matter for evolution are those that occur in gametes . Only these mutations can be passed to offspring. For any given gene, the chance of a mutation occurring in a given gamete is very low. Thus, mutations alone do not have much effect on allele frequencies. Mutations, however, provide the genetic variation needed for other forces of evolution to act.
::突变在基因库中创造了新的基因库。 所有新的异变最初都是如何产生的。 在性再生物种中, 进化重要的突变是星系中的变异。 只有这些变异可以传递给后代。 对于任何特定的基因来说, 在某个基因中发生突变的可能性非常低。 因此, 单是突变不会对异变频率产生多大影响。 然而, 变异提供了其他进化力量采取行动所需的基因变异。Gene Flow
::基因流动Gene flow occurs when individuals move into or out of a population. If the rate of migration is high, this can have a significant effect on allele frequencies. Allele frequencies may change in the population the migrants leave, as well as in the population the migrants enter.
::如果移徙率很高,这可能会对Alle频率产生重大影响,移徙者离开的人口以及移徙者进入的人口可能会发生变化。An example of gene flow occurred during the Vietnam War in the 1960s and 1970s. Many young American servicemen had children with Vietnamese women. Most of the servicemen returned to the United States after the war, leaving copies of their genes behind in their offspring. In this way, they changed the allele frequencies in the Vietnamese gene pool. Do you think the gene pool of the U.S. was also affected? Why or why not?
::20世纪60年代和70年代越南战争期间出现了基因流动的例子。许多年轻的美国军人怀有越南妇女的孩子。大多数军人战后回到美国,留下后代的基因副本。这样,他们改变了越南基因库中的异能频率。 你认为美国基因库也受到影响吗?为什么或为什么没有?Genetic Drift
::遗传漂流Genetic drift is a random change in allele frequencies that occurs in a small population. When a small number of parents produce just a few offspring, allele frequencies in the offspring may differ, just by chance, from allele frequencies in the parents. This is like tossing a coin. If you toss a coin just a few times, you may, by chance, get more or less than the expected 50 percent heads and 50 percent tails. Due to such chance variations in small populations, allele frequencies drift over time.
::基因漂移是少数人群中不同频率的随机变化。 当少量父母只生几个孩子时, 后代的异频率可能偶然地与父母的异频率不同。 这就像抛硬币一样。 如果你扔了一枚硬币几次, 你可能会偶然地得到比预期的50%头部和50%尾部多或少。 由于小人群中的机会变化, 异频频率会随时间而移动 。There are two special conditions under which genetic drift occurs. They are called the bottleneck effect and the founder effect .
::基因漂移有两个特殊条件,叫做瓶颈效应和创建效应。-
Bottleneck effect
occurs when a population suddenly gets much smaller. This might happen because of a natural disaster (such as a forest fire) or disease
epidemic
. By chance, allele frequencies of the survivors may be different from those of the original population.
::当人口突然变小时,就会产生瓶颈效应。 这可能因为自然灾害(如森林火灾)或疾病流行而发生。 偶然情况下,幸存者的频率可能与原始人口不同。 -
Founder effect
occurs when a few individuals start, or found, a new population. By chance, allele frequencies of the founders may be different from allele frequencies of the population they left. An example of the founder effect occurred in the Amish, as described in the figure
.
::创建者效应发生于几个人开始或发现新人口时。 偶然情况下,创建者的所有频率可能不同于他们离开的人口的所有频率。 如图所示,创建者效应的一个例子发生在阿米什(Amish ) 。
Founder Effect in the Amish Population. The Amish population in the U.S. and Canada had a small number of founders. How has this affected the Amish gene pool?
::美国和加拿大的阿米什人拥有少数创始人,这如何影响了阿米什人的基因库?Natural Selection
::自然选择自然选择Natural selection occurs when there are differences in fitness among members of a population. As a result, some individuals pass more genes to the next generation than do other members of the population. This causes allele frequencies to change over time. One case that can serve as an example is sickle anemia, which is shown in the following table and described . This shows how natural selection can keep even a harmful allele in a gene pool.
::自然选择发生在人口成员健康程度不同时。 因此,一些人将基因传给下一代比其他人口成员多。 这导致所有频率随时间变化。 一个可以作为例子的例子的例子就是镰状贫血,如下表所示并描述。 这显示了自然选择可以将一个有害的异象保存在基因库中。Sickle Cell Anemia and Natural Selection Genotype Phenotype Fitness AA 100% normal hemoglobin Somewhat reduced fitness because of no resistance to malaria AS Enough normal hemoglobin to prevent sickle-cell anemia Highest fitness because of resistance to malaria SS 100% abnormal hemoglobin, causing sickle-cell anemia Greatly reduced fitness because of sickle-cell anemia The allele ( S ) for sickle cell anemia is a harmful, autosomal recessive allele . It is caused by a mutation in the normal allele ( A ) for hemoglobin (the oxygen-carrying on red cells). Malaria is a deadly disease that is common in many African populations. Heterozygotes ( AS ) with the sickle cell allele are resistant to malaria. Therefore, they are more likely to survive and reproduce. This keeps the S allele in the gene pool.
::镰状细胞贫血的止血球(S)是一种有害、自动隔热的止血球。它是由血红蛋白(含氧于红细胞)正常止血球菌(A)的突变造成的。疟疾是许多非洲人口常见的致命疾病。 使用镰状细胞的异性螺旋球(AS)抗疟疾。因此,它们更有可能存活和繁殖。这让S allele在基因库中。The sickle cell example shows that fitness depends on phenotypes , and also on the environment. What do you think might happen if malaria were to be eliminated in an African population with a relatively high frequency of the S allele? How might the fitness of the different genotypes change? How might this affect the frequency of the S allele?
::镰状细胞的例子表明,是否健康取决于苯型,也取决于环境。 如果疟疾在非洲人口中被消灭,其频率相对高的Selle人会怎么样?不同的基因型的适合性会如何变化?这如何影响Salle的频率?The sickle cell trait is controlled by a single gene. Natural selection for — which are controlled by multiple genes — is more complex, although it is less complicated if you consider just phenotypes for polygenic traits (rather than genotypes). There are three major ways that natural selection can affect the distribution of phenotypes for a polygenic trait. The three ways are shown in the graphs .
::镰状细胞特性受单一基因控制,自然选择(由多种基因控制)更为复杂,尽管如果只考虑多致性特征(而不是基因型)的苯型,自然选择(由多种基因控制)比较复杂。自然选择可以影响多致性特征的苯型类型的分布,有三种主要方法。三种方法在图表中显示。-
Stabilizing selection
occurs when phenotypes at both extremes of the phenotypic distribution are selected against. This narrows the range of variation. An example is human birth weight. Babies that are very large or very small at birth are less likely to survive, and this keeps birth weight within a relatively narrow range.
::选择稳定化选择时,要对视胎儿分布两个极端的苯型。这缩小了变异范围。例如,人的出生体重。出生时非常大或非常小的婴儿存活的可能性较小,这使出生体重保持在相对狭窄的范围之内。 -
Directional selection
occurs when one of two extreme phenotypes is selected for, shifting the distribution toward that extreme. This is the type of natural selection that the Grants observed in the
beak
size of Galápagos finches. Larger beaks were selected for during drought, so beak size increased over time.
::方向选择发生于选择两种极端苯型中的一种, 将分布转向极端。 这是在加拉帕戈斯角的嘴上观察到的自然选择类型。 大嘴是在干旱期间被选中的, 因此嘴的大小会随时间而增加 。 -
Disruptive selection
occurs when phenotypes in the middle of the range are selected against. This results in two overlapping phenotypes, one at each end of the distribution. An example is
sexual dimorphism
. This refers to differences between the phenotypes of males and females of the same species. In humans, for example, males and females have different average heights and body shapes.
::当选择范围中间的苯型时,就会出现干扰性选择。这导致两种重叠的苯型,在分布的每个端各出现一种。一个例子是性畸形。这是指同一物种中男性和女性的苯型之间的差异。例如,在人类中,男性和女性的平均高度和身体形状不同。
Natural Selection for a Polygenic Trait. Natural selection may affect the distribution of a polygenic trait. These graphs show three ways this can happen.
::多源Trait 的自然选择。 自然选择可能会影响多源特性的分布。 这些图表显示了三种可能的方式 。Feature: Human Biology in the News
::特著:《新闻》中的人类生物学Recently reported research may help solve one of the most important and long-lasting mysteries of human biology : why people with the AS genotype for sickle cell hemoglobin are protected from malaria. As you read above, their sickle cell hemoglobin gives them higher fitness in malaria areas than normal homozygotes (AA) who have only normal hemoglobin.
::最近报道的研究可能有助于解决人类生物学中最重要的、最持久的奥秘之一:为什么患有AS基因型镰状细胞血红蛋白的人可以免受疟疾。 正如你在上面所看到的,他们的镰状细胞血红蛋白使他们在疟疾地区比正常的同性人(AA)更健康,他们只有正常的血红蛋白。The malaria parasite and its mosquito vector were discovered in the late 1800s. The genetic basis of sickle cell hemoglobin anemia and the resistance to malaria it confers were discovered around 1950. Since then, scientists have assumed — and some evidence has suggested — that the few sickle-shaped red blood cells of heterozygotes make them less hospitable hosts for the malaria parasite than the completely normal red blood cells of AA homozygotes. This seems like a reasonable hypothesis , but is it the correct one? The new research suggests a different hypothesis.
::疟疾寄生虫及其蚊子病媒是在1800年代末发现的。镰状细胞血红蛋白血病的遗传基础及其带来的对疟疾的抗药性在1950年左右被发现。自那时以来,科学家们假定——而且一些证据表明——异体细胞中少数的镰状红血细胞使他们对疟疾寄生虫的热情比AA同质人完全正常的红血细胞更低。这似乎是一个合理的假设,但正确吗?新的研究提出了不同的假设。Working with genetically engineered mice as model organisms, researchers in Portugal discovered that an that produces the gas carbon monoxide is expressed at much higher levels in the presence of sickle cell hemoglobin than in normal hemoglobin. Furthermore, the gas seems to protect the infected host from developing the lesions and symptoms of malaria, even though it does not seem to interfere with the of the malaria parasite in red blood cells. These findings may lead to new therapies for treating malaria, which is still one of the most serious public health problems in the world. The findings may also shed light on other abnormal hemoglobin variants known to protect against malaria.
::葡萄牙的研究人员发现,生产一氧化碳气体的鼠作为模型生物,其表现水平比正常血红蛋白高得多;此外,该气体似乎保护受感染的宿主避免患上疟疾的损害和症状,尽管它似乎没有干扰红血细胞中的疟疾寄生虫;这些发现可能导致新的疟疾治疗疗法,疟疾仍是世界上最严重的公共健康问题之一;该结果还可能揭示已知的防止疟疾的其他异常血红蛋白变体。Summary
::摘要-
Microevolution refers to evolution that occurs over a relatively short period of time within a population.
::微进是指人口在相对较短的时间内发生的演变。 -
In this case,
a
population is the unit of evolution. Population genetics is the science that studies evolution at the population level. A population’s gene pool consists of all the genes of all the members of the population. For a given gene, the population is characterized by the frequency of different alleles in the gene pool.
::在这种情况下,人口是进化的单位。 人口遗传学是研究人口层面演变的科学。 人口基因库包含所有人口成员的所有基因。 对于特定基因而言,人口特征是基因库中不同基因群的频率。 -
The Hardy-Weinberg theorem states that, if a population meets certain conditions, it will be in equilibrium. In an equilibrium population, allele and genotype frequencies do not change over time. The conditions that must be met are: no mutation, no migration, very large population size, random mating, and no natural selection.
::Hardy-Weinberg理论指出,如果一个人口符合某些条件,它就会处于平衡状态。在平衡状态下,等离子体和基因型频率不会随时间而变化。必须满足的条件是:没有突变、没有移徙、人口规模很大、随机交配、没有自然选择。 -
There are four forces of evolution: mutation, which creates new alleles; gene flow, in which migration changes allele frequencies; genetic drift, which is a random change in allele frequencies that may occur in a small population; and natural selection, in which allele frequencies change because of differences in fitness among individuals.
::有四种进化力量:突变,它创造了新的变异物;基因流动,其中移徙改变了异变频率;基因漂移,这是在少数人口中可能发生的异变频率的随机变化;自然选择,其中异变频率因个人健康程度的差异而变化。
Review
::回顾1. Why are populations ( not individuals) the unit of evolution?
::1. 为什么人口(而不是个人)是进化的单位?2. What is a gene pool?
::2. 什么是基因库?3. Describe a Hardy-Weinberg equilibrium population. What conditions must be met for the population to remain in equilibrium?
::3. 描述Hardy-Weinberg均衡人口:人口必须满足哪些条件才能保持平衡?4. List and define the four forces of evolution.
::4. 列出并界定四种进化力量。5. Why is mutation needed for evolution to occur, even though it usually has little effect on allele frequencies?
::5. 为何演变需要突变才能发生,即使突变通常对异频影响不大?6. What is the founder effect? Give an example.
::6. 创建者的作用是什么?举一个例子。7. Identify three types of natural selection for polygenic traits.
::7. 确定三种类型的多种致病特征自然选择。8. Assume that a population of 50 individuals has the following numbers of genotypes for a gene with two alleles:
::8. 假定50人的人口具有下列基因型数,用于两种异位基因:B and b : BB = 30, Bb = 10, and bb = 10
::B和b:BB = 30,Bb = 10,bb = 10Calculate the frequencies of the two alleles in the population’s gene pool.
::计算人口基因库中两个异形的频率。9. Assume that a population is in Hardy-Weinberg equilibrium for a particular gene with two alleles, A and a . The frequency of A is p , and the frequency of a is q . Because these are the only two alleles for this gene, p + q = 1.0. If the frequency of homozygous recessive individuals ( aa ) is 0.04, what is the value of q ? Based on the value of q , find p . Then use the values of p and q to calculate the frequency of the heterozygote genotype ( Aa ).
::9. 假设人口处于Hardy-Weinberg均衡状态,具有两个等同基因,A和a。A的频率是p,a的频率是q。因为这些是该基因的唯一两个等同基因,p + q = 1. 0。如果同性休眠个体的频率(aa)为0.04,q的价值是多少?根据q的值,找到p。然后使用p和q的值计算异体基因型(Aa)的频率。10. Explain why genetic drift is most likely to occur in a very small population.
::10. 解释为什么基因漂移最可能发生在极少数人口中。11. In some species, females prefer to mate with males that have certain genetically determined characteristics, such as bright coloration or a large, showy tail. Explain why this would affect Hardy-Weinberg equilibrium.
::11. 在有些物种中,雌性倾向于与具有某些遗传特性的雄性交配,如亮色或大、亮尾巴,解释这为何会影响Hardy-Weinberg的平衡。12. Which of the following may cause genetic drift?
::12. 下列哪些因素可能导致遗传漂移?a. a natural disaster
::a. 自然灾害b. a large population where members mate with each other and also with new migrants that come into the population
::b. 大量人口,其中成员彼此交配,并与新移民一起进入人口c. an island with no birds that becomes populated by a small number of a bird species
::c. 岛上没有鸟类,没有少数鸟类种居住;d. both A and C
::d. A 和 C 两者13. True or False: Allele frequencies can change within an organism.
::13. 真实或假:Allele频率可在生物体内发生变化。14. True or False: Most populations on Earth are in Hardy-Weinberg equilibrium.
::14. 真实或假:地球上大多数人口处于Hardy-Weinberg均衡状态。15. True or False: Genotype frequency can change if there is migration into or out of the population.
::15. 真实或假:如果人口向外移徙,热亚型的频率就会改变。Explore More
::探索更多Check out this tutorial on solving problems using the Hardy-Weinberg equation:
::使用 Hardy- Weinberg 方程式解决问题时, 请查看此教程 :Did you know that many genetic diseases are the result of relatively small population sizes? Many people love the idea of adopting purebred dogs, but the harsh reality is that pure breeding decreases genetic diversity. Learn more here:
::你知道吗,许多遗传疾病是人口规模较小造成的?许多人喜欢采用纯种狗的想法,但严酷的现实是,纯种会减少遗传多样性。 -
p
= number of
A
alleles/total number of alleles = 140/200 = 0.7
