小进化--生物进化的机制.ppt

小进化生物进化的机制,Chapter 2,Section 1,Hardy-Weinberg Law, Individuals can carry only two different alleles （等位基因）of a given gene. A group of individuals can carry a larger number of different alleles, giving rise to a reservoir of genetic diversity. The diversity contained in the population can be measured by the Hardy-Weinberg law. Mutation （突变） is the ultimate source of genetic variation, and other factors such as drift, migration, and selection can alter the amount of genetic variation in populations. Recombination （重组） is the immediate source of genetic variation.,SECTION CONCEPTS, Since 1900, biologists had begun a long effort to reconcile (使和解，使和谐，使顺从) Mendels concept of genes and alleles with the theory of evolution. In a key insight, biologists realized that changes in the relative frequency of different phenotypic traits in a population are tied to changes in the relative frequency of the alleles that influence the traits., The discipline within evolutionary biology that studies changes in allele frequencies is known as population genetics, 基因频率是居群遗传组成的基本标志。不同居群的同一基因往往频率不同。所谓基因频率就是在一个居群中，某一基因对其等位基因的相对比率。由于基因频率是一个相对的比率，是以百分率表示的，因此变动范围在01之间。一般都写成小数形式，没有负值。,A few of Basic concepts,gene frequency 基因频率,genotype frequency 基因型频率 phenotype frequency 表型频率, 在二倍体生物的体细胞中，基因都是成对存在的。一对或几对基因构成某个性状的基因型。一个性状的遗传组成不仅决定于基因，更直接地决定于基因型。一个居群中某一性状的各种基因型间的比率叫做基因型频率。 基因型不等于表型，基因型频率并不是表型频率。, An organism that has different alleles at a particular gene locus on homologous chromosomes. 杂合体：在同源染色体的某一特定基因位点上有不同的等位基因的有机体,Heterozygote 杂合体,Homozygote 纯合体, An organism that has same alleles at a particular gene locus on homologous chromosomes. 纯合体：在同源染色体的某一特定基因位点上有相同的等位基因的有机体, 经典定义是：居群中任何一个雌性或雄性个体与任何一个相反性别的个体交配的概率都相同，也就是说，任何一对雌雄的结合都是随机的，不受任何选配的影响。 自然交配不是随机交配。由于动物有性爱效应，植物居群中有邻接效应，严格的随机交配居群是很少的。,Random mating 随机交配, 基因型是由基因组成，两者的频率当然是密切关连的。下面以一对基因为例加以说明。设A和a是一对等位基因，它们的频率分别为p和q（p + q = 1）。这一对基因组成的基因型有三种，即AA、Aa和aa，它们的频率分别以D、H和R代表（D + H + R = 1）。在整个居群里有D份AA基因型，每个基因型有两个A基因，因此有2D份个A基因；另有H份Aa基因型，其中有H份个A基因和H份个a基因；还有R份aa基因型，包含有2R份个a基因。,基因频率与基因型频率的关系 Relationship between gene and genotype frequencies,A 基因的频率,a 基因的频率,Figure 2.1 The contents of Hardy-Weinberg Principle from American Heritage Dictionary, 1903年美国遗传学家W. E. Castle发表文章，指出居群中遗传的法则，提出了平衡定律的雏形。1908年英国数学家H. Hardy和德国医生W. Weinberg独立发表了居群遗传平衡的文章，将孟德尔定律用于随机交配的大居群，正式提出平衡定律，现称Hardy-Weinber定律，为居群遗传学的诞生奠定了第一块基石。,The contents of Hardy-Weinberg Principle, 随机交配的大居群若没有其它因素的影响，基因频率一代一代下去始终不变。 任何一个大居群，无论其基因频率如何，只要经过一代随机交配，一对常染色体基因的基因频率就达到平衡状态，若没有其他因素的影响，以后一代一代随机交配下去，这种平衡状态始终保持不变. 在平衡状态下，基因频率与基因型频率的关系是： D = p2 H = 2pq R = q2,Proving Hardy-Weinberg Principle, 设0世代的基因频率为p0和q0，基因型频率为D0，H0和R0；一世代的基因频率为p1和q1，基因型频率为D1，H1和R1；二世代的基因频率为p2和q2，基因型频率为D2，H2和R2，等等。 在一个大居群中，任何一个配子带有A基因或a基因的概率就等于该基因在这个居群中的频率。所以0世代的个体所产生的配子，带有A基因的概率为p0，带有a基因的概率为q0。也就是说，有p0份的配子带有A基因，q0份的配子带有a基因。,在随机交配下，各雌雄配子随机结合，一世代个体的各基因型频率为：,由表可知，一世代的基因型频率为： D1=p02 H1=2p0q0 R1=q02 由此计算一世代的基因频率： p1=D1+ H1 = p02 + p0 q0 = p0 ( p0 + q0 ) = p0 q1 = H1+R1 = p0 q0+ q02 = q0 ( p0 + q0 ) = q0,同样可以证明： P2 = p0 q2 = q0 Pn = p0 qn = q0 也就是一代一代下去，基因频率不变。 无论0世代的基因型频率如何，其基因频率总是p0和q0,在随机交配下，一世代的基因型频率为D1= p02 , H1= 2p0q0 , R1= q02 ，基因频率仍为 p0 和 q0。因而二世代的基因型频率仍为 p02，2p0q0，和q02。 由于 p0= p1= p2= pn，q0 = q1= q2= qn， 因此，足码可以取消，从一世代开始，每个世代的基因型频率都是 p2，2pq 和 q2，始终保持不变。,Figure. 2.2 The relationship between genotype and allele frequencies derived from the Hardy-Weinberg equation., In general, the frequencies of all three genotypes can be estimated once the frequency of either allele is known and Hardy-Weinberg assumptions are invoked (调用，援引, 援用). The relationship between genotype and allele frequency is shown in Fig. 2.2. It is important to note that heterozygotes (杂合体) increase rapidly in a population as the values of p and q move from 0 or 1. This observation confirms our conclusion that when a recessive trait such as cystic fibrosis ( 囊肿性纤维化，遗传性胰腺病) is rare, the majority of those carrying the allele are heterozygotes. In populations in which the frequencies of p and q are between 0.33 and 0.67, heterozygotes occur at higher frequency than either homozygote.,Biological Significance of Hardy-Weinberg Principle, Hardy-Weinberg定律可以说是居群生物学中的“遗传守恒定律”，它向人们揭示：居群中基因型频率和基因频率的一般关系；根据H-W定律，我们可以计算基因频率和基因型频率。 居群内个体间的遗传变异通常由基因型的差异引起，而同种不同居群间的遗传变异则主要起因于基因频率的差异；相对而言，改变居群的遗传结构比较容易。也就是说，我们只要改变基因频率就可以改变居群的遗传素质；改变居群的遗传素质比改变个体的遗传素质容易得多。 Hardy-Weinberg定律只有在特定的条件（居群中没有突变，没有自然选择，没有基因流动，并且是随机交配的大居群因而不发生随机遗传漂变）下才能成立，这样的特定条件在自然居群中从来也不可能满足，所以它反而证明，在自然条件下，生物居群的基因频率一直在改变，进化一直在进行。, We have noted (我们注意到) that the Hardy-Weinberg Law establishes an ideal population that allows us to estimate allele and genotype frequencies in populations in which the assumptions of random mating, absence of selection and mutation, and equal viability (生活力) and fertility（繁殖力） hold（拥有）. Obviously, it is difficult to find natural populations in which all these assumptions hold. In nature, populations are dynamic, and changes in size and gene pool are common. The Hardy-Weinberg Law allows us to investigate populations that vary from the ideal., The Hardy-Weinberg Law specifies what will happen to allele frequencies in a population under a set of simple assumptions（在一系列简单的假设下）. If there is no selection, no mutation, and no migration, if the population is large, and if individuals mate randomly, then allele frequencies will not change from one generation to the next. The population will not evolve. The Hardy-Weinberg Law can be used to investigate whether or not a population is in evolutionary equilibrium, and to estimate the frequency of heterozygotes in a population from the frequency of homozygous recessives.,Section Summary, By specifying the conditions（通过指定的条件） under which allele frequencies will not change, the Hardy-Weinberg Law identifies （确认）the forces that can cause evolution. Selection, mutation, migration, and genetic drift can cause allele frequencies to change and are thus forces of evolution. Nonrandom mating does not alter the frequencies of alleles, but by altering the frequencies of genotypes, nonrandom mating can indirectly affect the course of evolution., Natural selection is the most powerful of the forces that can alter a populations allele frequencies. The rate of evolution under natural selection depends on the initial allele and on the relative fitness of different genotypes. In