遗传变异及物种形成

会计学1遗传变异及物种形成遗传变异及物种形成Genes are pieces of DNA which contain the genetic code necessary to produce a specific protein.A gene can have many alleles and individuals can have a homozygous or heterozygous genotype.The phenotype expressed depends on whether alleles are dominant,recessive or codominant.The total set of genes and alleles present in a population is known as the gene pool.Key NotesGenes and alleles第1页/共16页测定遗传变异种群和物种内的遗传变异可直接由DNA或蛋白（别构酶）变异来估计。蛋白或DNA片段可通过凝胶电泳分离，呈现可刻划的带来决定个体的基因型。对个体DNA的详细调查产生了一种独特的遗传指纹，对建立父子关系很有用。多 型多型指的是种群中等位基因的存在。一个种群或物种可能颜色是多型的，如蜗牛，或者一些生化特性是多型的，如植物的毒性。一些多型可能是由自然选择保持下来的，但其他是由于许多基因控制多型性状的结果。第2页/共16页polymorphismThe term polymorphism refers to the presence of alleles in a population.A population or species may be polymorphic for color,as in snails,or for some biochemical function,as in plant toxicity.Some polymorphisms are maintained by natural selection,but others appear to result from the effect of many genes controlling the polymorphic character.Genetic variation within populations and species can be estimated directly from the DNA or from protein(allozyme)variation.Proteins or DNA fragments are separated by gel electrophoresis to give bands that can be scored to determine the genotype of individuals.Detailed investigation of the DNA of individuals yields a unique genetic fingerprint,useful for establishing paternity.Measuring genetic variation第3页/共16页遗传漂变遗传漂变是基因频率的随机变化，仅偶然出现。在小种群中，遗传漂变更明显。基因频率“漂离”起始值，增加、减少或上下波动。发生遗传漂变是因为在个体、产生后代的合子以及繁殖前死亡的个体中都有偶然要素。基因频率的随机变化导致来自种群的遗传变异的固定和逐渐丧失。遗传瓶颈当种群数量突然减少时，基因频率也会发生变化，总的遗传变异下降。瓶颈发生的时候，小种群的遗传漂变导致遗传变异丧失。尽管种群数量可以恢复，遗传变异在以后许多代仍会保持低水平。第4页/共16页Genetic driftGenetic drift is a random change in gene frequency arising through chance alone.It tends to be more apparent in small populations allele frequencies drift away from their starting values,increasing,decreasing or fluctuating up and down.It occurs because there is an element of chance in which individuals and which gametes will produce offspring and which individuals will die before reproducing.Random changes in allele frequency can lead to fixation and the progressive loss of genetic variation from the population.Genetic bottleneckWhen a population undergoes an abrupt contraction in numbers,this is accompanied by a change in gene frequencies and a decline in the total genetic variation.Genetic drift in the small population during the bottleneck results in the loss of genetic variation.Although population numbers may recover,genetic variation will remain low for many generations.第5页/共16页建立者效应建立者效应描述由一个或几个个体建立新种群，导致低水平的遗传变异和在亲本种群中稀少的等位基因的经常不成比例的数量。人类中特定遗传病的高发病率可归因于建立者效应。相关主题适应（B1）稀有物种、生境损失和灭绝（V1）第6页/共16页Related topicsAdaptation (B1)Rare species,habitat loss and extinction (V1)Founder effectThis term describes the establishment of a new population by one or a few individual(s),resulting in low levels of genetic variation and often a disproportionate number of alleles that are rate in the parent population.In human populations the high incidence of certain genetic diseases can be traced to founder effects.第7页/共16页繁殖种的概念繁殖种的概念，集中在如下思想：物种延续并保持其遗传的完整性和独特性是因为不进行异种杂交。生物种概念和识别种概念都根据相互杂交定义种。本观点与实际的物种定义不同的是后者以明显的形态学性状为基础。基 因 流基因流描述的是基因在种群内通过相互杂交、扩散和迁移进行的运动。高基因流使种群遗传上彼此相似。受到限制的基因流使种群间发生分化。第8页/共16页The reproductive species concept focuses on the idea that species exist and maintain their genetic integrity and distinctness because they do not interbreed.The biological species concept and the recognition species concept both define species in terms of interbreeding.His view contrasts with the practical definition of species on the basis of distinguishing morphological characters.Key NotesReproductive species conceptGene flow describes he movement of genes among populations through interbreeding,dispersal and migration.High gene flow causes populations to become genetically similar to one another.Restricted gene flow allows differentiation to occur among populations.Gene flow第9页/共16页生物种的概念生物种概念认为种是一组可以相互杂交的自然种群，它们与其他种群间具有繁殖隔离。该概念将没有明显形态学差别的种群分成分离的姊妹种，因为它们不能彼此交换基因。生物种由繁殖隔离机制来保持。识别种的概念识别种的概念定义种是一组具有共同的交配识别系统（SMRS）的个体。交配识别系统包括交配的所有方面，如繁殖器官和配子的兼容性、求偶鸣叫、行为和仪式。该种定义强调将种保持在一起的因素。第10页/共16页This concept views species as a group of interbreeding natural populations that are reproductively isolated from other such groups.It allows populations that are indistinguishable morphologically to be classified as separate,sibling species because they do not interchange genes.Biological species are maintained by reproductive isolating mechanisms.Biological species conceptThe recognition concept views a species as a group of individuals with a common Specific Mate Recognition System(SMRS).The SMRS consists of all aspects of mating such as compatibility of reproductive organs and gametes,courtship song and behavior and pheromones.This species definition emphasizes the factors that keep species together.Recognition species concept 第11页/共16页异域性物种形成与原来种由于地理隔离而进化形成新种，为异域性物种形成。异域性物种形成最易发生在边缘隔离、处在种分布区的极端边缘的小种群。小的非典型种群与极端环境条件的混合作用可产生迅速而广泛的遗传重组（遗传革命），从而导致物种形成。邻域性物种形成邻域性物种形成发生在分布区相邻，但分布区内不同地点环境（如气候）条件不同的种群。如在环形种中那样，可发现中间杂种，但包括的很大的距离使两种类型不能完全混合。第12页/共16页Allopatric speciation occurs when the new species evolves in geographic isolation from the parent species.It may take place most readily in peripheral isolates,small populations at the extreme edge of a species range.The combined effect of a small atypical population and extreme environmnental conditions can cause rapid and extensive genetic reorganization(a genetic revolution)leading to speciation.Allopatric speciationThis form of speciation occurs where the speciating populations are contiguous but subject to different environmental(e.g.climatic)conditions in different parts of the range,Intermediate hybrids are found,as in ring species,but the large distances involved prevent the two types from merging completely.Parapatric speciation第13页/共16页相关主题适应（B1）遗传变异（O1）同域性物种形成同域性物种形成可能发生在没有地理隔离，但具有宿主选择差异、食物选择差异或生境选择差异的种群。同域性物种形成是否发生尚有争议，尽管已观察到植食性昆虫宿主选择的快速变化。植物可以通过多倍体进行同域性物种形成。第14页/共16页Related topicsAdaptation (B1)Genetic variation(O1)Sympatric speciationSympatric speciation may occur where there is no geographical separation between the speciating populations of habitat preference.Whether sympatric speciation happens at all is a contentious issue,although rapid changes in the host preference of phytophagous insects has been observed.Plants can undergo sympatric speciation through polyploidy.第15页/共16页