定向选择 定向选择（ directional selection ）：寄主抗病品种有利于病原物相应的毒力基因的选择作用，称为定向选择。定向选择 (Directional selection) 是天择的一种形式。 对于一种特征 ( 性状 ) 的不同状态而言，因为不同状态带给生物体的适应度 ( 适存度 , fitness) 有差异，其中一种状态的适应度较高，具有此状态的有利个体就能产生较多的子代，造成此状态的等位基因 (allele) 便倾向于在族群中增加，并渐渐取代其他等位基因，此过程即称为定向选择。 假设，影响一特征 / 性状的等位基因有 A1 与 A2 两种，如果同型合子 A1A1 的适应度最高，而异型合子 A1A2 适应度中等， A2A2 适应度最低（ω 11 ω 12 ω 22, ω代表适应度），在天择的影响下，保留 A1 、并淘汰 A2 的过程就是定向选择。若环境维持稳定， A1 基因所代表的性状持续带给此物种较高的适应度，“有利的” A1 基因随着时间而渐渐散布到整个族群，基因频率提高，最后达到稳固 (fixation) 。 接续上例，适应度较低的 A2 基因可能会完全消失于族群中，此过程可称为净化选择 (purifying selection) 。对于一个突变的新基因型，而受到定向选择而被保留，可称为正向选择；反之，有害的基因受到淘汰称为负向选择。依照讨论的例子，有时负向选择与净化选择意义相同。 定向选择与平衡选择 (balancing selection) 相反，前者只保留有利的基因型，后者会倾向于保留多种基因型。值得考虑的是，不同的环境可能偏好不同的基因型，因此一个等位基因有利或有害，需要参考生物族群的生活环境才能决定。 定向选择造成的基因频率变化，由相对的适应度决定，并不会因为此基因是显性或隐性而受影响。虽然说，被保留的基因达到稳固的时间，会因为此基因是显性或隐性而有差异：若有利的基因是隐性，则在起始基因频率很低的状况下，只有很少的个体是隐性同型合子、很少个体表现出较高的适应度，因此需要累积很多世代才能增加基因频率。 以族群遗传学的方法，比较两世代之间的基因频率是否有变化，可以推知此基因所影响的性状是否正在受到天择的作用。 自然选择 正负中性选择 自然选择 Natural selection 生物在生存斗争中适者生存、不适者被淘汰的现象。最初由 C · R ·达尔文提出。达尔文虽借用了人工选择中的“选择”这个词，但这只是一种比喻，并非说有超自然的力量在进行选择。按照达尔文的意见，自然选择不过是生物与自然环境相互作用的结果。从进化的观点看，能生存下来的个体不一定就是最适者，只有生存下来并留下众多后代的个体才是最适者；又考虑到进化是群体而不是个体的现象，现代综合进化论从群体遗传学的角度修正了达尔文的看法，认为自然选择是群体中“不同基因型的有差异的（区分性的）延续”，是群体中增加了适应性较强的基因型频率的过程。 自然选择是为了使物种更好的适应环境，另一方面是为了抑制生物过度繁殖生物在生存过程中，既要与自然环境进行斗争，又要与其他生物进行斗争。在生存斗争中，具有有利变异的个体容易生存下来（适者生存），具有不利变异的个体则被淘汰，这便是自然选择的原因，自然选择促成了物种的进化。 达尔文的自然选择学说，其主要内容有四点：过度繁殖，生存斗争（也叫生存竞争），遗传和变异，适者生存。 正向选择，负向选择（纯化选择）和中性选择 Positive selection ， Negative （ Purifying ） selection and Neutral selection In population genetics, the relative fitness of a new mutant allele a relative to the predominant wild-type allele A is measured by the selective coefficient (Malthusian parameter) s. Let the relative fitness of genotypes AA, Aa, and aa be 1, 1 + s, and 1 + 2s, respectively. Then s 0, = 0, 0 correspond to negative (purifying) selection, neutral evolution, and positive selection, respectively. The frequency of the new mutant allele goes up or down over generations, affected by natural selection as well as random genetic drift. Whether random drift or selection dominates the fate of the mutation depends on Ns, where N is the effective population size. If |Ns| 》 1, natural selection dominates the fate of the allele while if |Ns| is close to 0, random drift will be very important and the mutation is effectively neutral or nearly neutral. 物种受外界环境的影响，进行基因的自我调节和转变，淘汰不适应环境的基因，产生可有效适应环境的基因。 参考： 《 Computational Molecular Evolution 》 定向选择 (Directional selection) 是天择的一种形式。 对于一种特征 ( 性状 ) 的不同状态而言，因为不同状态带给生物体的适应度 ( 适存度 , fitness) 有差异，其中一种状态的适应度较高，具有此状态的有利个体就能产生较多的子代，造成此状态的等位基因 (allele) 便倾向于在族群中增加，并渐渐取代其他等位基因，此过程即称为定向选择。 假设，影响一特征 / 性状的等位基因有 A1 与 A2 两种，如果同型合子 A1A1 的适应度最高，而异型合子 A1A2 适应度中等， A2A2 适应度最低（ω 11 ω 12 ω 22, ω代表适应度），在天择的影响下，保留 A1 、并淘汰 A2 的过程就是定向选择。若环境维持稳定， A1 基因所代表的性状持续带给此物种较高的适应度，“有利的” A1 基因随着时间而渐渐散布到整个族群，基因频率提高，最后达到稳固 (fixation) 。 接续上例，适应度较低的 A2 基因可能会完全消失于族群中，此过程可称为净化选择 (purifying selection) 。对于一个突变的新基因型，而受到定向选择而被保留，可称为正向选择；反之，有害的基因受到淘汰称为负向选择。依照讨论的例子，有时负向选择与净化选择意义相同。 定向选择与平衡选择 (balancing selection) 相反，前者只保留有利的基因型，后者会倾向于保留多种基因型。值得考虑的是，不同的环境可能偏好不同的基因型，因此一个等位基因有利或有害，需要参考生物族群的生活环境才能决定。 定向选择造成的基因频率变化，由相对的适应度决定，并不会因为此基因是显性或隐性而受影响。虽然说，被保留的基因达到稳固的时间，会因为此基因是显性或隐性而有差异：若有利的基因是隐性，则在起始基因频率很低的状况下，只有很少的个体是隐性同型合子、很少个体表现出较高的适应度，因此需要累积很多世代才能增加基因频率。 以族群遗传学的方法，比较两世代之间的基因频率是否有变化，可以推知此基因所影响的性状是否正在受到天择的作用。 Futuyma, D. J. (2009). Evolution (Sunderland, MA USA: Sinauer Associates, Inc.).
知识发现平台 http://arrowsmith.psych.uic.edu/cgi-bin/arrowsmith_uic/edit_b.cgi 知识发现结果： Start A-Literature C-Literature B-list Filter Literature A-query: artemisinin C-query: Malaria The B-list contains title words and phrases (terms) that appeared in both the A and the C literature. 1588 articles appeared in both literatures and were not included in the process of computing the B-list but can be viewed here . The results of this search are saved under id # 3673 and can be accessed from the start page after you leave this session. There are 355 terms on the current B-list ( 192 are predicted to be relevant ), which is shown ranked according to predicted relevance. The list can be further trimmed down using the filters listed in the left margin. To assess whether there appears to be a biologically significant relationship between the AB and BC literatures for specific B-terms, please select one or more B-terms and then click the button to view the corresponding AB and BC literatures. Use Ctrl to select multiple B-terms. 发现192个相关知识单元： http://arrowsmith.psych.uic.edu/cgi-bin/arrowsmith_uic/view_b_txt.cgi?ID=3673 job id # 3673 started Mon Sep 12 23:54:07 2011 Max_citations: 50000 Stoplist: /var/www/html/arrowsmith_uic/data/stopwords_pubmed Ngram_max: 3 3673 Search ARROWSMITH A A_query_raw: artemisininMon Sep 12 23:54:34 2011 A query = artemisinin started Mon Sep 12 23:54:34 2011 A query resulted in 2500 titles 3673 Search ARROWSMITH C C_query_raw: Malaria Mon Sep 12 23:54:41 2011 C: Malaria 59590 A: pubmed_query_A 2500 AC: ( artemisinin ) AND ( Malaria ) 1588 C query = Malaria started Mon Sep 12 23:54:42 2011 C query resulted in 50000 titles A AND C query resulted in 1588 titles A_query_raw: Mon Sep 12 23:54:55 2011 3463 B-terms ready on Mon Sep 12 23:55:27 2011 Sem_filter: Living Beings 355 B-terms left after filter executed Tue Sep 13 00:05:00 2011 B-list on Tue Sep 13 00:08:17 2011 1 plasmodium falciparum isolate 2 plasmodium falciparum infected 3 beta arteether 4 susceptibility plasmodium falciparum 5 alpha beta arteether 6 beta artemether 7 isolate plasmodium falciparum 8 vitro susceptibility plasmodium 9 isolate plasmodium 10 mefloquine resistant plasmodium 11 quinine plasmodium falciparum 12 chloroquine resistant plasmodium 13 plasmodium falciparum vitro 14 derivative plasmodium falciparum 15 artemether plasmodium falciparum 16 susceptibility plasmodium 17 resistance plasmodium falciparum 18 babesia 19 resistant plasmodium 20 mechanism plasmodium 21 resistant plasmodium falciparum 22 plasmodium falciparum glutathione 23 artemisia annua 24 plasmodium falciparum drug 25 strain plasmodium 26 resistance plasmodium 27 plasmodium falciparum strain 28 vitro resistance plasmodium 29 vitro plasmodium falciparum 30 multidrug resistant plasmodium 31 mutant plasmodium 32 strain plasmodium falciparum 33 artesunate plasmodium 34 extract artemisia 35 plasmodium falciparum susceptibility 36 electrochemical detection 37 plasmodium yoelii 38 mefloquine resistance plasmodium 39 mefloquine human 40 plasmodium berghei 41 mefloquine plasmodium falciparum 42 vitro plasmodium 43 action plasmodium falciparum 44 neospora caninum 45 plasmodium yoelii nigeriensis 46 haematobium 47 action plasmodium 48 nigeriensis infected mice 49 haematobium infection 50 retinol plasmodium 51 schistosoma japonicum 52 derivative plasmodium 53 plasmodium berghei author 54 efficacy plasmodium 55 retinol plasmodium falciparum 56 pathogenesis plasmodium 57 drug resistance plasmodium 58 resistant plasmodium berghei 59 plasmodium falciparum 60 juvenile schistosoma 61 babesia microti 62 plasmodium gallinaceum 63 analysis plasmodium falciparum 64 induce apoptosis human 65 efficacy plasmodium falciparum 66 yoelii mice 67 malarial parasite 68 thai isolate 69 erythromycin plasmodium 70 schistosoma haematobium 71 dihydroartemisinin vietnamese 72 plasmodium falciparum thailand 73 parasite plasmodium falciparum 74 susceptibility thai 75 apicomplexan parasite 76 healthy vietnamese 77 multiresistant plasmodium falciparum 78 human liver microsome 79 plasmodium falciparum trophozoite 80 plasmodium 81 hepatitis b virus 82 apicomplexan 83 artemisia annua a 84 resistant strain plasmodium 85 combination plasmodium falciparum 86 infected plasmodium falciparum 87 schistosoma mansoni infection 88 artemisia 89 madurella mycetomatis 90 trypanosoma brucei rhodesiense 91 mefloquine quinine plasmodium 92 plasmodium falciparum eastern 93 resistance plasmodium berghei 94 line plasmodium 95 plasmodium falciparum four 96 parasite protein 97 human umbilical vein 98 leishmania donovani 99 apoptosis human 100 quinine mefloquine plasmodium 101 uptake plasmodium 102 cryptosporidium parvum 103 plasmodium inui 104 toxoplasma 105 dihydroqinghaosu human 106 healthy thai 107 sensitive resistant plasmodium 108 toxoplasma gondii 109 uptake plasmodium falciparum 110 line plasmodium berghei 111 extract plasmodium 112 medicinal plant 113 analysis plasmodium 114 plasmodium falciparum cultured 115 echinostoma caproni 116 papillomavirus 117 development plasmodium 118 vitro development plasmodium 119 stage plasmodium 120 dihydroartemisinin human 121 leishmania 122 activity plasmodium falciparum 123 hepatitis c virus 124 schistosoma 125 infected schistosoma japonicum 126 development plasmodium falciparum 127 human cytochrome p450 128 babesia gibsoni 129 resistant line plasmodium 130 schistosoma mansoni 131 helminth 132 oocyst 133 dimensional structure plasmodium 134 babesia microti infection 135 release plasmodium 136 genus artemisia 137 structure plasmodium falciparum 138 transgenic 139 release plasmodium falciparum 140 human hepatocyte 141 parasite plasmodium 142 flowering 143 treatment schistosoma 144 infected plasmodium 145 treatment schistosoma mansoni 146 combination plasmodium 147 kappa b 148 sporogonic stage plasmodium 149 hybrid potent 150 mansoni infected mice 151 echinostoma 152 antimalarial action plasmodium 153 plasmodium falciparum inhibition 154 malarial parasite plasmodium 155 morphology plasmodium 156 human malarial parasite 157 infected mice 158 cryptosporidium 159 cmv 160 trophozoite infected 161 structure plasmodium 162 coccidia 163 activity plasmodium 164 strain schistosoma 165 b virus 166 pneumocystis carinii 167 vietnamese 168 pneumocystis 169 schistosoma mansoni infected 170 transgenic plant 171 eimeria tenella 172 plasmodia 173 fasciola 174 pyronaridine retinol plasmodium 175 proteome analysis plasmodium 176 lpr mice 177 mrl lpr 178 mice infected schistosoma 179 influence plasmodium 180 traditional chinese 181 plasmodium falciparum northwestern 182 echinococcus 183 eimeria 184 mouse bone marrow 185 tetrahymena thermophila 186 strain schistosoma mansoni 187 arabidopsis thaliana 188 rat liver microsome 189 single dose 190 arabidopsis 191 blind randomized placebo 192 volunteer 实证研究实例论文： http://arrowsmith.psych.uic.edu/cgi-bin/arrowsmith_uic/show_sentences.cgi Start A-Literature C-Literature B-list Filter Literature AB literature B-term BC literature artemisinin plasmodium falciparum isolate Malaria 1: In vitro susceptibility of Plasmodium falciparum isolate s from Myanmar to antimalarial drugs.2001 Add to clipboard 2: In vitro susceptibility of Plasmodium falciparum isolate s in Vietnam to artemisinin derivatives and other antimalarials.1997 Add to clipboard 1: Genetic Diversity of Polymorphic Vaccine Candidate Antigens (Apical Membrane Antigen-1, Merozoite Surface Protein-3, and Erythrocyte Binding Antigen-175) in Plasmodium falciparum Isolate s from Western and Central Africa.2011 Add to clipboard 2: Sequence analysis of coding DNA fragments of pfcrt and pfmdr-1 genes in Plasmodium falciparum isolate s from Odisha, India.2011 Add to clipboard 3: Detecting mutations in PfCRT and PfMDR1 genes among Plasmodium falciparum isolate s from Saudi Arabia by pyrosequencing.2011 Add to clipboard 4: Genetic polymorphism at the C-terminal domain (region III) of knob-associated histidine-rich protein (KAHRP) of Plasmodium falciparum in isolate s from Iran.2011 Add to clipboard 5: Highly co-ordinated var gene expression and switching in clinical Plasmodium falciparum isolate s from non-immune malaria patients.2011 Add to clipboard 6: Molecular markers of antifolate resistance in Plasmodium falciparum isolate s from Luanda, Angola.2011 Add to clipboard 7: The NTS-DBL2X Region of VAR2CSA Induces Cross-Reactive Antibodies That Inhibit Adhesion of Several Plasmodium falciparum Isolate s to Chondroitin Sulfate A.2011 Add to clipboard 8: Increased pfmdr1 copy number and sequence polymorphisms in Plasmodium falciparum isolate s from Sudanese malaria patients treated with artemether-lumefantrine.2011 Add to clipboard 9: Susceptibility of Plasmodium falciparum Isolate s to Doxycycline Is Associated with pftetQ Sequence Polymorphisms and pftetQ and pfmdt Copy Numbers.2010 Add to clipboard 10: Evidence of selective sweeps in genes conferring resistance to chloroquine and pyrimethamine in Plasmodium falciparum isolate s in India.2010 Add to clipboard 11: High prevalence of sulfadoxine/pyrimethamine-resistant alleles of Plasmodium falciparum isolate s in pregnant women at the time of introduction of intermittent preventive treatment with sulfadoxine/pyrimethamine in Gabon.2010 Add to clipboard 12: Functional and immunological characterization of the var2CSA-DBL5varepsilon domain of a placental Plasmodium falciparum isolate .2010 Add to clipboard 13: Plasmodium falciparum isolate s from Angola show the StctVMNT haplotype in the pfcrt gene.2010 Add to clipboard 14: Large variation in detection of histidine-rich protein 2 in Plasmodium falciparum isolate s from Colombia.2010 Add to clipboard 15: In vivo and in vitro gametocyte production of Plasmodium falciparum isolate s from Northern Thailand.2010 Add to clipboard 16: Resistance-mediating polymorphisms of Plasmodium falciparum among isolate s from children with severe malaria in kumasi, ghana.2010 Add to clipboard 17: Distinct haplotypes of dhfr and dhps among Plasmodium falciparum isolate s in an area of high level of sulfadoxine-pyrimethamine (SP) resistance in eastern Sudan.2009 Add to clipboard 18: Parasites bearing a single copy of the multi-drug resistance gene (pfmdr-1) with wild-type SNPs predominate amongst Plasmodium falciparum isolate s from Malawi.2009 Add to clipboard 19: Sequence variation of PfEMP1-DBLalpha in association with rosette formation in Plasmodium falciparum isolate s causing severe and uncomplicated malaria .2009 Add to clipboard
标签: 生物体
