Most of the properties we associate with life are properties of the cytoplasm. Much of the mass of a cell consists of this semifluid substance, which is bounded on the outside by the plasma membrane. Organelles are suspended within it, supported by t.. of the cytoskeleton. Dissolved inluid are nutrients, ions, soluble proteins, and other materials needed for cell functioning.生命的大部分特征表现在细胞质的特征上。细胞质大部分由半流体物质组成，并由细胞膜包被。细胞器悬浮在其中，并由丝状的细胞骨架支撑。细胞质中溶解了大量的营养物质、离子、可溶蛋白以及维持细胞生理需求的其它物质。
The eukaryotic cell nucleus is the largest organelle and houses the genetic material (DNA) on chromosomes. (In prokaryotes the hereditary material is found in the nucleoid.) The nucleus also contains one or two organelles-the nucleoli-that play a role in cell division. A pore-perforated sac called the nuclear envelope separates the nucleus and its contents from the cytoplasm. Small molecules can pass through the nuclear envelope, but larger molecules such as mRNA and ribosomes must enter and exit via the pores.真核细胞的细胞核是最大的细胞器，为染色体上遗传物质（DNA）提供空间（原核生物遗传物质存在于拟核中）。细胞核含有一或二个核仁，在细胞分裂中发挥作用。一个穿孔的囊叫核膜，将细胞核和其中内含物与细胞质分开，小分子物质可以自由通过核膜，但大分子物质如mRNA 和核糖体必须通过核孔进出核膜。
All eukaryotic cells contain most of the various kinds of organelles, and each organelle performs a specialized function in the cell. Organelles described in this section include ribosomes, the endoplasmic reticulum, the Golgi complex, vacuoles, lysosomes, mitochondria, and the plastids of plant cells.所有的真核细胞都
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含有多种细胞器，每种细胞器在细胞中行使一项专门功能。本节主要介绍核糖体、内质网、高尔基体系、液泡、溶酶体、线粒体和植物细胞中的质体。
The number of ribosomes within a cell may range from a few hundred to many thousands. This quantity reflects the fact that, ribosomes are the sites at which amino acids are assembled into proteins for export or for use in cell processes. A complete ribosome is composed of one larger and one smaller subunit. During protein synthesis the two subunits move along a strand of mRNA, \"reading\" the genetic sequence coded in it and translating that sequence into protein. Several ribosomes may become attached to a single mRNA strand; such a combination is called a polysome. Most cellular proteins are manufactured on ribosomes in the cytoplasm. Exportable proteins and membrane proteins are usually made in association with the endoplasmic reticulum.细胞中核糖体的数量变化从几百到几千，核糖体数量如此之多反映了核糖体是氨基酸组装成蛋白质并被运出或在细胞过程中使用的位点这样一个事实。完整的核糖体由大亚基和小亚基组成。在蛋白质合成中两个亚基沿mRNA 链移动，阅读其中编码的遗传序列，将序列翻译成蛋白质。一条mRNA上可能有多个核糖体，这种组合称多聚核糖体。大多数细胞中的蛋白质由细胞质中的核糖体（游离核糖体）制造。运输蛋白和膜蛋白通常由附着于内质网上的核糖体制造。
The endoplasmic reticulum, a lacy array of membranous sacs, tubules, and vesicles, may be either rough (RER) or smooth (SER). Both types play roles in the synthesis and transport of proteins. The RER, which is studded with polysomes, also seems to be the source of the nuclear envelope after a cell divides.内质网是由膜状囊、管和小泡等排列在一起，有粗面内质网和滑面内质网两种类型。两种类型都与蛋白质的合成和运输有关。粗面内质网上散布着多聚核糖体，似乎也是细胞分裂后形成核膜
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的来源。
SER lacks polysomes; it is active in the synthesis of fats and steroids and in the oxidation of toxic substances in the cell. Both types of endoplasmic reticulum serve as compartments within the cell where specific products can be isolated and subsequently shunted to particular areas in or outside the cell.光滑内质网上无核糖体/光滑内质网上缺乏多聚核糖体，主要作用是脂肪和类固醇的合成以及细胞内有毒物质的氧化。两种内质网合成的产物在其中进行分流或运输到细胞外/这两种类型的内质网都担当细胞中的隔断，致使特殊产物能够分离并最终逃避到细胞内外的特定区域。
Transport vesicles may carry exportable molecules from the endoplasmic reticulum to another membranous organelle, the Golgi complex. Within the Golgi complex molecules are modified and packaged for export out of the cell or for delivery else where in the cytoplasm.
运输小泡能够将可运输分子从内质网运输到高尔基复合体上。在高尔基复合体中修饰，包装后输出细胞或传递到细胞质中的其他场所。
Vacuoles in cells appear to be hollow sacs but are actually filled with fluid and soluble molecules. The most prominent vacuoles appear in plant cells and serve as water reservoirs and storage sites for sugars and other molecules. Vacuoles in animal cells carry out phagocytosis (the intake of particulate matter) and pinocytosis (vacuolar drinking).细胞中的液泡好象是中空的，但实际上充满了液体和可溶性分子。最典型的液泡存在于植物细胞中，储备水，糖以及其它分子。动物中的液泡起吞噬（颗粒物的吸收）和胞饮作用。
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A subset of vacuoles are the organelles known as lysosomes, which contain digestive e(packaged in lysosomes in the Golgi complex) that can break down most biological macromolecules. They act to digest food particles and to degrade damaged cell parts.溶酶体是液泡亚单位，含有消化酶（在高尔基体中的溶酶体中包装而成），能够降解大部分生物大分子。消化食物微粒和降解损伤的细胞残片/行使消化食物颗粒和分解损坏的细胞组件的作用。
Mitochondria are the sites of energy-yielding chemical reactions in all cells. In addition, plant cells contain plastids that utilize light energy to manufacture carbohydrates in the process of photosynthesis. It is on the large surface area provided by the inner cristae of mitochondria that ATP-generating enzymes are located. Mitochondria are self-replicating, and probably they are the evolutionary descendants of what were once free-living prokaryotes.线粒体是所有细胞中发生产能化学反应的场所。另外，植物细胞中的质体在光合作用中利用光能产生碳水化合物，线粒体内嵴上提供了很大的表面积并分布着产ATP酶。线粒体自我复制，并且可能是自由生活的原核生物在进化中形成的后代。
There are two types of plastids: leucoplasts, which lack pigments and serve as storage sites for starch, proteins, and oils; and chromoplasts, which contain pigments.The most important chromoplasts are chloroplasts-organelles that contain the chlorophyll used in photosynthesis. The internal structure of chloroplasts includes stacks of membranes called grana, which are embedded in a matrix called the stroma.质体有两种类型：白色体，缺乏色素，是淀粉、蛋白质和油类的贮存场所；色质体，含有色素。叶绿体是最重要的色质体，含有与光合作用有关的叶绿素。叶绿体的内部结构是由多层膜垛叠形成的叶绿体基粒，其中包埋在基质中的基粒称子
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座。
All eukaryotic cells have a cytoskeleton, which is a convoluted latticework of filaments and tubules that appears to fill all available space in the cell and provides support for various other organelles. A large portion of the cytoskeleton consists of threadlike microfilaments composed mainly of the contractile protein actin. They are involved in many types of intracellular movements in plant and animal cells. A second protein, myosin, is involve in the contraction of muscle cells. Another main structural component of the cytoskeleton consists of microtubules, which are composed of the globular protein tubulin and together act as scaffolding that provides a stable cell shape. Cytoskeletal intermediate filaments appear tensile strength to the cell cytoplasm. Mechanoenzymes such as myosin, dynein, and kinesin interact with the cytoskeletal filaments and tubules to generate forces that cause movements.所有的真核细胞都有细胞骨架，它是由细丝和管组成的回旋状的网格，似乎充满了细胞中的所有空间并为其它各种细胞器提供了支撑作用。细胞骨架大部分由线状的微丝组成，微丝主要由可收缩的肌动蛋白组成。动物和植物细胞中多种类型的细胞运动与肌动蛋白有关。第二类蛋白是肌球蛋白，它与肌肉细胞的收缩有关。细胞骨架的另一个主要结构组分是微管，由球形蛋白质—微管蛋白组成，象脚手架那样使细胞维持稳定的形状。细胞骨架的中间丝提供了细胞质伸缩动力。一些动力酶如，肌球蛋白，动力蛋白，驱动蛋白与微丝、微管相互作用产生动力而引起细胞运动。
Although the cytoskeleton provides some stability to cells, its microtubules and filaments and their associated proteins enable cells to move by creeping or gliding . Such movements require a solid substrate to which the cell can adhere and can be guided by the geometry of the surface. Some cells also exhibit
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chemotaxis, the ability to move toward or away from the source of a diffusing chemical.尽管细胞骨架提供了细胞的某些稳定性，微丝，微管及相关蛋白能使细胞爬行或滑动。这种运动需要固体基质依托并通过表面几何形状的改变而运动。某些细胞具备趋化性，即趋向或逃离扩散开的化学源。
Certain eukaryotic cells can swim freely in liquid environments, propelled by whiplike cilia or flagella. Both cilia and flagella have the same internal structure: nine doublets (pairs of microtubules) are arranged in a ring and extend the length of the cilium or flagellum, and two more microtubules run down the center of the ring. Every cilium or flagellum grows only from the cell surface where a basal body is located. Movement is based on the activities of tiny dynein side arms that extend from one of the microtubules of each doublet.某些真核细胞能在液体中自由运动，由纤毛或鞭毛推动。纤毛和鞭毛具有同样的内部结构：9个联管（微管对）排列成环状，沿纤毛或鞭毛纵向延伸，环中心是两个或两个以上微管组成。纤毛或鞭毛从细胞表面的基体长出，双微管的动力蛋白臂从一侧延伸到另一侧而引起运动。
Nutrients, proteins, and other materials within most plant cells are moved about via cytoplasmic streaming. The process occurs as myosin proteins attached to organelles push against microfilaments arrayed throughout the cell. Microfilaments and microtubules are responsible for almost all major cytoplasmic movements. During cell division, microtubules of the spindle assembled from tubutin subunits near organelles called centrioles move the chromosomes.大部分植物细胞的营养物质、蛋白质和其它物质通过胞质流动进行运动。这个过程是由于依附在细胞器上的肌球蛋白推动微丝在细胞中重排形成的。绝大部分细胞质运动由微丝和微管完成。在细胞分裂过程中，中心粒周围的由微管蛋白亚基装配形成的纺锤体微管移向染色体。
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Photosynthesis occurs only in the chlorophyllchlorophyll-containing cells of green plants, algae, and certain protists and bacteria. Overall, it is a process that converts light energy into chemical energy that is stored in the molecular bonds. From the point of view of chemistry and energetics, it is the opposite of cellular respiration. Whereas cellular respiration is highly exergonic and releases energy, photosynthesis requires energy and is highly endergonic.光合作用只发生在绿色植物，海藻，某些原生动物和细菌等含有叶绿素的细胞中。总体来说，这是一个将光能转化成化学能，并将能量贮存在化学键中。从化学和能量学的角度来看，它是细胞呼吸作用的对立面。细胞呼吸作用是高度放能的，而光合作用是需要能量并高吸能的过程。
Photosynthesis starts with CO2 and H2O as raw materials and proceeds through two sets of partial reactions. In the first set, called the light-dependent reactions, water molecules are split(oxidized), 02 is released, and ATP and NADPH are formed. These reactions must take place in the presence of light energy. In the second set, called light-independent reactions, CO2 is reduced (via the addition of H atoms) to carbohydrate. These chemical events rely on the electron carrier NADPH and ATP generated by the first set of reactions.光合作用以二氧化碳和水为原材料并经历两步化学反应。第一步反应称为光反应，水分子分解（氧化），放出氧气，形成ATP和NADPH。这些反应必须有光能的存在才能进行。第二步称为非光依赖型反应（暗反应），二氧化碳被还原（通过加氢）成碳水化合物，这步反应依赖于第一步反应产生的NADPH和ATP 作为电子载体
Both sets of reactions take place in chloroplasts. Most of the enzymes and pigments for the lightdependent reactions are embedded in the thylakoid membrane of chloroplasts. The dark reactions take place in the stroma. 以上两步
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反应都发生在叶绿体中。光反应需要的大部分酶和色素包埋在叶绿体的类囊体膜上。暗反应发生在基质中。
The energy in light photons in the visible part of the spectrum can be captured by biological molecules to do constructive work. The pigment chlorophyll in plant cells absorbs photons within a particular absorption spectrums statement of the amount of light absorbed by chlorophyll at different wavelengths. When light is abt alters the arrangement of electrons in the absorbing molecule. The added energy of the photon boosts the energy condition of the molecule from a stable state to a less-stable excited state. During the light-dependent reactions of photosynthesis, as the absorbing molecule returns to the ground state, the \"excess\" excitation energy is transmitted to other molecules and stored as chemical energy.生物分子能捕获可见光谱中的光能用于建设性的工作。植物细胞中叶绿素在不同光波下吸收特定吸收光谱。当光线被吸收时，光的作用使分子中的电子发生重排/它改变了吸收分子的电子排列。由光子增加的能量推进分子的能量状况从稳定状态（基态）转变为不稳定的激发态。在光合作用的光反应过程中，吸收分子又回到基态，多余的激发能传递到其它分子中，以化学能的形式储存起来。
All photosynthetic organisms contain various classes of chlorophylls and one or more carotenoid (accessory) pigments that also contribute to photosynthesis. Groups of pigment molecules called antenna complexes are present on thylakoids. Light striking any one of the pigment molecules is funneled to a special chlorophyll a molecule, termed a reaction-center chlorophyll, which directly participates in photosynthesis. Most photosynthetic organisms possess two types of reaction-center chlorophylls, P680 and P700, each associated with an
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electron acceptor molecule and an electron donor. These aggregations are known respectively as photosystem Ⅰ (P700) and photosystem Ⅱ (P680).所有的光合作用生物含有不同等级的叶绿素和一种或多种类胡萝卜素（辅助色素），这些色素在光合作用中发挥作用。称作天线复合体的色素分子群存在于类囊体中。到达任何一个色素分子的光能汇集到称为反应中心叶绿素的特殊的叶绿素分子上，它直接参与光合作用。大部分光反应细胞器拥有两套反应中心，P680和P700，每个光系统与电子受体分子和电子供体分子相关联。这些集合体就是大家熟识的光合系统Ⅰ (P700)和光合系统Ⅱ(P680) 。
The photosystems of the light-dependent reactions are responsible for the packaging of light energy in the chemical compounds ATP and NADPH. This packaging takes place through a series of oxidation reduction reactions set in motion when light strikes the P680 reaction center in photosystem Ⅱ. In this initial event water molecules are cleaved, oxygen is released, and electrons are donated. These electrons are accepted first by plastoquinone and then by a series of carriers as they descend an electron transport chain. For each four electrons that pass down the chain, two ATPs are formed. The last acceptor in the chain is the P700 reaction center of photosystem Ⅰ. At this point incoming photons boost the energy of the electrons, and they are accepted by ferredoxin. Ferredoxin is then reoxidized, and the coenzyme NADP+ is reduced to the NADPH. The ATP generated previously and the NADPH then take part in the light independent reactions.光反应的光系统将光能转化成化学复合物ATP和NADPH。当光激活光系统Ⅱ的光反应中心时，通过一系列的氧化还原反应实现能量的传递。/这一包装过程从光线到达光合系统II的活性中心P680这一步骤开始，包括了一系列氧化还原反应反应。在第一个步骤中，水分子被分解，放出氧气，提供电子。电子首先传递给质体醌，然后通过一系列载体形成的电子传递链。每传递4个电子，形成2个ATP。链的最终受体是光合系统I的
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P700 活性中心。此处光子激活电子，电子传递给铁氧还蛋白。铁氧还蛋白再氧化，并且辅酶NADP+还原成NADPH。前面产生的ATP和NADPH进入暗反应。
The production of ATP from the transport of electrons excited by light energy down an electron transport chain is termed photophosphorylation. The one-way flow of electrons through photosystems II and I is called noncyclic photophosphorylation; plants also derive additional ATP through cyclic photophosphorylation, in which some electrons are shunted back through the electron transport chain between photosystems Ⅱ and Ⅰ.由光能激发产生的电子沿电子传递链传递并产生ATP 的过程称为光合磷酸化。电子通过光合系统Ⅱ流经光合系统Ⅰ的电子路径称非循环式光合磷酸化；植物通过循环式光合磷酸化获得额外的ATP，在这一过程中一些电子在光合系统Ⅰ和Ⅱ之间的电子传递链中回流。
In the light-independent reactions of photosynthesis, which are driven by ATP and NADPH, C02 is converted to carbohydrate. The reactions are also known as the Calvin-Benson cycle. Atmospheric CO2, is fixed as it reacts with ribulose biphosphate (RuBP), a reaction that is catalyzed by the enzyme ribulose biphosphate carboxylThe reduction Of C02 to carbohydrate (fructose diphosphate several more steps of the cycle. Finally, RUBP is regenerated so that the cycle may continue.由ATP和NADPH驱动的暗反应中，CO2转化成碳水化合物。即卡尔文-本森循环。在二磷酸核酮糖羧化酶的催化下，二磷酸核酮糖固定空气中的CO2。CO2通过循环中的几步反应还原成为糖类（二磷酸果糖），最终核酮糖二磷酸再生，这样循环能够持续进行。
High levels of oxygen in plant cells can disrupt photosynthesis and can also cause photorespiration-an inefficient fun of the dark reactions in which 02 is fixed
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rather than C02 and no carbohydrate is produced.植物细胞中高浓度的氧气能够破坏光合作用，同时也能够引起光呼吸，这是一种效率差的暗反应， 02而不是CO2被固定，不产生糖类。
Most plants are C3 plants; they experience decreased carbohydrate production under hot, dry conditions as a result of the effects of photorespiration. Among C4 plants, however, special leaf anatomy and a unique biochemical pathway enable the plant to thrive in and conditions. Thus C4 plants lessen photorespiration by carrying out photosynthesis only in cells that are insulated from high levels of CO2. They also possess a novel mechanism for carbon fixation.许多植物是C3植物，在高温干旱条件下，由于光呼吸作用而使碳水化合物的合成降低。而在大多数的C4 植物中，由于叶脉的特殊构造和独特的化学路径使植物依然很茂盛。这样C4 植物并非在细胞中存在高浓度的CO2条件下才能进行光合作用，这是碳固定的一个新机制。
The cell nucleus is the main repository of genetic information. Within the nucleus are the chromosomes tightly coiled strands of DNA and clusters of associated proteins. Long stretches of the continuous DNA molecule wind around these clusters of proteins, or histones, forming beadlike complexes known as nucleosomes. More coiling and supercoiling produces a dense chromosome structure. Each long strand of DNA combines with histones and nonhistone proteins to make up the substance chromatin.细胞核是贮藏遗传信息的主要场所。DNA盘绕成螺旋线以及相关的成簇蛋白质。DNA螺旋线缠绕成簇的组蛋白形成珠链状的核小体。这些螺旋或超螺旋形成致密的染色体组结构。每个长链DNA与组蛋白和非组蛋白一起构成染色质。
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A pictorial display of an organism's chromosomes in the coiled, condensed state is known as a karyotype. Karyotype reveal that in most cells all but sex chromosomes are present as two copies, referred to as homologous pairs. Non-sex chromosomes are called autoso.Organisms whose cells contain two sets of parental chromosomes are called diploid; those with cells containing a single set of parental chromosomes are called haploid.以图示方式展示出来的螺旋的、浓缩状态有机体染色体，称为染色体组型。除了性染色体外，大多数细胞的染色体组成对出现，称同源染色体对。非性染色体称常染色体。生物细胞含有两套亲本染色体的有机体称为二倍体；含有单套染色体的有机体称单倍体。The cell cycle is a regular sequence in which the cell grows, prepares for division, and divides to form two daughter cells, each of which then repeats the cycle. Such cycling in effect makes single-celled organisms immortal. Many cells in multicellular organisms, including animal muscle and nerve cells, either slow the cycle or break out of it altogether.细胞周期是一个有顺序的过程：细胞生长、准备分裂、分裂形成两个子细胞，子细胞再循环。此循环使得单细胞生物永生。多细胞生物中的许多细胞，包括动物肌肉和神经细胞，能够使细胞周期时间延长或完全脱离细胞周期。
The normal cell cycle consists of four phases. The first three include G1, the period of normal metabolism; S phase, during which normal synthesis of biological molecules continues, DNA is replicated, and histones are synthesized; and G2, a brief period of metabolism and additional growth. Toget G1, S, and G2 phases are called interphase. The fourth phase of the cell cycle is M phase, the period of mitosis, during which the replicated chromosomes condense and move and the cell divides. It is believed that properties of the cell cytoplasm control the cell cycle, along with external stimulators and inhibitors such as chalones.正常的细胞周
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期包括四步。前3步包括G1(正常的代谢阶段)；S期（持续进行生物分子的正常合成，DNA复制，组蛋白合成）；G2期（代谢和再次生长的短阶段）。G1期、S期和G2期合起来称为间期。细胞周期的第四步为M 期（进行有丝分裂的阶段），在这一阶段，复制的染色体浓缩、运动，细胞分裂。被认为是细胞质的特性和外部的刺激因子和抑制剂例如抑素控制了细胞周期。
Biologists divide the mitotic cycle into four phases. At the beginning of prophase the chromosomes each consist of two highly condensed chromatids attached to each other at a centromere. As prophase ends and metaphase begins, the condensed chromosomes become associated with the spindle. Eventually thein a plane (called the metaphase plate) at a rig spindle fibers. Next, during anaphase , the two sister chromatids of each chromosome split, and one from each pair is drawn toward each pole of the cell. During telophase nuclear envelopes begin to form around each set of chromosomes, and division of the cytoplasm takes place.生物学家将有丝分裂划分为4个阶段。分裂前期，每一条染色体包括两条高度浓缩的染色单体，两个染色单体通过着丝粒连接在一起。当前期结束时，分裂中期开始，浓缩的染色体与纺锤体相连，最后以与纺锤丝成垂直角度排列在赤道板上。在分裂后期，每个染色体上的两个姊妹染色单体分开，分别移向细胞两极。在分裂末期，围绕每一套染色体开始形成核膜，然后开始细胞质的分裂。
As mitosis proceeds, the spindle microtubules play a crucial role in ensuring that both paired and separated chromatids move in the right directions at the proper times. Each half of the spindle forms as micros extend from each pole of a dividing cell to the region of the metaphase plate. During prophase, other microtubules, the centromeric fibers, extend outward from the spindle poles to
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structures on the chromosomes called kinetochores. During anaphase the fibers begin to shorten, and the chromatids begin to move apart.在有丝分裂过程中，是纺锤体的微管确保了染色单体适时的以正确方向进行分离。当微管从每个分裂细胞的两极延伸至赤道板时，形成了纺锤体的一半。在有丝分裂前期，另外一种微管，着丝粒纤维从纺锤体的两极向外延伸到染色体的动粒上。在分裂后期，纤维开始缩短，染色单体开始分开。
The spindle forms differently in plant and animal cells. In animals it is associated with centriole , while in plant and fungal cells spindle formation is associated with reions called microtubule organizing centers.在植物和动物细胞中纺锤体的形成不同，动物细胞中，与中心体有关。在植物和真菌细胞中纺锤体的形成与叫做微管组织中心的区域有关。
The division of the cell cytoplasm at the end of mitosis is called cytokinesis . In animal cells it takes place as a ring of actin filaments contracts around the cell equator, pinchin the cell in two. In plant cells, which are by a cell wall, cytokinesis involves the building of a new cell plate across the dividing cel Cell wall material is then deposited in the region of the cell plate. 有丝分裂结束时细胞质的分裂称为胞质分裂。在动物细胞中，环形肌动蛋白丝延赤道板收缩而使细胞一分为二。植物细胞有细胞壁包被，通过在赤道板形成新的细胞板实现胞质分裂。
Meiosis is a special form of cell division that takes place in the reproductive organs that produce sex cells. Like mitosis, it takes place after DNA replication has occurred and involves two sequ nuclear divisions (meiosis I and meiosisⅡ). These divisions result in four cells, each with half the number of chromosomes of the parent cell. The phen of crossing over during meiosis results in exchanges of
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genetic infon between chromosomes. Hence, the homologous chromosomes distributed to different progeny cells are not identical.减数分裂是发生在产生生殖细胞的生殖器官中的特殊的细胞分裂方式。如有丝分裂，它发生在DNA复制后，包括两个有顺序的核分裂阶段（减数分裂I期和减数分裂II期）。这种分裂产生4个子细胞，每一个子细胞的染色体数是母细胞染色体数的一半。减数分裂交叉现象的出现导致了染色体之间互换遗传信息，这样分配到不同后代细胞中的同源的染色体并不相同。
As in mitosis two chromatids exist for each chromosome at the beginning of prophase 1. During this phase the homologous chromosomes undergo sy-napsis, or pairing, which is brought about by a bridging structure of proteins and RNA called the synaptonemal complex. The hom pairs stay together when they align on the met plate. Unlike the anaphase of mitosis, however, during anaphase I the two chromatids of each chromosome stay joined at the centromere and move together to one of the two poles of the cell. It is this event that results in the halving of the chromosome number in the four daughter cells that result from meiosis.象有丝分裂那样，在分裂前I期的开始，每个染色体含有两条染色单体。在这一阶段，两个同源染色单体通过蛋白质和RNA桥配对形成联会复合体。当同源染色体对排列于赤道板上时，这两条染色体同时存在。然而，与有丝分裂后期不同的是，减数分裂后期I每个染色体的两条染色单体在着丝粒处相连，一起移向细胞的一极。正是由于这一事件导致减数分裂中4个子细胞的染色体数目减半。
During telophase I nuclear envelopes enclose the chromosomes in nuclei, and in most species cytokinesis (the first nuclear division) follows. The second nuclear division begins with metaphase Ⅱ, in which the chromosomes in each daughter cell again align on a metaphase plate. The centromeres finally divide, and each
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sister chromatid moves to one of the poles of the spindle. The next phase is telophase , followed again by cytokinesis. The result of the entire process is four haploid cells in which parental chromosomes are randomly distributed.在减数分裂末期I，核被膜将染色体包裹在细胞核中，接着多数种类开始胞质分裂（第一次细胞核分裂）。第二次细胞核分裂开始于减数分裂中期II，每个子细胞中的染色体重新排列排列在赤道板上。着丝粒最终分离，每个姊妹染色单体移向纺锤体的一极。接着胞质分裂。整产生4个单倍体细胞，亲本的染色体被随机分配。
Mitosis and meiosis, respectively, make simple cell division and sexual reproduction possible. Each means of passing on hereditary information has advantages. In asexual reproduction the parent organism gives rise to offspring that are genetic clones of the parent. The advantages of this type of reproduction are that it preserves the parent's successful genetic complement , requires little or no specialization of reproductive organs, and is more rapid than sexual reproduction. A major disadvant of the asexual mode is that a single catastrophic event or disease may destroy an entire population of genetically identical organisms. A prime benefit of sexual reproduction is that it provides genetic variaby and a ready mec for the elimina of deleterious mutations. It also allows \"new\" gene forms to arise and spread through populations.有丝分类和减数分裂分别使简单的细胞分裂和有性生殖成为可能,每一种方式在传递遗传信息过程中各有优势。无性生殖产生后代时克隆（完全复制）了中亲本的遗传信息，这种生殖类型的优点是它保存了亲本成功的遗传信息，基本或完全不需要特化出生殖器官，与有性生殖相比更迅速。但一个简单灾难性事件或疾病都可能摧毁一个细胞群体。有性生殖的主要好处是它能够提供遗传的多样性，具有迅速的清除有害变异的机制。它也使新基因形式出现、增加并扩展到整个种群。
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Early ideas of inheritance included Hippocrates' theory of pangenesis and August Weismands germ plasm theory. Based on experiments with mice, Weismann proposed that hereditary information in gametes transmitted traits to progeny. Both of these early views incorporated the blending theory: they held that heritable traits of the two parents blend, so that the distinct characteristics of each are lost in offspring.早期的遗传学思想包括希伯克拉底的泛生论和魏斯曼的种质理论。在小鼠实验的基础上，魏斯曼提出遗传信息储存在配子中并将遗传信息传递给后代。这些早期的观点合称为混合理论：子代持有的可遗传性状是两个亲本性状的混合，所以在后代中每一个亲本失去了独特的性状
Gregor Mendel, an Augustinian monk in the monastery at Brunn, Austria, is known as the \"father of genetics.\" Having been exposed to theories of the particulate nature of matter while a university student and having a background in mathematics, Mendel carried out a series of carefully planned experiments that demonstrated the particulate nature of heredity. His revolutionary ideas were neither understood nor accepted until many years after Mendel died.孟德尔是奥地利布鲁恩修道院奥古斯丁教的修士，被誉为“遗传学之父”。当他还是大学生时就提出了物质的粒子属性，同时还有数学的背景。他开展了一系列精心设计的实验，证明遗传的物质本质。他的革命性的思想直到他去世后许多年才被人理解与接受。
Mendel studied genetics through plant-breeding experiments with the garden pea, a plant species that is self-fertilizing and breeds true (each offspring is identical to the parent in the trait of interest). To test the blending theory, he focused his research on seven distinct characters. Each of these characters, such as seed color and plant height, present only two, clear-cut possibilities. He also
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recorded the type and number of all progeny produced from each paia plants, and followed the results of each cross for two generations.孟德尔通过花园中的豌豆开展了植物育种试验而研究遗传学，豌豆是自花授粉植物和纯品系。为验证融合理论，他将研究重点放在7个明显的特征上。这些特征的每一个，例如，种子颜色和植株高度，代表了仅有两个且易于区分的可能性。他记录了产生的每一个子代的类型和数量，以及两代杂交的结果。
For each of the characters he studied, Mendel found that one trait was dominant while the other was recessive. In the second filial (F2) generation, the ratio of dominant to recessive was 3:1. Mendel deduced that this result was possible only if each individual possesses only two hereditary units, one from each parent. The units Mendel hypothesized are today known as alleles, alternative forms of genes. Genes are the basic units of heredity. An organism that inherits identical alleles for a trait from each parent is said to be homozygous for that trait; if different alleles for a trait are inherited, the organism is heterozygous for that trait. When an organism is heterozygous for a trait, the resulting phenotype for that trait expresses only the dominant allele.Thus, the organism’s phenotype—its physical appearance and properties-differs from its genotype, which may include both a dominant and a recessive allele. A pictorial representation of all possible combinations of a genetic cross is known as a Punnett square.对于他研究的每一种特性，要么显性，要么隐性。在子2代中显形与隐性比为3∶1。孟德尔推断只有在每个个体仅拥有两个研究遗传单元，并每个单元来自一个亲代时，实验结果才成立。孟德尔假设的单位如今称为等位基因（基因二中选一的形式）。基因是遗传的基本单位，一些有机体继承了来自父本、母本的同一性状的两个相同的等位基因，被称为某一性状的纯合体。如果继承的是某一性状的不同的等位基因，有机体被称为某一性状的杂合体。当有机体是某一
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性状的杂合体时，它的表型由显性基因决定。因此，有机体的表型（它的物理形状和特性）与它的基因型不同，基因型同时包括一个显性和一个隐性等位基因。遗传杂交的所有可能的组合的图示形式被称为庞纳特方格。
The results of Mendel's experiments on dominant and recessive inheritance let to Mendel's first law: the law of segregation.This law states that for a given trait an organism inherits one allele from each parent. Together these alleles form the allele pair. When gametes are formed during meiosis, the two alleles become separated (halving of chromosome number).To gain evidence for his thMendel performed test crosses, mating plants of unknown genotype to plants that were homozy recessive for the trait of interest. The ratio of dominant phenotypes (if any) in the progeny makes clear whether the unkno genotype is heterozygous, homozygous dominant, or homozygous recessive.孟德尔关于显性和隐性遗传实验的结果称为孟德尔第一定律：分离定律。这一定律认为对于某一特定形状，有机体继承了每一个亲本的等位基因。这些等位基因在一起形成了等位基因对。当减数分裂形成配子时，两个等位基因分离（染色体数目减半）。为验证此理论，孟德尔做了测交实验，即基因型未知的植物与某一隐性纯合体的植物杂交。根据后代中显性基因型的比例可以搞清未知基因型是杂合、纯合显性还是纯合隐性。
Mendel also performed dihybrid crosses, which enabled him to consider how two traits are inherited relative to one another. This work let to the law of independent assortment, which states that the alleles of genes governing different characters are inherited independently. An apparent exception to Mendel's laws is incomplete dominance, a phenomenon in which offspring of a cross exhibit a phenotype that is intermediate between those of the parents. However,
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incomplete dominance reflects the fact that both alleles for the trait in question exert an effect on the phenotype. The alleles themselves remain separate. 孟德尔也开展了双因子杂合体杂交实验，双因子杂合试验，使得他考虑两个性状如何有关联的遗传。试验结果产生独自分配定律，即控制不同性状的等位基因独立遗传。孟德尔定律的一个明显例外情况是不完全显性。杂交后代的表型是亲本的中间类型。然而，不完全显性说明了两个等位基因对表型都有影响。等位基因自身仍然是独立的。
Mendel presented his ideas in 1866 in a scientific paper published by the Brunn Society for Natural History. Unfortunately, the meaning of his research was not understood by other scientists of the day. His work was rediscovered in 1900 by Carl Correns and Hugo de Vries.
1866年，孟德尔在布鲁恩自然历史学会出版的科学报上发表了他的观点。不幸的是，他的研究不被当时科学家接受。在1900年，他的著作被 Carl Correns 和Hugo de Vries 重新发现。 。
Soon after Mendel's work was rediscovered, Walter Sutton and Theodor Boveri independently proposed that the hereditary units might be located on chromosomes. Experiments to prove this hypothesis were carried out by Thomas Hunt Morgan and his students at Columbia University, in research on the sex chromosome of fruit flies. Morgan's studies were also the first exploration of sex-linked traits. It also led to the discovery in 1916 by Calvin Bridges of the phenomenon of nondisjunction, in which a chromosome pair fails to segregate during meiosis.
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孟德尔的工作被重新发现不久，Walter Sutton 和Theodor Boveri分别提出，遗传单位可能位于染色体上。这一假设被哥伦比亚大学的Thomas Hunt Morgan 和他的学生用果蝇的性染色体实验进行了证实。摩根的研究也是性连锁特性的首次探索。它导致了1916年Calvin Bridges 的不分离现象的发现，即在减数分裂中，染色体对不分离。
The first scientist to investigate the question of how genes affect phenotype was Sir Archibald Garrod, whose studies of alkaptonuria implied a relationship between genes and enzymes. Thirty years later Beadle and Ephrussi showed a relationship between particular genes and biosynthetic reactions responsible for eye color in fruit flies. Next, in a series of classic experiments on the effects of mutations in the bread mold Neurospora crassa, Beadle and Tatum explored the one-gene-one-enzyme hypothesis-the idea that each gene codes for a particular enzyme. Their work paved the way for other researchers to elucidate the precise ways in which enzymes affect complex metabolic pathways.In 1949, in research on the role of hemoglobin in sickle cell anemia, Linus Pauling helped refine the one-gene-one-enzyme hypothesis into the one-gene-one-polypeptide hypothesis. Archibald Garrod爵士是第一个研究基因如何影响表型的科学家，他对尿黑酸症的研究揭示了基因与酶之间的关系。Beadle 和Ephrussi在三十年发现关于特定基因和控制果蝇中复眼颜色的生物合成反应之间的关系。接着，在一系列关于面包霉粗糙脉孢霉突变效应的经典实验中，Beadle 和Tatum 探究了一基因一酶假设，这一假设认为每一个基因编码一个特定的酶。他们的工作为其他研究者以精确方式阐明酶影响复杂代谢途径铺平了道路。 在1949年，在研究了镰刀形红细胞贫血症中血红蛋白的作用后，Linus Pauling 帮助将一基因一酶假设精炼成为一基因一多肽假设。
Nuclei acid, originally isolated by Johann Miescher in 1871, was identified as a
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prime constituent of chromosomes through the use of the red-staining method developed by Feulgen in the early 1900s. Frederick Griffith's experime with the R and S stains of pneumococci showed that an as yet unknown material from one set of bacterial could alter the physical traits of a second set. In the 1940s the team of Avery, MacLeod, and McCarty showed that this unknown material was DNA. At about the same time P.A. Levene discovered that DNA contained four nitrogenous bases, each of which was attached to a sugar molecule and a phosphate group-a combination Levene termed a nucleotide.
核酸最早由Johann Miescher 在1871年分离。在20世纪早期，采用由Feulgen 建立的红染方法，核酸被鉴定为组成染色体的基本成分，Frederick Griffith 采用R和S菌株对肺炎双球菌进行染色，表明来自于一个细菌的未知物质能够改变另一细菌的物理性状。在20世纪40年代，Avery、MacLeod和McCarty 组成的团队，表明这一未知物质是DNA。大约是同一时代，P.A. Levene 发现DNA 包括四种含氮碱基，每一种碱基与一个核糖分子和一个磷酸基相连，Levene称这种组合为核苷酸。
Disagreement over whether DNA could carry complex genetic information was ended in the early 1950s by Martha Chase and Alfred Hershey, whose work with E. coli showed clearly that DNA, and not protein, is the bearer of genetic information.在二十世纪50年代早期 Martha 和Alfred Hershey 关于大肠杆菌的工作清楚地表明遗传信息的载体是DNA，而不是蛋白质，这样就结束了关于ＤＮＡ是否能够携带复杂的遗传信息的争论。
Each DNA nucleotide contains a five-carbon sugar, deoxyribose, attached to one of four bases: adenine, guanine, cytosine, or thymine. Adenine and guanine
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molecules are double-ring structures called purines, while cytosine and thymine are single-ring structures called pyrimidines. The molecule made up of a base plus a sugar is termed a nucleoside. In each molecule of DNA a phosphate group links the five-carbon sugar of one nucleoside to the five-carbon sugar of the next nucleoside in the chain. This phosphate bonding creates a sugar-phosphate backbone.每一个核苷酸都含有一个五碳糖，脱氧核糖，分别连在四种碱基的其中一种之一上，即：腺嘌呤、鸟嘌呤、胞嘧啶和胸腺嘧啶。腺嘌呤和鸟嘌呤分子是被称为嘌呤的双环结构，而胞嘧啶核胸腺嘧啶是被称为嘧啶的单环结构。由一个碱基和一个糖分子构成的分子称为核苷。DNA的每个分子中磷酸基团连接了链上一个核苷的五碳糖和相邻核苷的五碳糖。磷酸键形成磷酸骨架。
Chargaff’s rules describe the fact that (1) the amount of adenine is equal to the amount of thymine in DNA, with amount of cytosine equal to that of guanine, and (2) the ratios of A to T and of C to G vary with different species.Chargaff 规则定义了如下事实，(1) DNA 中腺嘌呤数量与胸腺嘧啶的数量相同，胞嘧啶和鸟嘌呤的数量相同；（2）腺嘌呤与胸腺嘧啶，胞嘧啶与鸟嘌呤的比例随物种不同而不同。
In the late 1940s and early 1950s, researchers looking for the structure of DNA drew upon Chargaff s insight, Levene's ideas on DNA components, and two other lines of evidence. One was the suggestion of Linus Pauling that DNA might have a helical structure held in place by hydrogen bonds, and the other was X-ray diffraction photos of DNA, showing a helical structure with distance between the coils, taken by Franklin and Wilkins.直到40年代末50年代初，研究者利用Chargaff 的观点、Levene 关于DNA组分的思想和其他两条证据去研究DNA结构。一个是Linus Pauling 的观点，认为DNA可能是由氢键在确定位置形成的螺旋结构，另一个证据是由
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Franklin 和Wilkins 拍摄的DNA X-衍射照片，在每一圈之间存在距离的螺旋结构。
Based on this information Watson and Crick proposed the double helix model of DNA-A twisted ladder-like molecule with two outer sugar phosphate chains and rungs formed by nucleotide pairs. Paired nucleotides, which always occur as A-T or G-C, are linked by hydrogen bonds. Watson and Crick also proposed that genetic information is encoded by the sequence of base pairs along the DNA molecule.在这一信息的基础上，Watson 和Crick 提出了DNA双螺旋模型，一个象盘旋曲折的梯子样的分子，外面含有2条糖-磷酸基链，里面是核苷对形成的横档。成对的核苷通常以A-T或G-C 以氢键相连。Watson 和Crick 也提出遗传信息是由DNA分子上的碱基对序列进行编码的。
In their model of DNA structure and function, Watson and Crick hypothesized that DNA replicates itself by \"unzipping\" along the hydrogen bonds joining A to T and C to G. This process would produce two opposite halves that could then serve as templates for the construction of new, complementary strands. This model of semiconservative replication conservative because each new molecule has one half of the former parent molecule-was later confirmed by the work of在DNA结构与功能的模型中，Watson 和Crick 假设，DNA通过将连接A和T、C和G的氢键解开来自我复制。这一过程将产生两条互补链，它们方向相反，将作为组建新链的模板。这是半保留复制的模型，叫做半保留是因为每一个新分子具有原来亲本分子的一半，后来被Meselson 和Stahl的工作所证明。
In E. coli DNA replication begins with the formation of a bubblelike structure on the circular chromosome that is produced by replication forks. Studies of
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bacterial DNA replication have shown that a growing DNA chain lengthens only in the 5' to 3' direction (from the 5' carbon of one sugar to the 3' carbon of the next). The leading strand is synthesized continuously, while the lagging strand is synthesized in short stretches known as Okazaki fragments. The enzyme DNA polymerase links free nucleotides as they line up on the template formed by the original strand of the parent molecule.在大肠杆菌中，DNA的复制从环形染色体上的泡状结构开始，环形染色体由复制叉开始。细菌DNA 复制的研究表明DNA链长度只在5’至3方向（从5’端一个核糖的碳原子，到相邻核糖的3’端碳）延长，前导链连续合成，后随链以被称为冈崎片段的短片段方式合成。游离的核苷排列在父本分子的原初链形成的模板上，由DNA聚合酶催化。
In eukaryotes DNA replication follows the same general principles as in prokarotes. On the long DNA molecules replication proceeds (in two directions at once) from hundreds or thousands of points of origin.真核生物复制与原核生物复制相似，但有几百到几千个复制原点（原核一般只有一个复制原点）。
The story of life's origins begins with the formation of the earth. The sequence of events that gave rise to our planet began, in turn, with the cosmic explosion physicists call the Big Bang. The sun at the center of our solar system condensed from a cloud of primordial matter roughly 5 billion years ago; the planets, including the earth, condensed about 4.6 billion years ago. The earth is composed of a number of layers: a solid crust, a semisolid mantle, and a largely molten (liquid) core that has a solid center. Basic physical features of Earth that may have made the emergence of life possible include the planet's size, temperature, composition, and distance from the sun. The major current
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hypothesis holds that life arose spontaneously on the early earth by means of chemical evolution from nonliving substances.生命起源的故事看开始于地球的形成。大爆炸是我们行星形成的开始。大约50亿年前，位于太阳系中心的太阳由云状的原始物质（星际物质）收缩形成。包括地球在内的行星在46亿年前形成。地球由多层组成：固态的地壳，半固态的地幔，很大程度上熔化（液态的）的地核，地核含有一个固态的中心（固态内核）。地球的基本特征包括行星的大小、温度、组成以及离太阳的距离，使生命出现成为可能。当前主要假设认为，在早期地球上非生命物质通过化学演化自发地产生生命。
Evidence for prelife stages of chemical organization comes from laboratory experiments that try to duplicate the physical environment and chemical resources of the early earth. These experiments, including the pioneering work of Miller and Urey, have successfully produced organic monomers including amino acids, simple sugars, and nucleic acid bases. The probable next step toward life was the spontaneous linking of such monomers into polymers such as proteinoids and nuclei acids. Current research suggests that likely sites for this polymerization were clay or rock surfaces.通过创造出地球早期的自然条件和化学资源条件，科学家在实验室中已经获得了化学有机体生命前阶段的证据。这些实验包括Miller 和Urey的开创性的工作，已成功地制造出包括氨基酸、简单糖类和核酸碱基等有机单体。这些单体自发的连接成多聚体，如类蛋白和核酸，使进入生命状态成为可能。如今的研究工作表明这些聚合作用可能发生在泥土或岩石表面。
Researchers have found that, when energy is available to a system, they can generate three kinds of organic molecular aggregates. The Russian Aleksandr Oparin obtained polymer-rich droplets, called coacervates from solutions of polymers. Sidney Fox generated proteinoid microspheres from mixtures of amino
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acids and water. A third laboratory structure is the liposome, a spherical lipid bilayer that forms from phospholipids. A structure similar to one or more of these aggregates may have been the precursor of true cells.研究者业已发现，当一个系统获得能量时，它们将产生3种有机分子聚合体。俄国科学家Aleksandr Pparin 从聚合体溶液中得到了富含多聚体的小滴，被称为团聚体。Sidney Fox 用氨基酸和水制造了类蛋白质微球。实验室第三个结构物质是脂质体，即由磷脂形成的球形脂双层结构。
Further steps in the appearance of cells on the earth included the development of RNA and DNA as biological information molecules. Evidence suggests that RNA, which can form spontaneously under conditions mimicking those of the early earth, was the first informational molecule. The discovery of RNA ribozymes-RNA that can act as an enzy that such catalytic RNA also could have assembled new RNAs from early nucleotides. Certain cc RNAs can also carry out sexlike exchanges of pieces of RNA.导致地球上细胞的出现的接下来的几步包括：作为生物信息分子RNA和DNA的变化。证据表明RNA能够在模拟早期地球条件下自发形成, 是地球早期形成的第一个贮藏信息的物质。RNA 酶的发现（能够行使酶的催化功能的RNA），说明起催化作用的RNA也能够将早期的核苷酸组装成新的RNA。对于RNA物种而言，某些催化RNA具备类似于性交换的功能。
Following the development of a lipid-protein surface layer and replicating RNA and DNA informational molecules, the events leading to the emergence of living cells would have included the origin of the genetic code; the sequestering of RNA or DNA into cell-like structures; and the development of metabolic pathways接下来脂蛋白表层的形成，RNA的复制,DNA信息分子的形成，最终导致活细胞的出现，包括最初的遗传密码，RNA或DNA被包裹进细胞样的结构中；及代谢途径的建立。
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The oldest fossils that may represent living cells are found in rocks that are about 3.5 billion years old. The cells were probably anaerobic heterotrophs, with autotrophs arising much later. The first autotrophs produced their own nutrients and released O,-a metabolic by-product that had a crucial impact on later life forms. The resulting ozone layer in the earth's atmosphere reduced the penetration of ultraviolet light. As a result, cells would survive in shallow water and on the land surface. The increasing quantity of atmospheric oxygen also permitted the evolution of aerobic cells and cellular respiration, which in turn signaled the beginning of the global carbon cycle.Although the earliest cells were all prokaryotes, by about 1.5 billion years ago eukaryotes appeared.能说明活细胞存在的最古老化石最早发现于35亿年前的岩石中。最早出现的细胞可能是厌氧异氧生物，很久以后出现了自养生物。最早的自养生物自己生产营养并释放氧气，氧气是新陈代谢副产品，对后期生命的形成有一个深远的影响。在地球大气层中最后出现的臭氧层降低了紫外辐射。结果，细胞就能够在浅水区和陆地上生存了。大气中氧气数量的增加使得需氧细胞进化并产生细胞呼吸，这预示着全球碳循环的开始。最早期的细胞都是原核生物，大约15亿年后，真核细胞才出现。
Changes in land masses, the seas, and climate have greatly affected the evolution of life on the earth. The basic parts of the planet include a light, solid crust over a hot, semisolid mantle and an inner, partially molten core. Massive segments or plates of the crust move over the mantle in the process of continental drift. Over the past 500 million years, continental drift has sculpted the earth's crusts to produce the form and distribution of present-day continents. Climatic changes that greatly affected living organisms accompanied these plate movements; the period was marked by occasional waves of mass extinctions of
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living creatures. Organisms were also affected by periods of glaciation that followed variations in the earth's orbit and in the output of energy by the sun.大陆板块、海洋及气候的变化对地球生命的进化都有深远影响。地球的基本构件包括：亮的、固态的地壳，半固态的地幔，内部是特殊的地核。大块的地壳板块在地幔上运动，称为大陆漂移。在过去的5亿年中，大陆漂移将地球的地壳雕刻产生了如今这样组成和分布的大陆。伴随板块运动，气候改变对活有机体有深远影响。在特定时期的生物大量灭亡高峰是这个时期的见证。生物也受冰河期影响，在冰河期，地球轨道和太阳能的输出都发生了很大变化。
Biologists use the binomial system of nomenclature developed by Linnaeus to categorize the varieties of life on the earth. The system assigns each type of organism to a genus and species. Organisms are then further classified into higher taxonomic categories-family, order, class, division (plants), phylum (animals), and kingdom. Evidence from many subfields of biology, such as biochemistry and anatomy, helps define species and higher taxa (taxon). And whereas species were originally defined in terms of morphological traits, today biologists generally use the criterion of a reproductively isolated population.生物学家采用林奈的双名法对地球上的生物进行分类。这一系统对每一生物分配了一个属名和一个种名。然后将生物进一步分类更高级类别中，即科，目，纲，门，界。从生物学的许多分支领域例如生物化学和比较解剖学中得到的证据有助于划分物种和更高级的分类单位。虽然物种最初根据形态学特征进行定义，如今生物学家通常采用生殖隔离种群的规则。 从生物学的许多分领域例如生物化学和比较解剖学中得到的证据，帮助定义种和更高级的分类单位。虽然物种最初根据形态学特征进行定义，今天生物学家大体上使用孤立多产的群体作为标准。
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Taxonomy reveals a great deal about the evolutionary relationships among organisms. A clade is a taxonomic unit whose members are derived from a common ancestor.分类学揭示了关于有机体进化关系的许多内容。进化枝是一个分类学的单位，成员来自一个共同的祖先。
A phylogenetic tree is a graphic representation of evolutionary relationships. Your text uses a common five-kingdom arrangement: organisms are grouped into the kingdoms Monera, Protista, Fungi, Plantae, and Animalia. Although this system is a convenient organizational tool, the kingdoms are probably no true clades.进化系统树是进化关系的图解表现。教材中采用了常用的五界分类系统：原核生物界、原生动物界、真菌界、植物界和动物界。尽管这个系统是一个便利的组织工具，5界划分可能不是正确的进化枝。
The approximately 175,000 species of fungi include some of the simplest multicellular organisms. Fungi have a variety of lifestyles. They may be saprobes that decompose dead organic matter; they may be parasites which obtain nutrients from living hosts; or they may live in symbiotic relationsh with algae or with the roots of hig pla. In spite of these variations, however, all fungi carry out extracellular digestion: they secrete enzymes that digest organic matter, and then they absorb the resulting nutrients.包括最简单的多细胞生物在内，真菌大概有175，000种。真菌有多种生命形式。他们可能是分解无生命的腐生菌；也可能是寄生菌；或者与藻类共生，或者与高等植物共生。不管如何变化，所有真菌都是细胞外消化。他们分泌酶消化有机物，然后吸收有效的营养物质。
Most fungi have the same basic body structure consisting of a main body or
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thallus composed of filaments called hyphae. In most species the walls of hyphal cells contain chitin. Hyphae in certain species may become specialized to form rhizoids, which serve as rootlike anchors, or they may become the feeding structures known as haustoria. Finally, hyphaee septate cross walls that segregate independent cells, each with at least one nucleus. Lower fungi are coenocytic; that is, they are one mass of cytoplasm that contains multiple nuclei.大多数真菌有同样的基本结构，即由菌丝组成的菌体。大多数真菌的细胞壁含有明角质。某些种类的菌丝可能特化形成假根，具有根样的固定作用，或者他们成为饲喂结构称吸器。菌丝或有细胞隔膜或没有细胞隔膜，但至少有一个细胞核。低级真菌是多核体的，即一大团细胞质含有多个细胞核。
Hyphae grow and branch to form a filamentous network called a mycelium. Food is digested and absorbed at the tip of each hyphae; more hyphae are generated as this process continues. As a result, fungi may grow very rapidly. Growth depends on mitosis and the rapid manufacture of cytoplasm; fungal mitosis is unique in that it occurs within the nucleus. Hyphae from genetically distinct individuals may fuse to form a heterokaryon-a single cytoplasm with dissimilar nuclei.菌丝生长形成分支，进而形成网状结构称为菌丝体。在菌丝尖端，食物被消化并吸收；这个过程能持续产生更多菌丝。结果，真菌生长很快。生长靠有丝分裂和细胞质的快速产生。真菌的有丝分裂很独特，即它只发生在细胞核里。遗传学明显不同的菌丝个体融合形成异核体——同一个细胞质中含有不同的细胞核。
As nonmotile heterotrophs, fungi must eventually be able to find new sources of nutrients. This function is fulfilled by spores, the fungal reproductive bodies. Spores may be home on aerial hyphae, which discharge spores into the air, and
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depending on the species they may haploid or diploid. There are two main categories of spores: (1) dispersal spores, which are usually short-lived and are produced in large numbers during active fungal growth; and (2) survival spores, which are usually produced in smaller numbers and at a time in the life cycle when the fungus is under some kind of environmental stress.作为不会移动的异养生物，真菌最终能够发现营养新资源的。这个过程由孢子来完成，即真菌的生殖体。孢子可能是在气生菌丝的中心，将孢子释放到空气中，依赖物种不同，可能是单倍体也可能是二倍体。有两种主要的孢子：（1）分散的孢子，通常生命期短，数量大，主要存在于活跃的菌丝生长阶段；（2）存活孢子，通常产生很少的数量，在生存压力条件下的一种生命循环。
As with some of the other groups you have studied, it is difficult to group fungi according to actual evolutionary relationships. In general, they are classified according to morphology, methods of reproduction, and modes of spore production. On the basis of these features, the single division of the kingdom Fungi, Mycota, is divided into six principal classes.对真菌依据实际进化关系进行分类是很困难的。总体来说，它们是依据形态学，生殖方式，孢子产生的模式来分类。基于这些特性真菌界的门被分成6个基本纲。
The lower fungi comprise the groups Chytridiomycetes, Oomycetes, and Zygomycetes. All lack septate hyphae and are commonly coenocytic; spores are formed by asexual means. Of the six fungal classes, only the oomycetes usually have a diploid vegetative state. Oomycetes and chytrids (sometimes called water molds) produce motile, flagellated spores in sporangia. They also produce gametes in gametangia; oomycetes are distinguished by their large, immobile egg cells. In fact, both these groups have such distinctive features that some
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biologists rather than fungi. Zygomycetes wo classes in this group but have nonmotile spores. They are also completely terrestrial, and some form mycorrhizal associations with plant roots.低级真菌包含壶菌，卵菌纲和接合菌。所有缺乏隔膜的菌丝是通用的多核细胞的；孢子通过非性方式产生。只有卵菌纲通常有二倍体的营养状态。卵菌纲和壶菌在孢子囊中产生能动的带鞭毛的孢子。在配子囊中产生配子；卵菌纲具有非寻常大的静止的卵细胞。实际上，这两大群具有不同的特征，生物学家宁愿把它们归为原生动物而不是真菌。接合菌与其它两纲相似，但它的孢子不会动。它们完全是陆生的，一些菌根与某些植物的根形成共生关系。
The higher fungi include the class Ascomycetes, the largest class of fungi. Most ascomycetes are either saprobes or parasites. Asexual reproduction produces spores called conidia, which develop on the tips of specialized aerial hyphae. In the ascomycete sexual cycle, hyphae of different mating strains fuse, giving rise to ascospores that form in a small, saclike ascus. Groups of asci form fruiting bodies. Ascomycetes of interest to humans include truffles, yeasts, and Penicillium species.高级真菌包括子囊菌纲，最大的真菌纲。大多数子囊菌要么是腐生菌，要么是寄生菌。非性繁殖产生的孢子称分生孢子子实体，其依赖特化的气生菌丝尖。在子囊菌性循环过程中，不同的菌丝通过株融合进行匹配，产生的子囊菌存在于小类子囊中。子囊群形成果实体。对人类有利的子囊菌包括块菌，酵母和产青霉素菌。
Most members of the class Basidiomycetes—the second group of higher fungi form visible fruiting bodies. A prime characteristic distinguishing basidiomycetes from ascomycetes is the dense hyphae called the basidiocarp-the \"mushroom\" seen on damp lawns and the forest floor. Club-shaped basidia, each beahaploid basidiospores, line the surfaces of the gills on the underside of the mushroom cap.
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Members of this group undergo both sexual and asexual reproductive processes at different times in their life cycle, and in response to varying environmental influences.纲中大多数菌为担子菌，第二大类高级真菌，能形成可见的果实体。担子菌与子囊菌相比，最基本不同是它具备浓密的双核菌丝，称担子果。棒形担子，每个都含有4个单倍体的担子孢子，线形地排列在蘑菇帽下的菌褶表面。大多数成员有性和非性繁殖两种，在不同生命循环中有不同形式，并依据环境变化而变化。
The class Deuteromycetes, or Fungi Imperfecti, includes a variety of fungi that lack modes of sexual reproduction. Most are known to reproduce asexually by means of conidia. Deuteromycetes imhumans include those used to ferment soybeans and rice to make soy sauce and sake respectively, and those responsibl for producing citric acid and the highly dangerous aflatoxin.半知菌纲，包括大量无性繁殖的真菌。分生孢子完成非性繁殖。对人类很重要的半知菌纲包括发酵大豆成酱油和大米发酵成清酒的菌类，它们也产生柠檬酸和有毒的黄曲霉毒素。
Lichens are composite organisms in which about 90 percent of the lichen mass consists of one species of fungus, while the remaining 10 percent is made up of one or two species of algae. The algal portion of the lichen provides the fungal portion with essential nutrients, while the presence of the fungal component may enable the alga to exploit an otherwise unavailable ecological niche. Lichen fungi are usually ascomycetes, although the other two higher fungi are sometimes found in lichens. Lichens have a number of remarkable features, including their ability to become almost completely desiccated without drying and their ability to absorb inorganic nutrients. Reproduction in lichens is not well understood. 地衣是复合生物，大概90%的地衣团由一种地衣组成，10%含有一两种藻类。藻类为真菌提供
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必要的营养，真菌为藻类提供了难得的小生态环境。地衣真菌通常是子囊菌，有时在地衣中也能发现其他两类真菌。地衣有大量的显著特征，包括在几乎脱水条件下仍能吸收无机养分。地衣繁殖方式还不清楚。
The various fungi may have arisen independently from prokaryotes, since some evidence suggests that ascomycetes and basidiomycete did not evolve from known lower forms. However, all fungi do show the same dependence on nutrients produced by plants, animals, or algae.
各种真菌可能都独立进化于原核生物，有证据表明，子囊菌和担子菌没有进化。但所有真菌都依赖由植物，动物或藻类产生的营养。
In sexually reproducing org males and fems pro sex cells, known as gametes. These are swimmsperm in males and ova (eggs) in females.
有性生殖的雄性和雌性个体产生性细胞，称为配子。雄体产生游动的精子，雌体产生卵
The process of sperm production, spermatogenesis, takes place in testes. The sperm originate in gonial cells (spermatogonia) in the walls of seminiferous tubules. Spermatocyt produced by mitosis in spermatogonia divide meiotically to generate haploid spermatids. The mature sperm has a tail, a nucleus cont haploid chrom, and a front end with an acrosome, the storage site for enzymes that will aid fertilization.精子发生的过程，称为精子发生，发生于睾丸中。精子由位于输精管壁上的性腺细胞（精原细胞）产生。由精原细胞有丝分裂产生的精母细胞进行减数分
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裂，产生单倍体的精细胞。成熟的精子具有一个尾巴，一个含有单倍染色体组的核，一个含有顶体的头部，顶体是贮存帮助受精的酶的场所，有助于受精。
Ova, which are produced during oogenesis, are generated in gonial cells (oogonia) of the female's ovaries. Oocytes then enter a stage of arrest in early meiosis. At a species-specific later point, a final ripening (ovulation) and the first meiotic division occur. A second meiotic division, followed by development of the embryo, takes place if the egg is fertilized.卵是在卵子发生过程中产生，由雌体卵巢中性腺细胞（卵原细胞）生成。然后卵母细胞进入早期减数分裂的抑制阶段。在后续具有物种特异性的阶段中，最终的成熟（排卵作用）和第一次减数分裂发生。如果卵受精，随着胚胎的发育，第二次减数分裂发生。
Eggs vary greatly in size from species to species and have complex structures. Virtually all developing animal ova are surrounded by helper cells, either follicle cells or nurse cells. Depending on the species, eggs also store varying amounts of yolk, a reservoir of nutrients produced by digestive-gland cells in the mother's body. Finally, follicle cells or cells of the maternal oviduct provide protective coatings for the egg, including albumen (egg white) and various types of outer membranes and shells.卵的大小随物种而异，具有复杂的结构。实际上，所有发育过程中的卵都辅助细胞环绕，要么是滤泡细胞，要么是抚育细胞。实际上，依赖物种的不同，卵黄贮备也不同，卵黄是由母体的消化腺细胞产生的营养物质。最后，滤泡或母体输卵管细胞产生保护性的卵膜，包括白蛋白（卵清）、各种类型的外层膜或壳。
Frog oocytes have served as model systems for studies of oocyte development. During maturation they produce huge numbers of ribosomes
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through gene amplification. Large quantities of mRNA may also be made and stored.蛙卵母细胞常作为研究卵母细胞发育的模式系统。在成熟过程中，通过基因扩增产生大量的核糖体。同时也产生和储备了大量的mRNA。
Fertilization unites male and female gametes and initiates development. In some species fertilization is external; in others (including most terrestrial animals) it takes place internally. The first contact of the sperm head with the egg's jelly coat triggers the acrosome reaction, in which enzymes are released to digest a hole through the egg's protective layers, and the plasma membrane of the sperm is brought into position to bind to the ovum's surface. After fusion of the egg and sperm plasma membranes, the haploid male nucleus with its chromosomes moves into the egg cytoplasm. Fusion also triggers the egg's final meiotic reduction divisions. When sperm and egg nuclei unite, the two sets of chromosomes mingle to create a diploid set. The fertilized egg is now a zygote.受精将雌雄配子连在一起，启动发育。有些种类为体外受精，另外一些种类，包括大部分陆生动物，是内部受孕。当精子和卵子的胶状膜发生接触时，触发了顶体反应，其中释放出的酶在卵的保护膜溶解出一个小孔。精卵原生质膜融合后，单倍体雄性核进入卵细胞质中。融合也触发了卵最后的减数分裂。当精卵核结合时，两套染色体混合产生一个二倍体，即受精卵。
The egg's cortical reaction serves as a barrier to the entry of more than one sperm. Initially, there is a temporary change in the egg's electrical state, and the egg cell is activated. The final stage of the reaction, the rapid elevation of the fertilization membrane, prevents further sperm penetration.卵的皮层反应作为屏障，防止更多的精子进入。首先，卵的膜电位出现短暂性的变化，卵细胞被激活。反应最后阶段，受精卵的膜快速隆起，阻止更多精子进入。
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In some species fertilization is not necessary. Instead, parthenogenesis takes place: the egg is spontaneously activated and proceeds to normal embry development. 对某些物种而言，受精并不是必须的。而是发生孤雌生殖，即卵自发地被激活并进入正常的胚胎发育过程中。
Cleavage, the major developmental event immediately following fertilization, is a special form of cell division (mitosis). Cleavage produces a blastula, a sheet of cells rounded into a sphere that in most species surrounds a cavity. In the process, the single-celled zygote is divided into many small cells, and yolk, mRNA, ribosomes, and other materials arc distributed to each cell in precise ways. The cells of the blastula, called blastomeres, also each receive a full diploid set of chromosomes.受精后，立即发生卵裂，是细胞分裂（有丝分裂）的一种特殊方式。卵裂产生囊胚，多数种类中一层细胞围成球形，形成一个空腔。在这一过程中，单细胞的受精卵分裂为许多小细胞，卵黄、mRNA、核糖体和其他一些物质以精确的方式分配到每一个细胞中。 这些囊胚细胞，称卵裂球，同样每个分裂球中得到一个全套的二倍体染色体。
There are different patterns of cleavage in different species. The amount of yolk present in the egg is a major factor in determining the pattern: in species having little yolk (such as mammals) the zygote cleaves completely through, forming cells that are roughly equivalent in size. In frogs, in which the egg has somewhat more yolk, cleavage proceeds more rapidly in regions of the embryo having less yolk. In bird eggs the yolk is so massive that cleavage divisions are restricted to a tiny area of cytoplasm.不同物种，卵裂方式不同。卵中卵黄含量多少是决定卵裂类型的最主要因素：含有很少卵黄（例如哺乳动物），合子分裂得很彻底，子细胞大致平均分裂，形成的细胞大小大体上相同。对于卵黄较多蛙类而言，在含有较少卵黄的
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胚胎区域卵裂速度较快。鸟类卵黄较多，卵裂被限制在细胞质很小区域内。
In many species the precise distribution to blastomeres of molecular determinants in the cytoplasm is crucial to proper development of different cell types in the embryo. In mammal and bird species the fate of cells is determined by the position of a cell late in cleavage.细胞质中分子遗传因素精确分配到卵裂球中，对许多物种而言，是发育成胚中不同类型细胞关键所在。对哺乳动物和鸟类而言，分裂球细胞的命运最终由细胞分裂后所处的位置决定的。
The rearrangement of the blastula into a three-dimensional organism with inner, middle, and outer layers occurs during gastrulation. The resulting gastrula consists of an outer ectoderm, an inner endoderm, and a mesoderm layer positioned between them.囊胚在原肠形成过程中进行重排形成含有内层、中层和外层的三维结构。因而发生的原肠胚含有外面的外胚层、内部的内胚层和位于两者之间的中胚层。
Each layer gives rise to specific tissues during embryonic development. A variation in gastrulation, involving the movement of cells into endodermal and mesodermal positions through the thickened primitive streak, arose in reptiles and can still be seen in bird and mammalian embryos, lending support to the theory that birds and mammals evolved from reptiles.
在胚胎发育过程中每一胚层将生成特定的组织。原肠胚形成过程中的变化包括：通过加厚的原条导致细胞移动到内胚层和中胚层位置，爬虫、鸟类、哺乳动物的胚胎都如此。鸟类和哺乳动物是由爬虫进化而来的。
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The organs and tissues of the embryo arise during organog as cells inside the embryo and on its surface become specialized. Organogenesis actually includes two closely linked processes, morphogenesis and differentiation. During morphogenesis cells and cell populations change shape: an example is neurulation in vertebrate embryos, in which the edges of the flat neural plate fold upward and fuse, forming the beginnings of the hollow brain and spinal cord. During differentiation cells mature so that they may perform separate functions. This maturation may include taking on a function-related shape, such as the long, spindly shape of skeletal-muscle cells. Cell differentiation also results in responsiveness-the ability of a cell to be regulated within the organism through the action of hormones, neurons, and other signals.
胚经过内部细胞和表面细胞特化后，器官形成，发育成器官和组织。器官形成实质上包括两个联系紧密的过程，即形态发生和分化。在形态发生期间，细胞和细胞群体形状发生改变：例如，脊椎动物的神经胚，扁平神经板的边缘向上折叠并融合，形成中空的脑部和神经索的起始端。在分化过程中，细胞成熟，执行各自功能。成熟包括功能相关的成型过程。例如，细长的、纺锤形的骨骼肌细胞。细胞分化也产生应答能力，即细胞受生物体内激素、神经和其他信号的调控过程。
The embryos of land vertebrates are enclosed within four extraembryonic membranes that afford protection while still permitting the exchange of gases, nutrients, and other materials.
陆地脊椎动物的胚胎由四层胚胎外膜包被，以此提供保护，同时仍允许气体、营养物质和其他物质的交换。
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Growth in embryos is largely due to an increase in the number of cells rather than to an increase in the size of individual cells. In many species the extent of embryonic growth is limited by the availability of food (yolk). In animals that develop entirely free of the maternal body, such as frogs and insects, the embryo give rise to a larval stage that can feed itself and later undergo metamorphosis to attain the adult stage. In many species the most spectacular growth phase takes place during the juvenile and adolescent phases of the life cycle. Actual growth generally stops once the organism reaches its typical adolescent phases of the life cycle. Actual growth generally stops once the organism reaches its typical adult size, although replacement of dead cells may continue.胚的生长很大程度取决于细胞数量的增加而不是单个细胞大小的改变。在大多数物种中，胚的生长是受卵黄的量限制的。一些动物胚胎发育完全脱离母体，例如蛙和昆虫，胚胎发育后为幼虫阶段，幼虫能够自我摄食，稍后经历了形态发生达到成体阶段。在许多动物中的幼年和未成年阶段，发生特别显著的生长。一旦细胞进入成年状态和成年大小，生长就停止了，尽管死细胞的替代不断发生。
A special type of growth, regeneration of lost body parts, can take place in adults of some species. Prior to such regeneration cells in stump tissue undergo dedifferentiation. They lose their functional phenotype, divide rapidly, and generate a population of cells that will regenerate the lost part. Compensatory hypertrophy is a different, temporary growth response in which residual tissue increases in mass and cell number: cells undergo mitosis but do not dedifferentiate.一种特殊的生长类型，再生失去的部分躯体，能够发生在某些物种的成体中。在再生之前，残肢组织细胞经历了去分化过程。它们失去了其功能性的表型，能够迅速分裂，产生细胞群来再生失去的部分。代偿式肥大是一个不同的、暂时的生长应答过程，
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残余组织在体积和细胞数量上增加：细胞经历了有丝分裂，但没有去分化过程。
Aging is an ongoing, time-dependent developmental process in which body parts deteriorate. Proposed causes include the degeneration of collagen (the fibrous proteins of the connective tissues) and limits on the number of times cells can divide. Other theories focus on a decline of the immune system or on the accumulation of lipofs (aging pigments).衰老是一个持续过程，随时间发育，身体部分恶化。可能原因包括胶原质（结缔组织的纤维蛋白）的退化和细胞分裂次数的限制。其他的理论认为，衰老是由免疫系统的免疫下降或脂褐素的堆积造成的。
Modern biology generally define a species as group of actually or potentially interbreeding populations that is reproductively isolated from the such groups. Members of a species can interbreed with each other, but they cannot breed with organisms belonging to another species. One advantage of the standard of reproductive isolation is that it is very precise. Notice, however, that it can only be applied to organisms that reproduce sexually. Asexual reproducers, including most prokaryotes, many plants, and some animals, must be classified into species on the basis of physical (biochemical or morphological) traits.现代生物学大体将物种定义为实际上的一群或潜在的杂交群体，即从这个群体隔离繁殖的后代。种族中个体可以互相交配，但种间不能。这种繁殖隔离的一个优势是很精确。但仅适用于性繁殖的生物。非性繁殖，包括大多数原核生物，许多植物，某些动物，需要通过自然特征进行归类。
Two general types of mechanisms operate to block the exchange of genes between individuals of related groups. The first general type is made up of prezygotic isolating mechanisms that prevent the formation of zygotes.
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Prezygotic isolation falls into two categories: ecological and behavioral. In the first case, two related group may become adapted to slightly different environments-perhaps varying soil types or food sources. Over time, these genetic differences become so great that successful cross-fertilization can no longer take place. In behaviorisolation, related groups evolve differing behaviors such as specific mating rituals-that restrict the exchange of genes to members of the same group.
两种基因型机制阻碍了相关群体中个体的基因交换。第一种基因型由前合子机械隔离机制阻止合子的生成。前合子隔离分两类：生态学的和行为学的。第一种情况，两个相关群体可能分别适应稍微不同的环境，如土壤类型或食物来源的变化。长时间，这些遗传差异变得很大而很难异体受精。而行为学隔离，相关的群组进化成不同的行为方式，例如，结婚仪式，从而限制了同群成员基因交流。
Sometimes the differences that produce prezygotic isolation involve mechanical isolation. That is, mating is physically impossible between members of different species because genitals of males and females are structurally incompatible or because molecu surfaces of sperm and egg fail to bind. A final type of prezygotic mechanism is temporal isolation, in which time-related environmental cues that trigger reproductive processes are different for related species.有时，产生前合子隔离的差异涉及了隔离机制。即，由于生殖器结构不匹配或精卵分子表面不结合而使不同种群成员间不能自然结合。最后一类前合子机制是暂时隔离，与时间有关的环境因素触发了相关物种的不同繁殖过程。
In postzygotic isolating mechanisms mating occur, but the resulting hybrid
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organism is inviable or sterile. In a special case of hybrid sterility termed hybrid breakdown, the second and subsequent generations after a cross show reduced reproductive success. Contrast this fact with the very different outcome of crossbreeding between two genetically distant members of the same species, where the result is often heterozygote advantage (hybrid vigor).在合子后隔离机制中，匹配可以进行，但或不产生杂种或杂种不育。杂种不育在某些特殊情况下，是指第二代或以后几代显示出杂交繁殖能力的降低。正是同种远基因成员间的杂交产生不同的结果，而产生杂种优势。
Populations of a species that are spread out over a broad geographical range are often arrayed in a cline-a gradual change in one or more characteristics as each population evolves adaptations to its own local environment. Along a cline, subspecies with distinct characteristics may arise. Often, individuals at either end of a cline are reproductively isolated.地域上分布很广的种的群体中，经常存在变异群，因为适应当地的环境，每个群体逐渐产生一个或多个与众不同的特点。在变异群中，带有明显特征的亚群可能产生。通常，在两个变异群中的个体是繁殖隔离的。
Ernst Mayr's model of allopatric speciation proposes that species can originate in a two-stage process. In the first stage, populations of existing species are separated by a physical or geographical barrier. As a result, over time genetic differences leading to pre- or postzygotic isolation arise between the two groups. In the second stage, the diverged populations may again come into contact. If this happens, speciation becomes complete through the action of natural selection.异地物种形成模型认为，物种可能起源于两个阶段。第一阶段，已经存在的物种群体可能由于自然或地理屏障被隔离。结果，长时间后，在两个群体间遗传差异导致前合子或后合
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子隔离的产生。第二阶段，这些趋异群体可能再次接触，物种发生可能通过自然选择发生。
The extent of differences between populations that are diverging into separate species or between species that have already diverged is represented by a statistic called genetic identity-the relative proportion of the same structural genes present in members of groups being compared. In general, biologists believe that the genetic events leading to speciation take place gradually. Once a new species has arisen, it tends to diverge genetically from related species at a more rapid pace. In some cases, such as the primate order, major differences in body form are not reflected by corresponding divergences in structural genes. This has led Biologists to hypothesize that small changes in regulatory genes may acc for many of the large-scal chan responsible for sepciation and the origin of higher taxonomic groups.能够趋异的群体或已经趋异的物种，他们之间差异的扩大，由一个稳定的遗传识别代表。即在可比群体成员中相同基因所占相关比例。大体而言，生物学家相信，遗传导致的物种形成是逐渐发生的。在某些情况下，身体主要差别并不能反映结构基因的趋异，生物学家因此提出，调控基因很少的改变可能导致很大规模物种改变和更高分类学上群体的产生。
One mechanism that may rapidly split populations genetically is polyploidization—the sudden multiplication of an entire complement of chromosomes. This can result in sympatric speciation, in which new species arise even though no geographical isolation has taken place. A phenomenon similar to polyploidization involving the rearrangement of chromosomes has been proposed to explain the evolutionary origin of giant pandas. Clearly, species can originate in a variety of ways.遗传学上可以快速分离群体的一个机制是多倍体，一个完全互补染色
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体组的突然倍增。结果导致同域物种形成，即使没有地理隔离也会产生新的物种。与多倍体相似的一个现象是染色体重排，可以解释巨大熊猫的进化起源。显然，物种可以以多种方式进化。
The changes that generate species are sometimes termed microevolution; those that produce the major phenotypic differences that separate genera, classes, orders, and so on are termed macroevolution. Some lines of descent can be traced by studying the fossil record. In other cases relationships must be inferred by comparison of related living organisms. When lines of descent over evolutionary time are constructed, the result is a phylogeny.遗传物种的改变有时称微进化；那些产生很大差异用来区分属，纲，目的表型变化称大幅进化。某些品系的后代可以通过化石追根溯源。而其他相关性可以通过现存活体的比较来推论。当品系后代以进化钟构建时，形成系统发育树。
The rationale for building a phylogeny is simple: it assumes that similarities in body structure, biochemistry, reproductive strategies, and other features of organisms can be used to trace lines of common descent. The process is complex because evolution proceeds in different patterns. In cases of parallel evolution, two or more lineages evolve along similar lines. In convergent evolution, very distantly related lineages become more alike as similar adaptations take hold in response to demands of the environment. Thus similar structures in different organisms may reflect homology (derivation from a common ancestor) or analogy (independent origin of structures used for similar purposes).建立系统发育树的基本理论很简单：把身体结构，生物化学，繁殖策略以及其他特征相似的生物划分进同一个品系。这个过程很复杂，因为进化经历不同模式。就平行进化而言，两个或更多的血统沿相
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似品系进化。在相似生存条件下，亲源关系很远的血统也可能更相似。不同生物间的相似结构可能说明它们具有同源性。
One of the most common evolutionary patterns that can be constructed from the fossil record is divergent evolution or radiation. It is represented by the branching and rebranching of a single line. Another common featu of evolution is extinction-the comple loss of a species or group of species. Mass extinctions have occurred at least five times in the earth's history.从化石证据构建的最普通进化模式是趋异进化。由分支和次级分支的简单线条代替。另一个进化的共同特征是灭绝。地球历史中至少发生过5次大规模的灭绝。
Gaps in the fossil record have led some paleontologists to propose the punctuated equilibrium theory of evolution. The theory holds that evolution proceeds by spurts-radical changes over short (in geological time) periods of time-with intervening periods of equilibrium. The theory is controversial and tends not to be supported when an abundant fossil record is available.
化石证据上的断代使古生物学家们提出间断平衡进化理论。这个理论认为进化在短期内经历了根本突变，中间又介入均衡阶段。这个理论很有争议，一旦发现丰富的化石证据，理论就难支撑了。
Biologists have no certain answers to a number of questions about large-scale evolutionary changes. These questions range from whether novel higher taxa result from as-yet undescribed radical genetic processes, to whether known processes such as genetic drift and small gene changes can plausibly account for
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the evolution of new genera, families, and orders. Investigators are exploring these areas using traditional methods as well as the newer techniques of molecular biology.生物学家还无法回答关于大规模进化的许多问题。许多问题是新类别的物种可能起源于还不确定的遗传过程，或者知道遗传过程，例如，遗传漂移和少量基因的改变可能是新属，科，目产生的原因。科研工作者不但利用传统方法对这些领域进行研究，还利用分子生物学新科技。
Populations-groups of individuals belonging to the same species-all have three very significant statistical characteristics: per capita birth rate or natality; per capita death rate or motality; and number of individuals per unit area or density.
种群中的每个个体都属于同一个物种，具有三个非常显著的统计学特征：出生率；死亡率；种群密度。
As first described by Malthus, a population theoretically can grow exponentially (geometrically) if there are no limits on resources such as food or hiding places and no predation (ac of population growth is represented by an exponential growth curve. The condition exponential growth rarely occur in nature, however. The finite levels of resources in any environment set an upper limit to population size—termed carrying capacity (K)—that can be reached but never long exceeded. A logistic growth curve plots the leveling-off of growth when population size reaches equilibrium with available resources.马尔萨斯首次阐述了人口理论，他指出，如果没有类似于食物，庇护所等资源的限制，没有被捕食，群体总量的增长符合指数或几何关系。即指数生长曲线。但，这种指数生长在自然界中很少发生。环境中有限的资源限制了群体规模，即环境的容纳量，只能接近而不能超过。当群体
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规模接近可获得资源的最大平衡态时，逻辑生长曲线呈现负增长。
When resource limits are approached or exceeded, time is required for the birth rate to fall and for the death rate to rise. This response time is known as reproductive time lag. It is one reason for the fluctuations in numbers that are seen in every population. In many natural populations, carrying capacity (and hence population size) seasonally. If a population drastically exceeds capacity of its environment temporarily, damage may occur that permanently lowers environmental carrying capacity.当接近或超过资源限制时，出生率下降，死亡率升高。这段时期称繁殖滞后时期。这种群体数量的波动存在于每一个群体中。在自然群体中，群体数量随季节波动。若暂时超出环境的容纳量，可能造成对环境持久的伤害而保持很低的容纳量。
Besides environmental carrying capacity, a population's age structure and reproductive strategy also affect the rate at which the population grows. Age structure reflects the relative numbers of young, middle-aged, and older individuals in a give population. In a population having many members at or nearing reproductive age, significant growth may occur. Age structure may also be represented by a survivorship curve.除环境容纳量外，群体的年龄结构和繁殖方式也影响群体增长率。年龄结构反映中青老的比例关系。若生育年龄的比例大，则显著增长。存活曲线也可以反映年龄结构。
The reproducing members of a population follow a complex adaptive reproductive strategy that has evolved over millennia. Reproductive strategies generally fit into one of two categories: those of r-selected species and those of
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K-selected species. In r selection individuals reach reproductive age quickly and produce many offspring., Each offspring is small and enters the world with relatively few resources. Out of the many produced, only a few will survive until reproductive age. In K selection, a strategy related to environmental carrying capacity and the need to compete for resource, individuals mature slowly and produce few offspring. However, parents invest a great deal of resources in each offspring; and after a long period of growth to large size, each offspring's chances of survival until reproductive age are high.
几千年来，进化形成两个繁殖策略，一是繁殖周期快，后代数量多，每个后代小并可获得资源少。仅有少量存活到生育年龄。一是和环境的容纳量及可匹配资源有关，个体成熟慢并产生很少的后代。但，父母给后代提供很多资源，需要很长的生长期，存活到生育年龄的机遇很高。
The size of a population is measured in terms of its density. Whether population density is high or low, the distribution of individuals within the population is usually uneven. Common distribution patterns include clumped, uniform, and random. Negative consequences are often attached to high (or rising) population density. These density-dependent factors include increased predation, parasitism, disease, and intraspecific and interspecific competition. Population size may also be reduced by densityindependent factors, a category that includes natural catastrophes.群体的规模也可以用密度来衡量。不论群体密度高与低，群体中个体的分布是不均一的。通用分布模式包括群体的，均匀的，随机的。密度依赖因子包括增加了的捕食，寄生，疾病，种内竞争和种间竞争。群体规模也可因为密度无关因子而降低，包括自然灾难。
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The interactions of predators and their prey affect population size in complex ways. Such populations sometimes cycle regularly between growth and decline, in part from the effects of reproductive time lags. In general, predation may slow or stop the growth of a prey population only when many reproducing individuals are eliminated. If only weak, sick, or very young prey are taken, the effect of predation on the density of the population as a whole may be slight.捕食和被捕食的相互复杂关系影响着群体的规模。例如群体规模规律的增长和下降，部分是由于繁殖滞后期的影响。总的来说，当被捕食的是能繁殖的个体时，可能减慢或停止群体的增长。反之，影响就很微弱。
An area of controversy among ecologists is the question of whether species diversity in a community tends to generate stability of whether stability encourages species diversity. One aspect of this argument is the hypothesis that a complex food web is more stable than a simple one. In nature, however, many stable, highly diverse comthe presence of numerous simple food webs. It may be that stable environments beget diversity because they allow rare species to persist.生物学家一直在争论，是否是物种差异产生稳定性还是稳定性促成了物种差异。一种假设认为，复杂的食物链要比简单的食物链更稳定。实际上，许多稳定的，高差异的社会群体是由众多简单食物链支撑的。可能是稳定的环境引起差异，它只允许很少的物种存留。
Just as competition, predation, and other elements interact to determine the size of a population within a community, population distribution is the result of many interrelated factors. Overall, the distribution of a population in its potential range depends on locations of food and suitable habitat, interspecific competition for resources, and other variables. Among plants, one of the most effective forms
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of interspecific competition for resources is allelopathy. Among species that share similar or identical habitat niches, resource partitioning is often seen. In character displacement, closely related species have evolved physical differences in body structures used for exploiting a limited resource. Eventually, such solutions to the need for dividing up a scarce resource may lead to speciation.正如竞争，捕食，和其他因素相互作用影响着群体大小，群体分布是许多相关因子相互作用的结果。总之，潜在的群体分布取决于食物的位置和稳定的住所，种间对资源的竞争，以及其他可变因素。对于大多数植物，种间最有效的资源竞争是异种相生相克关系。许多物种分享相似或相同的生存环境，而进行资源划分。对于特征替换，关系更近的物种已经在进化过程产生了很大体证差异。最终，这种对珍贵资源的划分方式可能导致物种形成。
The awesome rate of increase that is now a feature of the human population started about 10,000 years ago during the agricultural revolution. At that time, world human population is estimated to have totaled 133 million people; today it has reached a staggering 5 billion. Viewed on a graph, this increase closely resembles the classical exponential growth curve-a growth pattern that cannot be sustained.自从1万年前农业革命以来，人类人口以惊人的速度增长着。当时，世界人口估计在133百万；今天已经达到50亿。几乎接近典型的指数增长曲线，一个不稳定的增长模式。
While the birth rate in many developed countries has slowed, it remains high in less well developed areas of the world. Demographers predict that, if this rate continues, the human population level will reach 30 billion before the end of the twenty-first century. Yet most biologists believe that the earcarrying capacity for humans is only 10 billion. Clearly, we humans face a serious problem in
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managing the earth's precious resources while sustaining such a high population density.尽管许多发达国家的出生率降低，但也有高出生率的。人口统计学预测，若出生率持续增加，21世纪末世界人口将达到300亿。然而，生物学家普遍认为，地球的容纳量仅为100亿。显然，人类将面临严重的考验，如何利用如此珍贵的资源维持如此高的人口密度。
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