专英三课文译文

How Biologists Define a Species
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。

Preventing Gene Exchange
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. 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 behavioral isolation, 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 molecules on the 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 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。

Becoming a Species: How Gene Pools Become Isolated
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 Genetic Bases of speciation
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 account for many of the large-scale changes 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。

Explaining Macroevolution: Higher-order Changes
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 feature of evolution is extinction-the complete 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。

The Role of microevolution in Macroevolution
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 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。

Lesson Ten The Ecology of Populations
Population Growth
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) fluctuates seasonally. If a population drastically exceeds the carrying 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 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。

Limits on Population Size
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。

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 communities are characterized by the 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。

How Populations Are Distributed
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 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。

Human Population: A Case Study in Exponential Growth
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 earth's carrying capacity for humans is only 10 billion. Clearly, we humans face a serious problem in 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。

注：下面是本人自己翻译的，不是老师给的。

存在很多错误，仅供参考，不能当作答案！
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