参考译文-第9课

Lesson NineThe Origin of SpeciesHow Biologists Define a SpeciesModern biologists generally define a species as group of actually or potentially interbreeding populations that is reproductively isolated from other 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。

但仅适用于有性繁殖的生物。

无性繁殖，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 ExchangeTwo 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 occurs, 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 IsolatedErnst 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 orgeographical 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 speciationThe 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。