Evolutionary biologists believe that speciation, the formation of a new species, often begins when some kind of physical barrier arises and divides a population of a single species into separate subpopulations. Physical separation between subpopulations promotes the formation of new species because once the members of one subpopulation can no longer mate with members of another subpopulation, they cannot exchange variant genes that arise in one of the subpopulations. In the absence of gene flow between the subpopulations, genetic differences between the groups begin to accumulate. Eventually the subpopulations become so genetically distinct that they cannot interbreed even if the physical barriers between them were removed. At this point the subpopulations have evolved into distinct species. This route to speciation is known as allopatry (“allo-” means “different”, and “patria” means “homeland”).
Allopatric speciation may be the main speciation route. This should not be surprising, since allopatry is pretty common. In general, the subpopulations of most species are separated from each other by some measurable distance. So even under normal situations the gene flow among the subpopulations is more of an intermittent trickle than a steady stream. In addition, barriers can rapidly arise and shut off the trickle. For example, in the 1800s a monstrous earthquake changed the course of the Mississippi River, a large river flowing in the central part of the United States of America. The change separated populations of insects now living along opposite shores, completely cutting off gene flow between them.
Geographic isolation can also proceed slowly, over great spans of time. We find evidence of such extended events in the fossil record, which affords glimpse into the breakup of formerly continuous environments. For example, during past ice ages, glaciers advanced down through North America and Europe and gradually cut off parts of populations from one another. When the glaciers retreated, the separated populations of plants and animals came into contact again. Some groups that had descended from the same parent population were no longer reproductively compatible – they had evolved into separate species. In other groups, however, genetic divergences had not proceeded so far, and the descendants could still interbreed – for them, reproductive isolation was not completed, and so speciation had not occurred.
Allopatric speciation can also be brought by the imperceptibly slow but colossal movements of the tectonic plates that make up Earth’s surface. About 5 million years ago such geologic movements created the land bridge between North America and South America that we call the Isthmus of Panama . While previously the gap between the continents had allowed a free flow of water, now the isthmus presented a barrier that divided the Atlantic Ocean from the Pacific Ocean. This division set the stage for allopatric speciation among populations of fishes and other marine species.
In the 1980s, John Graves studied two populations of closely related fishes, one population from the Atlantic side of isthmus, the other from the Pacific side. He compared four enzymes found in the muscles of each population. Graves found that all four Pacific enzymes function better at lower temperatures than the four Atlantic versions of the same enzymes. This is significant because Pacific seawater is typically 2 to 3 degrees cooler than seawater on the Atlantic side of isthmus. Analysis by gel electrophoresis revealed slight differences in amino acid sequence of the enzymes of two of the four pairs. This is significant because the amino acid sequence of an enzyme is determined by genes.
Graves drew two conclusions from these observations. First, at least some of the observed differences between the enzymes of the Atlantic and Pacific fish populations were not random but were the result of evolutionary adaptation. Second, it appears that closely related populations of fishes on both sides of the isthmus are starting to genetically diverge from each other. Because Graves’ study of geographically isolated populations of isthmus fishes offers a glimpse of the beginning of a process of gradual accumulation of mutations that are neutral or adaptive, divergences here might be evidence of allopatric speciation in process.
地理隔离也可以在很长时间内缓慢进行。我们在化石记录中发现了这样的扩展事件的证据，这让我们看到了以前持续不断的环境的分裂。例如，在过去的冰河时代，冰川通过北美和欧洲向下推进，逐渐将部分人口相互切断。当冰川退缩时，分离的动植物种群再次接触。一些来自同一父母本族的群体不再具有生殖上的兼容性 - 他们已经演变成了不同的物种。然而在其他群体中，遗传分化还没有进行到底，后代仍然可以进行杂交 - 对于它们来说，生殖隔离没有完成，所以没有发生物种形成。
The word "promotes" in the passage is closest in meaning to
原文定位：Though these bees do no sting, rival colonies of some species fight fiercely over potential nesting sites. 大部分同学可能都熟悉rival作为名词词性的意思为“对手，竞争者”。那么从这个名词词性也不难猜测其形容词词性。再者，也可以从这句话的语境来推测，主要说的是这些物种会争夺地盘，那么rival就是形容这些物种之间的关系的，很明显，将这一层意思表达出来的只有B选项。rival: 竞争的。
Physical separation between subpopulations promotes the formation of new species 亚群的身体区别促进产生新的物种，promote在句子意思中是促进的意思。选项A 描述 B 鼓励，促进 C 延迟 D 要求， 排除ACD，选B