在一些地方和群落中，存在着数以百计的物种，而在另一些地方和群落中，只有很少的物种。例如，相对于高纬度地区，热带地区是一个复杂的生物天堂。生物学家正在努力找出其中的缘由。环境和生命机体间的相互作用本身就在促进或者抑制生物多样性上扮演着重要角色，同样地，对生物多样性起着重要作用的还有人类的干扰，捕食者与被捕食者的关系以及其他食物网的联系。但是上述因子和其他因子之间究竟是如何正确地协同作用来塑造生物的多样性，目前仍是个大大的谜团。

这个挑战是令人望而生畏的。基线数据非常匮乏，比如：我们不知道地球上有多少种类的植物和动物，研究者甚至连预测构成微生物世界的有机体的数目和种类都无从下手。探究多样性的进化以及限制的研究者们也缺少一个标准的时间尺度，因为进化发生经历的时间从数天到数百万年不等。另外，同一个物种间的变异可以与两个密切相关的不同物种间的变异一样多。遗传变异能否产生一个新的物种，或者它们对物种形成的真实影响，都还是不清楚的。

理解形成物种多样性的原因需要大量的跨学科的努力，包括古生物学的诠释、野外调察、实验室试验、基因组比较以及有效的统计分析。现有的为数不多的详尽的记载，如联合国千禧年计划和对环球海洋微生物的基因评估，应该会提高基线数据，但它们仅能触及到问题的表面。一个能够预测出一个物种何时分化为两个物种的模型将会对解决问题有所帮助。还有一个新兴的学科，称为发育进化学，正在研究基因是怎样参与到有助于进化的发育中的。这些努力加在一起，将对生命史的阐明大有裨益。

古生物学家们在追踪过去千年间不同有机体分布范围的扩张与收缩方面已经取得进展。他们发现，地理分布在物种形成中扮演重要的角色。未来的研究将继续揭示物种的大规模分布模式并且可能会在物种大量消亡的起源以及这些大灾难对于新物种进化的影响方面带来更多的曙光。

从植物和动物的野外调查中，研究者们发现生境可以影响形态和行为——尤其是性选择——从而加快或者减慢了物种的形成。另外进化生物学家们也已经发现物种形成可能会停滞，例如，由于分离的种群重新获得联系，将使本来会发生分歧的基因组均质化。而特定的等位基因通过分子力，例如低变异率或减数分裂驱动，有更高的可能从一代传递到下一代，从而影响了物种形成的速率。

在某些情况下，即使在一个生态系统中，其多样性也有差异。比如生态系统的边缘有时要比其内部所支持的物种要少。

进化生物学家才刚刚开始找出在不同生物群体中这些因素是如何以不同方式交织在一起的。这个任务非常紧迫：找出塑造多样性的推手对于理解当今世界所正在经历的物种灭绝浪潮的本质并制定出对策去减缓它是非常重要的。

原著：伊丽莎白ž潘妮西

翻译：郭子骁、李宇明、云莎

修订：李刚

原文如下：

Countless species of plants,animals, and microbes fill every crack and crevice on land and in the sea. Theymake world go' round, convertiing sun-light into energy that fuels the rest oflife, cycling carbon and nitrogen between inorganic and organic forms, andmodifying the landscape.

In some places and some groups,hundreds of species exist, whereas in others, very few have evolved; thetropics, for example, are a complex paradise compared to higher latitudes.Biologists are striving to understand why. The interplay between environmentand living organisms themselves play key roles in encouraging or discouragingdiversity, as do human disturbances, predatorprey relationships, and other foodweb connections. But exactly how these and other forces work together to shapediversity is largely a mystery.

The challenge is daunting. Baselinedata are poor, for example:  we don'tknow how many plant and animal species there are on earth, and researcher can'teven begin to predict the numbers and kinds of organisms that make up themicrobial world. Researchers probing the evolution of, and limits to, diversity   also lack a standardized time scale becauseevolution  takes  place over periods lasting from days tomillions of years. Moreover, there can be almost as much variation within aspecies as between two closely related ones. Nor is it clear what geneticschanges will result in a new species and what their true influence onspeciation is.

Understanding what shapes diversitywill require a major interdisciplinary effort, involving paleontologicalinterpretation, field studies, laboratory experimentation, genomic comparisons,and effective statistical analyses. A few exhaustive inventories, such as theUnited Nations’Millennium Project and an around-the-world assessment of genesfrom marine microbes, should improve baseline data, but they will barely scratchthe surface. Models that predict when one species will split into two willhelp. And an emerging discipline called evo-devo is probing how genes involvedin development contribute to evolution. Together, these efforts will go a longway toward clarifying the history of life.

Paleontologists have already madeheadway in tracking the expansion and contraction of the ranges of variousorganisms over the millennia. They are finding that geographic distributionplays a key role in speciation. Future studies should continue to reveallarge-scale patterns of distribution and perhaps shed more light on the originsof mass extinctions and the effects of these catastrophes on the evolution ofnew species.

From field studies of plants andanimals, researchers have learned that habitat can influence morphology andbehavior—particularly sexual selection—in ways that hasten or slow downspeciation. Evolutionary biologists have also discovered that speciation canstall out, for example, as separated populations become reconnected,homogenizing genomes that would otherwise diverge. Molecular forces, such aslow mutation rates or meiotic drive—in which certain alleles have an increasedlikelihood of being passed from one generation to the next—influence the rateof speciation.

And in some cases, differences indiversity can vary within an ecosystem: Edges of ecosystems sometimes supportfewer species than the interior.

Evolutionary biologists are justbeginning to sort out how all these factors are intertwined in different waysfor different groups of organisms. The task is urgent: Figuring out what shapesdiversity could be important for understanding the nature of the wave ofextinctions the world is experiencing and for determining strategies tomitigate it.

                                                                                                                                                                     –ELIZABETH PENNISI