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What Determines Species Diversity?

Science  01 Jul 2005:
Vol. 309, Issue 5731, pp. 90
DOI: 10.1126/science.309.5731.90

Countless species of plants, animals, and microbes fill every crack and crevice on land and in the sea. They make the world go 'round, converting sunlight to energy that fuels the rest of life, cycling carbon and nitrogen between inorganic and organic forms, and modifying the landscape.

In some places and some groups, hundreds of species exist, whereas in others, very few have evolved; the tropics, for example, are a complex paradise compared to higher latitudes. Biologists are striving to understand why. The interplay between environment and living organisms and between the organisms themselves play key roles in encouraging or discouraging diversity, as do human disturbances, predator-prey relationships, and other food web connections. But exactly how these and other forces work together to shape diversity is largely a mystery.

The challenge is daunting. Baseline data are poor, for example: We don't yet know how many plant and animal species there are on Earth, and researchers can't even begin to predict the numbers and kinds of organisms that make up the microbial world. Researchers probing the evolution of, and limits to, diversity also lack a standardized time scale because evolution takes place over periods lasting from days to millions of years. Moreover, there can be almost as much variation within a species as between two closely related ones. Nor is it clear what genetic changes will result in a new species and what their true influence on speciation is.

Understanding what shapes diversity will require a major interdisciplinary effort, involving paleontological interpretation, field studies, laboratory experimentation, genomic comparisons, and effective statistical analyses. A few exhaustive inventories, such as the United Nations' Millennium Project and an around-the-world assessment of genes from marine microbes, should improve baseline data, but they will barely scratch the surface. Models that predict when one species will split into two will help. And an emerging discipline called evo-devo is probing how genes involved in development contribute to evolution. Together, these efforts will go a long way toward clarifying the history of life.

CREDITS (LEFT TO RIGHT): STEPHEN FRINK/CORBIS IMAGES; KEN LUCAS/VISUALS UNLIMITED; HANS PFLETSCHINGER/PETER ARNOLD

Paleontologists have already made headway in tracking the expansion and contraction of the ranges of various organisms over the millennia. They are finding that geographic distribution plays a key role in speciation. Future studies should continue to reveal large-scale patterns of distribution and perhaps shed more light on the origins of mass extinctions and the effects of these catastrophes on the evolution of new species.

From field studies of plants and animals, researchers have learned that habitat can influence morphology and behavior—particularly sexual selection—in ways that hasten or slow down speciation. Evolutionary biologists have also discovered that speciation can stall out, for example, as separated populations become reconnected, homogenizing genomes that would otherwise diverge. Molecular forces, such as low mutation rates or meiotic drive—in which certain alleles have an increased likelihood of being passed from one generation to the next—influence the rate of speciation.

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

Evolutionary biologists are just beginning to sort out how all these factors are intertwined in different ways for different groups of organisms. The task is urgent: Figuring out what shapes diversity could be important for understanding the nature of the wave of extinctions the world is experiencing and for determining strategies to mitigate it.

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