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New engines of evolution

Natural History, Feb, 2003 by Peter Brown

Last month I noted that part of what many people don't like about science stems from its conclusion that we human beings don t occupy the center of the universe. But another great source of discomfort about science is its insistence that change is a pervasive feature of the world. Evolutionary change, of course, has long been a thorn in the side of many conservative Christians. Change in the heavens, the very model of permanence and order, seems to fly in the face of common sense. Maybe it's fortunate that our resistance to change is balanced by the fact that one human lifetime is too short for anyone to notice many of the grandest changes in nature. Yet the capacity of science to take the long view, to study events that take far more than a lifetime to unfold, often makes science the bearer of unwelcome tidings that undermine our yearning for stability.

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Take evolution. One consequence of Darwinian evolution by natural selection is that as the world changes, what lives and what dies can change as well. In this month's cover story, Christian Ziegler and Egbert Giles Leigh Jr. document the subtle but pervasive effects of one world-changing event--the construction of the Panama Canal--on the ecology and biodiversity of the Panamanian tropical forest (see "Biosphere III," page 50). Other changes with substantial effects on the world's genetic history--agricultural breeding, the transport of species from one region to another--are too slow to be perceived without specialized techniques. Daniel G. Bradley, in his "Genetic Hoofprints" (page 36), describes how the magic lantern of DNA analysis has shed some surprising light on the evolutionary history of cattle since their domestication 10,000 years ago.

But what about the mechanisms of evolutionary change? What gives rise to the changes that individuals present for testing by natural selection? For much of the history of Darwinism the answers have been genetic mixing through sexual reproduction and random genetic mutation. But random mutation has seemed to many the Achilles' heel of the theory: the known rates of random mutation have seemed inconsistent with the time available for the observed biodiversity on Earth to have evolved.

Now the scientific understanding of change itself is changing. In his "Invasion of the Gender Benders" (page 58) John H. Werren describes one way that microorganisms are playing a key role in evolutionary change. The microorganisms that Werren studies are parasitic bacteria that survive by changing the reproductive process in their hosts. Some of these bacteria change their male hosts into females. Some kill all their hosts that happen to be male. Some make their hosts parthenogenetic, rendering the need for sexual reproduction irrelevant. Bacteria with such diabolical talents are not confined to some small and obscure corner of nature. They affect at least a fifth of all insect species, and perhaps as many as 70 percent. And their activities have broad implications for humanity. In a companion piece to Werren's article, T.V. Rajan eloquently recounts how microbiologists discovered that some of the same bacteria play a role in several of the most devastating pestilences that afflict humankind: elephantiasis and river blindness (see "The Worm and the Parasite," page 32).

The lesson of Werren's story is an apt one for our times (with apologies to Geoff Mack): "Change is everywhere, man."

New engines of evolution

Natural History, Feb, 2003 by Peter Brown

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