Social Spiders Hold Key To Evolutionary Questions

Social spiders hold key to evolutionary questions

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(EDITORS: Aviles leaves for her field season in Ecuador in late March. Her work will be profiled on television at 8 p.m. Feb. 25 on The Desert Speaks, a production of KUAT, Tucson's Public Broadcasting System station, and on April 28 on PBS' Scientific American Frontiers (8 p.m. on KUAT).

The British Broadcasting Corp. will be filming her work in Ecuador this year for a future broadcast.)
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Social spiders. For most of us, the incongruous phrase might conjure up a Gary Larsonesque image of spindly-legged creatures standing upright holding drinks at some surreal arachnid cocktail party.

For Leticia Aviles, though, social spiders hold the key to understanding some of the most important and controversial topics in modern-day evolutionary biology.

Aviles, an assistant professor in The University of Arizona's Department of Ecology and Evolutionary Biology, has spent her career studying an unusual group of animals -- spider species that live in groups and hunt prey cooperatively in large communal webs. She has investigated not only the evolution of sociality-- how and why organisms live in groups and cooperate -- but also unequal sex ratios, population dynamics, and the level at which natural selection occurs. And her unique work has managed to integrate these usually disparate areas of research. "The pieces fall together," she says, "because the biology of the social spiders demands that they do."

Aviles first became interested in social spiders as a college student in her native Ecuador, when she observed the spiders in a field course while simultaneously taking a sociobiology class. Rereading E.O. Wilson's landmark book, "The Insect Societies," she found that the tome was dominated by findings from studies on ants and bees, but "only had a page and a half on social spiders." Fifteen years, one Harvard doctorate and many scientific papers later, Aviles is an international authority on social spiders and the scientific questions they inspire.

Only a few dozen of the world's 34,000 spider species show social behavior. Traits such as group-living, cooperation in hunting, and communal feeding and brood care occur among distantly related species, so it's thought that sociality has evolved independently in spiders perhaps 12 times in evolutionary history. Most social spiders live in the tropics -- for instance, Ecuadorian rainforest where Aviles does most of her work -- but a few species reach north into the United States.

Biologists often contrast two approaches to research: one in which an interest in certain questions of broad significance leads to study of particular plant or animal groups that best address those questions; and another in which a love of a particular organismal group leads to study of the broadly important questions that group best exemplifies.

Aviles has taken the latter course; the spiders have led to the questions. "When you work on a particular group of organisms, one thing leads to another," she explains. "You never get bored. You never have to grasp around for new questions because the organisms themselves take care of proposing the questions. If you listen to the organisms they will tell you where to go."

"Working on a particular organism, you actually end up getting to do more diverse things" than with the question-driven approach, she says. "Some people may say, oh, you're narrow because you work only on social spiders, but you're really not narrow at all (because of the diversity of questions to which you're led.)"

Her spiders first led Aviles to wonder why they were producing ten females for every one male. Research into this biased sex ratio led to the realization that natural selection was acting at the level of the colony rather than the level of the individual. This notion of "group selection" was juicy and controversial. Traditional Darwinian biology held that the fittest individuals survive and reproduce to pass on their genes through the generations.

But group selection held that this process of selection can also act between groups of individuals. After years of contentious debate, biologists have now approached consensus that individual selection is the rule, but that group selection may al! so occur under a certain restricted set of conditions. Since social spiders meet all those conditions, Aviles' work is recognized as an important example of group selection in nature.

>From here, the biology of the spiders led her to study population dynamics. The spider colonies are essentially small independent populations that show drastic changes in population size due to various factors. Figuring out what drives population fluctuations in these clearly delineated populations, Aviles says, may help biologists better understand how these processes work in larger, more unwieldy populations, including ours.

Determining what drives social evolution is of interest to anyone who wants to know how and why animals -- including humans -- evolve to cooperate, especially since traditional evolutionary thinking suggests that competition between individuals should be the rule in nature.

Aviles' focus on spiders gives a new perspective to the field of sociality, which is still dominated by people studying ants and bees. The odd biology of ants and bees, with their unique patterns of genetic relatedness within groups, "may have overemphasized the importance of [such factors] to the detriment of understanding the costs and benefits of group living," Aviles says. The cost-benefit approach, she points out, is commonly taken by scientists studying social vertebrates, which do not show such odd genetic patterns.

In addition, Aviles maintains, "sorting out the level at which selection acts is a very important question." There have been transitions in the level at which selection acts throughout life's history on earth. "Understanding these transitions is really fundamental to understanding how life evolved," she says. And social spiders are a good model, she adds, helping us understand other transitions, "such as that from cells to multicellular organisms."

With a major new grant from the National Science Foundation, Aviles will explore these issues in an integrated fashion, using both fieldwork in Ecuador and modeling and computer simulation in her Tucson laboratory. Students and collaborators will join her in raising spider colonies in the lab, following the ups and downs of hypothetical populations on computer screens, and canoeing through pristine Amazonian rainforest to find and monitor new spider colonies.

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