About 2 to 3 million years ago, a group of spiders pushed long silk threads into the wind and sailed across the Pacific to Hawaii, so to speak. These spiders were parasites of other spiders, invading their nets and flipping threads to steal trapped insects. But there were not many networks in Hawaii when they arrived. So they expanded their repertoire and looked for other ways to survive by capturing and eating other spiders. From these first spiders, a new species evolved after finding a way to live on rocks. And then another species developed to live under leaves. And then one more thing. And then 1
Charles Darwin first noticed this phenomenon, which is called adaptive radiation, in the beaks of the finches of the Galapagos Islands. His study of the variety of finches led to his theory of evolution through natural selection. But how the adaptive radiation and thus the evolution actually works, is still largely unknown today. Because of its isolation, Hawaii is even more of a hotbed for biodiversity than the Galapagos Islands.
In Hawaiian Ariamnes bar-spiders, adaptive radiation has led to 14 species that live in Hawaii today. They share a generally similar body type, but each is a separate species with different physical characteristics. Remarkably, spider mites with similar characteristics – for example, yellow and red – live on different Hawaiian islands, but are not the closest relatives of the other; According to a recent study by scientists from the University of California, Berkeley, this is a rare case in which a physical form has developed on each island. The study shows that evolution has led to a predictable and independently developed group of similar forms in spiders on each island.
"This highly predictable repeated evolution of the same shapes is intriguing because it sheds light on how evolution actually proceeds," said Rosemary Gillespie, Professor and Schlinger Chair of Systematic Entomology at UC Berkeley and lead author of the article. "Such excellent predictability is rare and can only be found in some other organisms that are similar to vegetation."
The study of Ariamnes bar-spiders is published on March 19 in the journal Current Biology . It was funded by the National Science Foundation and the William M. and Esther G. Schlinger Foundation.
"This study provides insights into a fundamental question about the origins of biodiversity, but also offers a remarkable story that points to the" We must preserve nature in all its forms, "said study co-author George Roderick, professor and chair of the department Hawaii is a chain of islands that has formed chronologically, so scientists were able to study the adaptive radiations of spiders over time as they move from old to newer.
The oldest island, Kauai, was founded 5 million years ago, followed by Oahu, Molokai, Lanai, Maui, and finally the Big Island of Hawaii, the youngest under 1 million years old.
The study found that That spider-spiders have evolved and differentiated from a single species on the same island, so spider species on an island were generally more narrow-minded different-looking spiders on the same island connected than spiders that looked the same on other islands. For example, a white spider on Oahu is closer to the brown spider on the same island than to a white spider on Maui.
"You can find these spiders in just about every habitat on every island," Gillespie said. "This really detailed and finely tuned repetition of the evolution of the same shape is really quite unusual."
The spiders can be divided into three different ecological species, the so-called ecomorphs: a brown that lives in rocks; a golden one that lives under leaves, and a white one that has a dull color and lives on lichen.
The analysis of bar spiders reflects Gillespie's earlier discovery in Hawaiian Tetragnatha spiders, another group that exhibits remarkable adaptive radiation. This group of spiny leg spiders does not spin webs and has repeatedly developed similar ecomorphs since their ancestor arrived in Hawaii. This study was presented on the cover of the journal Science in 2004.
The diverse habitat types in the Hawaiian Islands, cold and humid areas closely juxtaposed with hot and dry, have provided a rich tapestry of species diversity. The flipside of this extraordinary diversity, which has developed in isolation, is its vulnerability to change and invasive species, which are now flooded by human traffic, Gillespie said.
"We need to be able to understand and document this diversity and describe what's so special for people to hear about it," Gillespie said. "It's lost and it's a desperate situation."
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Rosemary G. Gillespie et al., Repeated Diversification of Ecomorphs in Hawaiian Stick Spiders, Current Biology (2018). DOI: 10.1016 / j.cub.2018.01.083