A Single Origin: The Diversification of Orb Weavers

Orb weavers, known for their spiral-shaped webs, are some of the most adept spiders on the planet: thousands of species exist on every continent except Antarctica. On October 11, 2011 Proceedings of the Royal Society B published “Tangled in a sparse spider web: single origin of orb weavers and their spinning work unraveled by denser taxonomic sampling”; a study completed by researchers at the University of Copenhagen that found orb-weaving spiders first emerged during the Middle Triassic period, about 230 million years ago. The spiders then rapidly diversified, as a large variety of orb weavers emerged around the middle of the Jurassic period, about 170 million years ago.

Scientists had previously assumed that orb weavers diversified when flying insects did, in response to the need to specialize to catch different prey. Dimitri Dimitrov, an evolutionary biologist at the University of Copenhagen and the study’s first author, explains that his team’s research suggest “there is a no real relationship” between these two diversifications. The team’s work has shown that orb weavers diversified millions of years before flying insects.

The team used fossil records and DNA samples of existing species to create an evolutionary tree and timeline to find that orbs have a single origin.

Results also indicate that orb weavers’ diversification was in response to environmental factors, not to food shortage. Dimitrov believes that orb weavers diversified to create specialized habitats depending on their surrounding environment. Certain species specialize in webs for tall grasses, while others create webs for tall tree branches.

Sources

http://rspb.royalsocietypublishing.org/content/early/2011/10/25/rspb.2011.2011.short?rss=1

http://www.pnas.org/content/106/13/5229.short