Spiders, webs and insects: A new perspective on evolutionary history

Spiders are one of the oldest and most diverse groups of species on earth, with a fossil register that dates back to the Devonian Period (some 380 million years ago). With almost 40,000 identified species, spiders are the predominant arthropod predators of microfauna in the natural environment. The study, which applied molecular biology and bioinformatic techniques for examining evolutionary patterns, focused on the phylogenetic analysis of DNA sequences. Specifically, the team of experts studied the molecular differences in six genetic markers taken from a taxonomic sample of 291 spider species, representing 21 of the 22 families of Orbiculariae (used in the study to refer to Deinopoidea, Araneoidea and Nicodamidae).

As Miquel A. Arnedo explains, "This scientific study looks at the most complete taxonomic sample examined to date, in terms of the number of species and families represented, to understand the phylogeny of spiders that weave orb webs, analysing the DNA sequences of all available genetic markers."

Why did spider webs diversify?

Orb weavers appeared approximately 200 million years ago, in the Middle Triassic, and underwent rapid diversification during the latter stages of the Triassic and the Early Jurassic. What are the causes of this evolutionary process?

Traditionally, the diversification of spider webs has been linked to the spread of insects, which are the spider's main prey, and flowering plants (angiosperms). The authors of the new study formulate a new hypothesis to describe this evolutionary phenomenon. "In the article, we suggest that the changes in spider webs are intended to facilitate the move to new habitats and to make better use of the trophic resources in different ecosystems. In other words, the abundance of prey and the structural complexity of the habitat are more influential factors than the actual diversification of the prey. Moreover, according to our study, the biological explosion of orb webs would not have coincided with the rapid diversification of insects," says Miquel A. Arnedo.

According to Arnedo, "Spiders are generalist predators, and few cases have been found in which they specialize in a particular type of prey. In addition, most spiders do not weave orb webs or produce more irregular forms. It is not the capacity to weave webs that has enabled spiders to diversify but rather their ability to produce silk, and this is not the same thing. Silk threads, which are produced by many arachnid species, can be used for anchorage, movement, nest-building, protecting eggs, and so on."

Traps that also evolved

Over their evolutionary history, spiders have come up with different strategies for catching prey. Orb webs, which are difficult to construct but extremely flexible and resistant, are the result of a complex stereotyped pattern of behaviour in spiders. In the article, the experts also consider a reconstruction of the evolution of webs, referring to examples such as the families Linyphiidae and Theridiidae, which weave simple webs that are easier to build, in which the spider is protected from predators, or the Mimetidae family, which prey on other spiders on their own webs by simulating the vibrations caused by trapped prey.

"The great diversification began with sticky silk, which is a more efficient material and easier for spiders to produce. In our study, we found that all of the evolutionary innovations that have occurred since the first orb webs emerge independently, require less energy to be expended by the spider, and demand fewer behavioural patterns," explains Arnedo. This suggests that spiders, rather than following the evolution of insects, design new strategies that allow them to occupy the largest possible ecological space.

Miquel A. Arnedo, winner of the 2009 ICREA Academia award, directs the UB's Arthropod Systematics and Evolution Laboratory, which focuses on the study of factors that determine the diversification of living species. The group works on various research areas based on the use of molecular markers to study the function and evolution of arachnids, a field in which next-generation sequencing technologies will pave the way for a more complete understanding of evolutionary processes.

The article published in the Proceedings of the Royal Society is also signed by the experts Gustavo Hormiga (George Washington University), Dimitar Dimitrov (University of Copenhagen), Lara Lopardo (University of Greifswald), Gonzalo Giribet (Harvard University) and Fernando Álvarez-Padilla (National Autonomous University of Mexico).