Spiders did not repeatedly gain, but repeatedly lost, foraging webs

Abstract

Much genomic-scale, especially transcriptomic, data on spider phylogeny has accumulated in the last few years. These data have recently been used to investigate the diverse architectures and the origin of spider webs, concluding that the ancestral spider spun no foraging web, that spider webs evolved de novo 10–14 times, and that the orb web evolved at least three times. These findings in fact result from a particular phylogenetic character coding strategy, specifically coding the absence of webs as logically equivalent, and homologous to, 10 other observable (i.e., not absent) web architectures. “Absence” of webs should be regarded as inapplicable data. To be analyzed properly by character optimization algorithms, it should be coded as “?” because these codes—or their equivalent—are handled differently by such algorithms. Additional problems include critical misspellings of taxon names from one analysis to the next (misspellings cause some optimization algorithms to drop terminals, which affects taxon sampling and results), and mistakes in spider natural history. In sum, the method causes character optimization algorithms to produce counter-intuitive results, and does not distinguish absence from secondary loss. Proper treatment of missing entries and corrected data instead imply that foraging webs are primitive for spiders and that webs have been lost ∼5–7 times, not gained 10–14 times. The orb web, specifically, may be homologous (originated only once) although lost 2–6 times.

Keywords

• Spider webs
• Orb webs
• Homology
• Character optimization
• Phylogenomics
• Araneae