From Life in the Undergrowth episode 3, 32:50 to 35:58

After watching this video I became curious about how within the class Araneae which is almost exclusively solitary, some animals have evolved to become social.

Despite it’s rarity, social spiders appear to have evolved many times

Out of the 39,000 known species of spider, only 23 appear social or quasi-social, and these spiders span 8 families and 11 genera, and scientists think that social species have evolved 18 or 19 times (1).

The group used molecular phylogeny techniques on a smaller subset of spiders–the clade above showing nine independent evolutions–the analosimus genus where 13 out of 53 species are social (2). They sequenced 6 genes from the species including mitochondrial DNA (which is passed down maternally) and nuclear DNA. Then they compared the sequences between species and used Baysian algorithms to calculate the most parsimonious (and therefore most likely) phylogenetic tree:

Sociality linked to prolonged maternal rearing, high female to male sex ratios, and inbreeding

Exactly what genetic and developmental changes are required to turn a species from a solitary one into a social one are still unknown, but it seems linked to prolonged maternal rearing, high female:male sex ratios, inbreeding (which promotes selfish altruism through increased kin selection). Studies of mitochondrial DNA (again passed only from the mother), indicate that colonies share a common matriarchal line and suggest that there may be inter-colony competition.

More work has been done on the ecological factors, that make social spiders advantagous. Most social spiders live in lower-elevation tropical regions with many insects and large insects, and towards the centers of forests where large webs can be supported.

Unusual social situation – species from 12 families of spiders share one massive web

Perhaps the ability to live socially (e.g. combine webs and not kill one-another) lies dormant in many species, as demonstrated by this massive Web fount in Texas. Texas A&M entomologist Allen Dean found most spiders were in the Tetragnathidae family, but he found species from 11 other families in the web, with species including: funnel web weavers, sac spiders, orb weavers, mesh web weavers, wolf spiders, pirate spiders, jumping spiders and long-jawed orb weavers. Three such webs were built in the park that season, and Dean speculate that they emerged from the right conditions of plentiful rains and over-abundant food supplies (4). A report of the incident was written up in the journal Southwestern Entomologist.

Arachnophobia

If this has made your skin crawl, you’re probably not alone. Arachnophobia is thought to be the most common of all phobias in the Western world. It’s clear phobias have some genetic element–we seem predisposed to fear animals or heights, but not cars even though they are much more likely to kill us.

Keep in mind though that just because you may be genetically predisposed to fear spiders, doesn’t mean there’s nothing you can do about it. Systematic desensitization, exposure therapy, and cognitive behavioral therapy are remarkably effective at treating phobias and can even be enhanced pharmacologically with d-cycloserine.

References

1. Ingi Agnarsson, Wayne P. Maddison, Leticia Avilés, The phylogeny of the social Anelosimus spiders (Araneae: Theridiidae) inferred from six molecular loci and morphology, Molecular Phylogenetics and Evolution 2007 43 (3), 833-851, 1055-7903, 10.1016/j.ympev.2006.09.01 – http://www.ncbi.nlm.nih.gov/pubmed/17236425

2. Ingi Agnarsson, Leticia Avilés, Jonathan A. Coddington, Wayne P. Maddison, and D. Funk SOCIALITY IN THERIDIID SPIDERS: REPEATED ORIGINS OF AN EVOLUTIONARY DEAD END Evolution 2006 60 (11), 2342-2351 www.bioone.org/doi/full/10.1554/06-078.1

3. AGNARSSON, I., MADDISON, W. P. and AVILÉS, L. (2010), Complete separation along matrilines in a social spider metapopulation inferred from hypervariable mitochondrial DNA region. Molecular Ecology, 19: 3052–3063. doi: 10.1111/j.1365-294X.2010.04681.x

4. http://www.laketawakoni.com/news/spider_web.html (accessed 5-11-12)