Family Values

I'm sure by now you've heard of the ginormous spider web that was spun in Texas. The thing was huge -- 200 yards long -- and it was spun by multiple different species. That interspecific collaboration got Bill Poser thinking, so he blogged about it at Language Log:

The web covers hundreds of square meters. Not only was it built by hundreds of spiders, who normally build isolated webs and eat each other if they get too close, but entomologist Allen Dean reports that they belong to twelve different families! We're talking massive inter-species communication here folks, and not particularly closely related species either. It is comparable to communication and collaboration between human beings (family Hominidae) and Lar Gibbons (family Hylobatidae).

Twelve different families! To show you how diverse these spiders are, Poser reveals what species we'd have to collaborate with to accomplish such inter-species team work. Only he underestimates the scale of the difference. You see, Poser falls victim to taxonomic bias, assuming that a family in one taxon (spiders) is equivalent to a family in another taxon (primates).

Before we get any further, allow me to remind you of the major taxonomic categories used for classifying animals:

     Kingdom

          Phylum

               Class

                    Order

                         Family

                              Genus

                                   Species

To use humans as an example, we're in the kingdom Animalia, aka the animals. Our phylum is Chordata, which is made up predominantly of the vertebrates (ie, things with backbones). Our class is Mammalia, our order is Primates, our family is Hominidae, and our species name (made up of the genus and species titles) is Homo sapiens. Many mnemonics have been developed to aid students in remembering the taxonomic groupings, playing off the first letter in each word. My favorite: King Plays Cards On Fat Girls' Stomachs.

Anyway, mammalian families aren't all that diverse. The family Hominidae contains only four extant species (humans, chimps, gorillas, and orangutans). If they were invertebrates, we'd probably classify them as a genus (or sub genus). But, because they're closely related to us, they get their own family (that's taxonomic bias). Other mammalian families are more diverse; Muridae (mice, rats, and gerbils) contains hundreds of species, and it's the largest of all mammalian families. As a point of comparison, the genus Drosophila has over a thousand species. Yes, a single insect genus is more specious speciose that the largest mammalian order.

But counting species isn't the only way to determine diversity within a taxon. We can also estimate the age of the taxon using fossils and DNA. Let's look again at the Muridae. The species that make up this family last shared a common ancestor approximately 25 million years ago (mya). To go back to our invertebrate example, the species in the genus Drosophila last shared a common ancestor approximately 60 mya. Once again, the invertebrate genus is more diverse than the mammalian family.

Hopefully I've shown you that comparing taxonomic categories across different taxa is fairly meaningless. What gets named as a family in one taxon contains less diversity than a genus in another taxon. But what about the spiders? What would be the equivalent collaborator for humans to accomplish the same amount of inter-family collaboration that we see in the spider web in Texas? A group of entomologists at Texas A&M sampled spiders from the web and classified them based on their families and species. The three most abundant families were:

1. Tetragnathidae (long-jawed orb weavers)
2. Salticidae (jumping spiders)
3. Araneidae (orb-weaver spiders)

All three of these families belong to the suborder Araneomorphae, which was thought to have radiated in the late Paleozoic or early Mesozoic Era (reference), or about 250 mya. Tetragnathidae have been observed as early as the Cretaceous Period, which came at the end of the Mesozoic Era, about 150 mya. We'll use this date as a lower-bound estimate of the divergence times for the spiders found in the web in Texas. What follows is therefore a search for a taxon with the minimum amount of divergence from humans to achieve the equivalent amount of divergence between the spiders in the huge web.

Mammals diverged from birds and reptiles about 300 mya. This date can be used to calibrate a molecular clock, which can then be used to estimate the divergence dates between different vertebrate lineages provided we can sequence DNA from extant representatives of those lineages. Kumar and Hedges used this approach to estimate the divergence times between humans and various other vertebrates. At about 150 mya, the human lineage (and all other eutherian mammals) diverged from the marsupials. This was followed by a rapid radiation of the eutherians, giving rise to the mammalian orders we see today. Humans and gibbons (Poser's chosen equivalent collaborator) diverged about 15 mya.

Contrary to what Poser claims, humans collaborating with gibbons is not equivalent to the various spider families working together to build I gigantic web. To accomplish such an inter-taxon collaboration, humans would have to work with kangaroos, koalas, or possums. I'm not sure how they do things in Australia, but I doubt they've got Tasmanian devils working construction.