Ceccarelli, F.S., Opell, B.D., Haddad, C.R., Raven, R.J., Soto, E.M., and Ramirez, M.J. (2016). Around the world in eight million years: historical biogeography and evolution of the spray zone spider Amaurobioides (Araneae: Anyphaenidae). PLoS ONE. 11(10): e0163740. http://dx.doi.org/10.1371/journal.pone.0163740
Why do they live here?
Broadly, the field of biogeography examines how and why species live where they do. Understanding the habitat of a species can tell us a lot about its evolutionary history. Geography can isolate a population of animals, such as on islands like Madagascar, leading to the development of unique, endemic species. The reverse can also occur: for example, the Bering Sea land bridge allowed many species, including early humans, to disperse from Asia into North America.
Biogeographers are often interested in comparing closely related groups of terrestrial species that live on different landmasses. Unlike remote islands, which are usually colonized by species after long-distance dispersal events (migrations), landmasses that were separated by tectonic events can also have “native” species that remained and evolved as the landmass moved. Distinguishing between such “native” and newcomer species is a big question in biogeography.
Spiders in historical biogeography
Spiders are often one of the first and most successful groups to colonize isolate oceanic islands. They can travel long distances by “ballooning,” a dispersal technique where spiders float in the breeze while anchored by a strand of silk (see below for a video demo!). Some species can even land and take off on choppy, salty water (Hayashi et al., 2015). Even without ballooning, spiders can travel great distances by using logs and other vegetation as rafts, as has been observed in the Amazon River system.
An international team of zoologists and biogeographers zoned in on members of the Amaurobioides genus of spiders. These arachnids live in coastal habitats, and are found on three Southern hemisphere continents (Africa, South America, and Australasia).These continents were born with the breakup of Gondawana during the Mesozoic era (210 to 180 million years ago).
The researchers used the DNA of 45 Amaurobioides species to investigate how the spiders spread across the Southern hemisphere. Did they all evolve together and then ride the continents as they broke apart? Or did they hop across the ocean, colonize new land masses, and follow unique evolutionary paths?
Constructing a biogeographical history
An organism’s DNA describes its entire evolutionary history. By analyzing the genetic information of the spiders, the team was able to reconstruct their evolutionary family tree, and retrace their movement around the world.
Constructing a phylogenetic tree assumes that the DNA of two populations of the same species becomes more different the longer they are isolated from each other. This is due, in part, to random mutations occurring in one population (but not the other). Making these trees is not an exact science, and often tree-making computer programs will produce multiple possible versions of evolutionary events with different likelihoods.
This study used the age of each species in a given area to estimate when they first arrived in that region. For example, if the spiders has “rode” the breakup of the Gondwana landmass to Australia, but had later crossed the ocean to reach South America, would expect the Australian species to be more different (diverged earlier) from the original African species than the South American species. This is because genetic divergence takes time, and it’s much fast to migrate, even across an ocean, than it is to follow tectonic drift of landmasses.
Spiders crossed the ocean in 8 million years
The oldest common ancestor of all present day Amaurobioides spiders is thought to have originated in southern Africa. This is supported by the early divergence of the African species from the other species on the phylogenetic tree, meaning their DNA is the most different of all the species examined. The African spiders have had the longest time to accumulate mutations.
From Africa, these tiny spiders (less than 1.5 cm in length) dispersed across the ocean. Since Amaurobioides do not balloon, the authors hypothesized that the spiders rode logs, algae, and other natural rafts across the ocean. They were pushed eastward by prevailing winds and the Antarctic Circumpolar Current to arrive first in New Zealand and Australia, and then South America. The completed this entire journey in 8 million ears – far shorter than the 50 million years it took from Africa, Australia, and South American to separate from each other.
Migrations and climate change
When given enough time, even humble spiders can accomplish amazing feats. While most studies in biogeography deal with past migrations and species distributions, there is growing interest in using biogeography as a predictive tool.
Species distributions are shifting with climate change as animals and plants flee increasing temperatures by moving towards the poles. For example, a recent large meta-analysis of 764 species (mostly arthropods) shows an average poleward migration of 16.9 km per decade (Chen et al., 2011). Another, earlier analysis including 99 species of birds, butterflies, and herbs reports a migration rate of 6.1 km/decade (Parmesan et al., 2003).
While moving poleward minimizes the impact of increasing temperatures on a species, it is not perfect solution. Migrants may have to deal with new predators and competitors that do not exist in their home range, and likely be less successful. By knowing why species live where they do, we can better understand the potential impacts of global climate change.