A new approach to organismal biogeography

Biogeography is a science that explores the prevalence and abundance of plants and animals in space and time. It attempts to reconstruct the history of organismal colonization of the Earth. A group of Slovenian biologists has developed a new, more accurate approach to determine the past distribution of organisms at the taxonomic level of the family and published its study in the Journal of Biogeography.

As part of her doctoral dissertation at the Scientific Research Center of the Slovenian Academy of Sciences and Arts and at the University of Ljubljana, Eva Turk developed a new method, which she published in open access on April 17 (https://onlinelibrary.wiley.com/doi/full/10.1111/jbi.13838). The study is co-authored by EZ Lab members, Dr. Simona Kralj-Fišer of ZRC SAZU, and Drs. Klemen Čandek and Matjaž Kuntner from the National Institute of Biology in Ljubljana.

In order to accurately determine the distribution and abundance of a group of organisms in the past, we need good data on the evolution of the group over time. With the development of new phylogenetic methods for reconstructing relatedness among biological species, we can fairly accurately determine how many millions of years ago evolutionary splits occurred between organisms. When we combine such data with knowledge of Earth's geological history, we can create probable scenarios as to what area the organisms originated from, how they colonized continents, and how new species came about.

Biogeography is evolving vividly as many studies on the history of continental settlement are being published. However, the accuracy of such reconstructions is often low and unreliable. That is why a group of Slovenian biologists has taken a new approach that, in addition to the relatedness among organisms, also considers their ability to overcome long distances in individual time slices of the Earth’s history.

The researchers were interested in the geographical distribution of golden orbweb spiders (the subtropical family Nephilidae) throughout their evolutionary history. In their prior publication in Systematic Biology that built on a combination of genomic and fossil data, this spider family was reconstructed to date to Cretaceous, 133 million years ago (https://academic.oup.com/sysbio/article/68/4/555/5229942).

The study by Turk and colleagues has now added to the reconstruction our knowledge of the dispersal biology of orbweb spiders that are capable of using silk sails for wind-associated travel over distances that exceed 4,000 kilometers. It assessed the probabilities of their dispersal during Earth history every 10 million years taking advantage of thoroughly reconstructed continental shifts that allowed measuring the distance between continents in time.

The results were unexpected. The authors refuted the hypothesis of the African origin of the Nephilidae family and found support for its Asian-Australasian origin. The history of continental settlements was diverse and contained many transoceanic crossings. According to the hypothesis, one such crossing is likely to have been from Australia via Antarctica to South America (panel b in the figure below).

The results of this study are interesting for understanding how some ancestors of certain groups of spiders traveled across continents and found open spaces for speciation there, while other branches of the same family more or less adhered to their ancestral areas. But perhaps the greatest value of the study is the new methodological approach we have developed. It takes into account the specific biology of organisms throughout Earth's history and adjusts the probability of their dispersion over time, which is completely new,

said Matjaž Kuntner.

The authors of the study proposed a similar approach for more accurate reconstructions of the biogeography of any organism. Modern biogeography should consider the different probabilities of intercontinental spread of organisms over periods of Earth's history.