Human colonization patterns are a common area of research by anthropologists, archaeologists and historians, but did you know that biologists regularly ask similar questions about non-human organisms? Understanding how species came to be in particular places is an essential aspect of their natural history. Did they evolve where they are currently found? Or did they evolve elsewhere and colonize from there? Who are they most closely related to? This area of study is called biological systematics – “the study of the diversification of living forms, both past and present, and the relationships among living things through time”.1 Just as studying ancestry in humans often involves a family tree, relationships among related non-human species are often visualized as an evolutionary tree.
In the past, an organism’s physical features (morphology) were used to resolve one species from another and to make educated guesses at which specimens were more closely related. But now we have another tool in the toolbox – DNA has become an essential research tool in biological systematics, helping determine evolutionary relationships for groups that have historically been difficult to resolve. This provides us with insight into the timing and causes of the evolution of new species. In effect, their ancestry!
Here at the Royal BC Museum, systematic research involving the common forest spider, genus Cybaeus, incorporated both morphology and genetics. These spiders have two major centres of diversity: Japan and North America. In North America, the greatest concentration of diversity is found in California, as with many other groups of organisms, whereas only four species are found east of the Mississippi River. Identification of Cybaeus spiders was traditionally based almost exclusively on their “naughty bits” – their genitalic morphology.
To see if the relationships predicted by morphology were matched by the genes, I undertook a genetic analysis of fifteen species, with interesting results. The Holarctic (temperate northern hemisphere) and Nearctic (strictly North American) genetic “family trees” were similar to the proposed morphological family tree, with some noteworthy exceptions. One species (Species A in Figure 1) does not appear to be related to either of the two broad groups (Holarctic or Nearctic) – and this leads to a new question: is it even a Cybaeus spider? Another species thought to be part of the Holarctic family tree appears to be Nearctic in its origin – so the genes are telling a very different story than the morphology (Species B). And last but not least, a species that had naughty bits that were not really similar to any of the other spiders from either the Holarctic or Nearctic groups is clearly part of the Nearctic family tree (Species C) – a perfect example of how genetic analysis can help to resolve some evolutionary quandaries. So how can we interpret these results? What could have caused the evolutionary pattern we see in both the genes and the morphology?
As one might expect, the geological history of North America plays a fundamental role in species distributions, evolution, and the presence of endemics (species that are only found in a certain area). For example, only one species of the four distributed east of the Mississippi River was examined genetically as part of my study, but it turned out to be most closely related to its European cousin (purple box, Figure 1). This is less surprising when you learn that spiders similar to species that are found on earth today had all appeared by the Tertiary Period (65 to 1.8 million years ago), a time when regions of western Europe were still joined to regions of eastern North America. Therefore, we might reason that the four species that occur in the Appalachian area of North America may have descended from a remnant population of European-origin Cybaeus, cut off from the others through glaciation and mountain building, and free to diversify.
And who can forget the glaciers! A series of ice sheets blanketed North America as far south as the 40ºN latitude during the Pleistocene Epoch (1.8 million years ago to 10,000 years ago), effectively eliminating much of the terrestrial flora and fauna beneath them. Impacts on speciation (when two or more species evolve from a shared ancestor) and colonization events by northern-inhabiting species of Cybaeus would have been greatest during this time period. The evolutionary history in the Holarctic species, as told by molecules and morphology, reflects this changeable epoch.
There are some recognized exceptions to the scouring effect of the glaciers. On the western coast of North America there are known locations of glacial refugia, including several in British Columbia. These refugia may have harboured some representatives of Cybaeus that were then able to recolonize portions of their former range and potentially evolve into new species following the retreat of the glaciers. Examples may include those Holarctic species whose distributions currently include British Columbia. Three of these species co-occur on Haida Gwaii, and they are very similar genetically, as one might expect based on this scenario (green box, Figure 1).
South of the extent of the glacial advance, the genus Cybaeus was likely undergoing speciation due to the effects of mountain building, sea water incursions and plate tectonic activity. Geological events such as these have been regular occurrences in the region of highest Cybaeus diversity in North America: California, but they also occur throughout the range of western Cybaeus. The restricted distributions of several species of modern-day Cybaeus is probably a result of a more recent event – a warm period known as the Holocene Climate Optimum during the interval roughly 9,000 to 5,000 years before present. It isn’t difficult to imagine moist forest-inhabiting spiders such as Cybaeus experiencing range reductions due to this overall warming trend.
Today, when the distributions of members of the genus Cybaeus are examined in relation to the impacts of habitat fragmentation and loss, as well as the current warming trend, some species are potentially at great risk of extinction. General concern for the unique biota of California may assist indirectly in preserving suitable habitat. Much more difficult to predict is how currently suitable habitats will change under the influence of this latest, anthropogenic warming trend. Further loss of wet forest ecosystems will surely result in a concomitant loss of the species that depend on them.
Tracing the history of species other than ourselves is still a view on their ancestry, but the short timescales used when we are thinking about human history is what sets us apart from much of the rest of life on earth. Evolutionary trees are not so different from family trees when you keep this in mind. And trying to determine the path of the evolution of species is fundamentally important to natural history collections in museums, just as genealogy is to human history collections.