Some of the latest evidence that we still don't know as much as we like to think about evolution comes from tree-borne animals who seem to be evolving in ways researchers never considered until now. One new study published in Ecology and Evolution shows that the common treeshrew, known taxonomically as Tupaia glis, appears to be upending two well-established rules of geographical variation referred to as Bergmann's rule and the island rule. Perhaps in even more blatant defiance, though, common ravens have recently been found to be reversing the evolutionary process outright according to a study published last Thursday in Nature Communications.
Geographical variation rules, for those who don't know, might seem less like evolutionary realities and more like bizarre, habitat decisions being made by individuals, but these are, indeed, adaptations on both the evolutionary and ecological levels. To understand how this is will require you to understand those rules. The island rule, for example, is basically a prediction that whole populations of small mammals will follow an adaptation trend over the long term that makes them successively bigger in body mass with each generation if they live on islands rather than on the mainland. The larger animals on an island, conversely, are predicted to adapt in the opposite direction, getting smaller while their mainland counterparts grow in size.
Already, you can see the evolutionary relevance and how defiance of this rule might prove revelatory to scientists. The Bergmann rule is similarly pertinent to both evolution and the environment in which an animal population cohabitates among other life. This rule dictates that populations in cold climates, which are usually found at higher latitudes, are fundamentally bigger than their counterparts of the same species in warm climates, typically found in the lower latitudes. On the Malay Peninsula and on 13 offshore islands, a team of researchers studied the treeshrew body size across a multitude of populations, and to do this, they measured specimens that have been collected over the course of the last 122 years by natural history museums in North America and Europe.
The research team ended up looking at 260 treeshrew specimens, testing for all kinds of variable data. They analyzed the maximal sea depth between each island and the mainland, the distances between said islands and said mainland, and the size of each island. They documented the latitude of each island, and they examined all this data in relation to the average body size of each treeshrew population. This is a rare kind of study in that Bergmann's rule and the island rule are rarely analyzed together, but the findings are still unexpected because they found that the two rules simply don't apply to common treeshrews.
"Determining the causes of geographical variation within a species is critical to understanding underlying mechanisms of evolutionary patterns," according to Eric J. Sargis, a Yale anthropology professor and the lead author on the study. "Our analysis demonstrates the need to assess multiple variables simultaneously when studying ecogeographical rules in a broadly distributed species like the common treeshrew, as multiple factors may have influenced how populations evolved." At the very least this rule reversal study illustrates that there may be better ways that these rules can be verified than the methodologies implemented in previous studies, especially if this prompts more studies to be conducted that find similar holes when measuring them together for known populations that were previously believed to conform.
The common raven, for that matter, has been found by the Nature Communications study to exemplify what researchers are calling speciation reversal. They aren't branching into new species the way evolution ordinarily dictates they would, and scientists discovered this by taking DNA samples from them — samples collected over two decades — and analyzing the evidence those samples yielded. That evidence showed that common ravens on North America's western coastline have, instead, split into three genetically unique lineages but that two of these groups seem to be reforming into one all over again. That's evolution going the opposite way.
Anna Kearns, a Smithsonian Conservation Biology Institute evolutionary biologist explains that this is odd because the scientific community views species evolution as a tree that constantly sprouts new branches that split from one another. "You see a split and then you see another split, and then you see another split, but you rarely see those two branches that are split come back together again," Kearns says. She was the lead author of the study. From time to time, though, this has been known to happen, and another term for it is "reticulate evolution," according to Kearns. She adds, though, that only a handful of species besides ravens have been seen to manifest this phenomenon, and those include two fish species and one other bird (finches).
Ultimately, these studies illustrate that emerging species can evolve in reverse, which begins to challenge the notions we learn about biological evolution in school like the interbreeding of different species, for example, which we learn isn't possible. Studies like this and that of the common treeshrew's reversal of rules depend largely on museum collections for requisite specimens, too. "Without well-documented and curated voucher specimens collected from numerous localities," according to the University of Alaska Museum's Link Olson, "and in large enough numbers to assess statistical significance, we simply could not have done this research."
Olson was a co-author on the treeshrew study.
[researchpaper 리서치페이퍼=Cedric Dent 기자]