New species usually arise when an existing species splits into separate populations that genetically diverge over time. According to the classical view, the driving force in this process is that the populations adapt to different ecological conditions after a spatial separation. A new study contradicts that. Accordingly, comparisons of thousands of sister species show that the separately developed species often have very similar biological adaptations. The most important role in the emergence of new species thus plays the spatial separation itself, not any ecological differences.
When Charles Darwin published his work "On the Origin of Species" in 1859, he described how new species develop from existing ones. He assumed that speciation takes place primarily through different adaptations to ecological conditions. A classic example is the finches of the Galapagos Islands, which have adapted to different ecological niches in a shared environment. More often, however, new species arise as a result of spatial separation. Ever since Darwin's time, there has been a controversial discussion in evolutionary research as to which effect plays the greatest role: geographic isolation per se or ecological differences at different locations.
Geography or Ecology?
"The effects of geographical separation and ecological divergence have been studied largely in isolation, and it is unclear whether and how they interact in the emergence of new species," explains a research team led by Sean Anderson from the University of Toronto in Canada. To answer this question, the scientists analyzed 15 large data sets, each containing up to a thousand pairs of different sister species of vertebrates. In each case, it was taken into account how many millions of years ago the related species developed separately from one another and what biological characteristics they have.
In the case of mammals and amphibians, Anderson and his colleagues included, among other things, climate preferences as well as body size, weight and shape in the analysis, and in the case of birds, also beak characteristics and song variations. The basic assumption of the research team: "If speciation in the case of spatial separation is generally caused by different ecological adaptations, then sister taxa should tend to differ in ecologically relevant characteristics." With the help of statistical models, the biologists examined pairs of related birds and mammals - and amphibian species, how clearly they differed in important ecological characteristics.
Separate development under similar conditions
The result: "In twelve of the 15 groups of sister pairs we analyzed, including global datasets of birds and mammals, differential adaptation was not confirmed as the dominant driving force," the researchers report. "So the vast majority of pairs in each data set diverged under similar rather than different selective pressures." New species often emerge with minimal ecological divergence - simply because the populations are spatially isolated from each other for a long time. Due to random mutations, they distance themselves genetically from each other until joint reproduction is no longer possible, even if the distribution areas later overlap again.
"Using new models to analyze trait differences between sister pairs, we find that adaptive ecological divergence in vertebrate allopatry appears to be the exception rather than the rule," the authors write. "A key takeaway from our result is that allopatry speciation in general does not require lineages to utilize new resources or otherwise adapt to different ecological pressures, but instead relies on their long geographic separation."
Source: Sean Anderson (University of Toronto, Canada) et al., Science, doi: 10.1126/science.abo7719