When Charles Darwin first put forward his revolutionary theory of evolution, he was met with considerable opposition and skepticism from both scientific and religious circles. While this is now entirely accepted by the scientific community, some of his ideas have continued to cause controversy for more than 150 years. One contentious hypothesis, for example, suggested that organisms could cross vast distances and oceans and then successfully establish themselves in a new geographic location.
To achieve this “jump dispersal,” it was proposed that organisms could hitch a ride on various objects such as mats of vegetation and icebergs, or even just blow in the wind. Although this idea has been largely dismissed by the scientific community, new research on island dwelling organisms suggests that he may have been right after all. Using statistical modeling to compare Darwin’s theory with a competing theory, strong evidence in support of jump dispersal was found. The work has been published in Systematic Biology.
Attempting to explain how certain species ended up in particular geographical locations across the globe becomes tricky when close relatives live on different continents, separated by vast oceans. Darwin thought that some organisms might have piggybacked to get to their destinations, but opponents thought that the idea of lizards floating on bits of wood for thousands of miles was a bit too far-fetched. Instead, it was suggested that organisms must have crossed “land bridges” to colonize new areas before the continents split apart. This “vicariance” theory was well-received by scientists, so much so that models used to estimate the biogeographic history of certain species completely excluded jump dispersal.
However, there are several lines of evidence to suggest that vicariance does not always cut the mustard. For example, some species’ ancestors are thought to have evolved millions of years after the continents separated, meaning that confidence in this theory has gradually waned over time. Because of this, the National Institute for Mathematical and Biological Synthesis researcher Nicholas Matzke took it upon himself to design a computer program to compare these two contrasting theories.
Matzke plugged in existing data from a large number of organisms living in Hawaii and several other archipelagos (a cluster of islands), and sure enough Darwin’s theory turned up trumps. He found that jump dispersal could explain the biogeography of the species examined with greater statistical probability than vicariance.
“Conventional biogeography said vicariance was a more scientific explanation than jump dispersal because vicariance relied on normal, predictable processes, and jump dispersal relied on extremely rare, near-miraculous events,” Matzke said in a news-release. “Now the shoe is really on the other foot because the jump dispersal pattern appears to be much more common. It looks like Darwin was right after all.”
Matzke goes on to remind us that just because an event is extremely rare on human timescales does not necessarily mean it is rare over geological timescales. Of course, this research is not dismissing vicariance, but does suggest that both methods have probably played roles in shaping biodiversity on Earth.