Newswise — Can evolution reverse itself?
Findings reported by two University of Illinois at Chicago biologists have reaffirmed a recently disparaged "law" that says a species trait, once lost to the sands of time through evolution, can never be regained.
Nineteenth-century Belgian paleontologist Louis Dollo argued that once natural selection for a complex functional trait is relaxed -- such as when a species moves into dark caves and loses its need for sight -- mutations that degrade the genes needed for the trait accumulate, and the sequence of mutations is unlikely to be exactly reversed.
But over the past two decades many biologists have challenged Dollo's Law, often by using statistical tools to reconstruct trait evolution and ancestry by looking only at existing species and their present-day traits.
Boris Igic, assistant professor of biological sciences, and Emma Goldberg, a post-doctoral student in Igic's UIC laboratory and now at both UIC and the University of Maryland, became suspicious of those methods -- which they also had used in evolutionary studies of plant fertilization. They found the methods flawed, prompting them to examine the challenges to Dollo's Law.
"We used computer simulations to demonstrate that this methodology consistently gives incorrect results when the loss of a trait truly is irreversible," said Igic. Fifteen years of studies have relied on these procedures to show that Dollo's Law is frequently violated, Igic said. "But they used what we found was faulty methodology."
Igic and Goldberg identified two problems with the challenges: a logical error in how the common ancestor of a group of organisms is treated mathematically, and a disregard for the likelihood that complex characters affect the chances either of speciation -- the origin of new species -- or of extinction.
The biologists suggest a better way of testing Dollo's Law. They used mathematical tools to refute two Dollo challenges which claimed certain animals regained traits such as winged flight and sexual reproduction, and say many other claims of regained complex traits should be re-evaluated.
Intuition suggests that Dollo was correct in saying that transitions between two character states are unidirectional -- go one way, and you can never return. Igic said despite challenges by respected evolutionary biologists, he and others suspected there was something amiss with the methods used. But the doubters couldn't point to what that misstep was.
"We put our finger on the problem," said Igic. It's exciting, he said, to show that "intuition can win over something that's counterintuitive."
Assessing the validity of Dollo's Law shows the difficulty in reconstructing evolutionary histories using limited available data, the UIC researchers said. But new research that combines evolutionary biology, genetics, genomics, geology, mathematics and paleontology will advance our understanding of biological processes.
The findings are published in the November issue of Evolution. Funding was provided in part by the National Science Foundation.
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