UCSD Biologists Find New Evidence for One-Way Evolution

By tracing the 30-million year history of variation in a gene found in plants such as tomatoes and tobacco, biologists at the University of California, San Diego have found new evidence to support an old idea — that some evolutionary changes are irreversible.

Their study, published this week in an early online edition of the journal Proceedings of the National Academy of Sciences, offers new support for the idea that the loss of complex traits, like eyes, wings or in this case a reproductive mechanism, is often irreversible. In other words, once lost, the traits never revert to their original state.

"This is the strongest evidence yet to support irreversibility,” said Joshua Kohn, an associate professor of biology at UCSD who headed the study. "If we had not used the genetic data coding for this reproductive mechanism and only inferred the pattern of evolution based on the traits of living species, we would have come to the opposite conclusion and with high statistical support — that the trait evolved more than once.”

The scientists examined existing variation in the gene used by many members of the Solanaceae family, which include tomatoes and tobacco, to recognize and reject their own pollen, thereby avoiding self-fertilization and the harmful effects of inbreeding. This ability is sometimes lost, as is the case for garden tomatoes, which can set seed by self-fertilization. Apparently, once lost, the ability to reject pollen in order to prevent self-fertilization is never regained.

Irreversible loss of complex traits, which result from the combined interaction of several genes, is an old and at times controversial scientific question. While the late evolutionary biologist Stephen Jay Gould popularized the hypothesis of irreversibility, known as Dollo's Law, studies that use current methods to reconstruct the evolution of complex traits often fail to support it. This is because it is often difficult to reconstruct characteristics of extinct ancestors with any certainty.

The study contradicts earlier studies of complex trait evolution, which have tended to favor multiple reappearances of complex traits after these organs were lost in ancestral species. The authors suggest that traditional methods for reconstructing the history of trait evolution may be inaccurate.

Discovering irreversible change for this sexual system trait highlights the importance of considering genetic data underlying the trait when reconstructing its evolutionary history.

“Our work implies that evidence for such evolutionary change in other cases may have been missed because the current methods aren’t sufficiently refined,” said Boris Igic, who conducted the study while a graduate student at UCSD and is now a postdoctoral fellow at Cornell University.

While lending support to the question of unidirectional evolution, the biologists’ findings also lead to new questions.

“Apparently, plants that have sex exclusively with other plants and not themselves, enjoy a greater evolutionary advantage,” Igic said. “Exactly why is unclear,”

Species capable of rejecting their own pollen in favor of pollen from other individuals, harbor more genetic variation than those that self-fertilize.

“An intriguing aspect of this study is that the mechanism for ensuring cross-fertilization is very old, often lost, and never regained,” Kohn said. “That it is still common despite frequent and irreversible loss implies that this trait confers an advantage to species that possess it, perhaps in terms of reduced rates of extinction.”

The study was supported by a grant from the National Science Foundation.

Source: University of California, San Diego

Related stories:

Despite Popularity, Researcher Finds Not Everyone Can Successfully Learn Through Online Courses
Since the 1990s, online courses have provided an opportunity for busy adults to continue their education by completing courses in the comfort of their own homes. However, this may not be the best solution for everyone. A researcher at the University of Missouri has found some students may find success in these types of courses more easily than others.
Researchers uncover new piece to the puzzle of human height
In studies involving more than 35,000 people and a survey across the entire human genome, an international team supported in part by the National Institutes of Health (NIH) has found evidence that common genetic variants recently linked to osteoarthritis may also play a minor role in human height. The findings were released today in the advance online publication of the journal Nature Genetics.
Disease-free mosquito bred to disease-carrier can have all disease-free progeny
A decade ago, scientists announced the ability to introduce foreign genes into the mosquito genome. A year ago, scientists announced the successful use of an artificial gene that prevented a virus from replicating within mosquitoes. But how does one apply what can be done with a small number of mosquitoes in a lab to the tens of millions of mosquitoes that spread disease worldwide?
Ancient Genetic Material Keeps Pups Pint-Sized
Soon after humans began domesticating dogs 12,000 to 15,000 years ago, they started breeding small canines. Now, scientists from the University of Utah and seven other institutions have identified a piece of doggy DNA that reduces the activity of a growth gene, ensuring that small breeds stay small.
Less sugary drinks during childhood may cut disease risk
Symptoms of heart disease and diabetes usually seen in adults are increasingly being found in adolescents according to a longitudinal study, which suggests that reducing the intake of sugar-sweetened beverages during childhood may lessen the risk of chronic disease in later life.
Political scientists examine the largest voting gaps in America
Recent studies of the 2004 election data by political scientists assess the role and impact of major sets of differences in the voting behavior of Americans--known in popular parlance as "voting gaps." Based on differences in support for George W. Bush and ranked in descending order, the studies confirm the largest voting gaps in the electorate were race and ethnicity, religion, class, region, gender, age, and education.
Evolution Study Tightens Human-Chimp Connection
Scientists at the Georgia Institute of Technology have found genetic evidence that seems to support a controversial hypothesis that humans and chimpanzees may be more closely related to each other than chimps are to the other two species of great apes – gorillas and orangutans. They also found that humans evolved at a slower rate than apes.

Healthy people and enhancement drugs
Healthy people are more willing to take drugs to enhance traits that are not fundamental to their identity. According to a new study in the Journal of Consumer Research, people's willingness to take a pill or drug depends on whether the trait the drug promises to enhance is one they consider fundamental.

News discussion:

General Science news