Iconic Southern Hemisphere trees 13-113 million years old

A new study of southern beeches shows that estimates of their evolutionary timescale can vary drastically, depending on the way we calculate the rate of evolution.

Unfortunately, not every species that once lived has left a trace in the fossil record. This makes it difficult to estimate evolutionary timescales. But this information can often be important – for instance, we might be interested in whether ancient bursts of speciation were caused by changes in climate.

This is why biologists are increasingly using a combination of genetic and fossil information to find out when species diverged in the past. This can be done by studying the DNA of living species and using the fossil record to estimate how quickly the DNA has changed over time.

The latest study is the fruit of an international collaboration among 14 researchers led by Dr Hervé Sauquet from Université Paris-Sud, France, and Dr Simon Ho from the School of Biological Sciences at the University of Sydney.

Dr Sauquet points out that “For most groups of plants and many groups of animals, fossils are very scarce if known at all. For instance, only three fossils are known for orchids, one of the largest families of flowering plants with more than 22,000 living species”.

The team focused on the southern beeches (Nothofagus), an iconic plant group from the Southern Hemisphere. These trees make up most of the canopy of rainforests in Chile, Argentina, southeastern Australia, New Zealand, and New Guinea.

“We compiled a comprehensive list of 38 fossils known for Nothofagus and its closest relatives (beeches, oaks, and birches), which are exceptional in having more fossils than any other plant group”, said Dr Sauquet.

The researchers also collected DNA sequences for most of the 35 living species of Nothofagus and 21 of their relatives.

Dr Ho explained that the team “tried many different ways to combine these fossils with the DNA data and were surprised by how much our results differed from one experiment to another.”

The age of the group containing all living species of Nothofagus could be as young as 13 million years or as old as 113 million years.

Dr Sauquet points out that “this is a very significant result because thousands of published studies are potentially affected. It clearly shows that we should worry a lot more about how we use fossils when we combine them with DNA. This is still a new field of research so studies that improve our methodology are welcome.”

Read the scientific publication here:
Testing the impact of calibration on molecular divergence times using a fossil-rich group: The case of Nothofagus (Fagales)
Sauquet, Ho, Gandolfo, Jordan, Wilf, et al. (2012) Systematic Biology, 61: 289–313