SU zoologist helps compile comprehensive family tree for mammals

A Stellenbosch evolutionary geneticist has added his knowledge of Africa’s most ancient mammals to an international study in which the most comprehensive family tree for mammals yet has been compiled. The study, which shows just how different families of mammals relate to one another, was published in the leading academic journal Science.

Prof Terry Robinson

Prof Terry Robinson of the Department of Botany and Zoology at Stellenbosch University (SU) was part of a team of biologists lead by the University of California Riverside (UC Riverside) and Texas A&M. Prof Robinson is an A-rated scientist who is acknowledged as the world expert on the most primitive of African mammals, the Afrotheria.

Afrotheria contains six orders of mammals of probable African origin that lie at the root of the eutherian or placental mammal evolutionary tree.

“The study greatly improves the tree of life for mammals as we know it, and provides the evolutionary backbone to understand the history of the unique changes in the genome that underpin the impressive morphological diversity observed in living species of mammals,” says Prof Robinson.

He says that although his contribution was “small in comparison to the size of the rest of the study”, he was privileged to be part of the five-year long project.

It is the first time that such a large and robust DNA matrix representing all mammalian families has been drawn up. The matrix has representatives for 99% of mammalian families, and covers not only the earliest history of mammalian diversification but also all the deepest divergences among living mammals.

“Until now, no one has been able to assemble this kind of matrix, based on DNA sequences from many different genes, to examine how the different families of mammals are related to each other,” said Prof Mark Springer, a professor of biology UC Riverside, who co-led the research project with Prof William Murphy, an associate professor of genetics at Texas A&M.

The study includes 164 of the 5400 living mammals. 

Prof Springer says that the dataset provides a large and reliable springboard for biologists to progress further. “We can now progress from phylogeny that has representatives for all the different mammalian families to phylogenies that have representatives for genera and species,” he says.

Phylogeny is the history of organismal lineages as they change through time. A vast evolutionary tree, called the Tree of Life, represents the phylogeny of organisms, the genealogical relationships of all living things.

To date divergence times on their phylogeny of mammalian families, Prof Springer and colleagues used a “relaxed molecular clock” that allows for the use of multiple rates of evolution instead of using one rate of evolution that governs all branches of the Tree of Life. They also used age estimates for numerous fossil mammals to calibrate their time tree.

“We need to have calibrations to input into the analysis so that we know, for example, that elephants and their nearest relatives have been separate from each other since at least the end of the Paleocene – more than 55 million years ago,” Prof Springer said. “We were able to put together a diverse assemblage of fossil calibrations from different parts of the mammalian tree, and we used it in conjunction with molecular information to assemble the most robust time tree based on sequenced data that has been developed to date.”

The study is the beginning of a larger plan to use large molecular data sets and sophisticated techniques for dating and estimating rates of diversification to resolve much larger portions of the mammalian tree. It will ultimately include all described species, as well as those that have gone recently extinct or for which only museum material may be available.

“Only then can we really begin to understand the role of the environment and events in earth history in promoting the generation of living biodiversity,” says Prof Murphy.

Prof Springer explained that the research team looked for spikes in the diversification history of mammals and used an algorithm to determine whether the rate of diversification was constant over time or whether there were distinct pulses of rate increases or decreases. The researchers found an increase in the diversification rate 80-82 million years ago, which corresponds to the time – specifically, the end of the Cretaceous Terrestrial Revolution – when a lot of different orders were splitting from each other.

“This is when flowering plants diversified, which provided opportunities for the diversification of small mammals,” Springer said.

Springer and colleagues also detected a second spike in the diversification history of mammals at the end of the Cretaceous – 65.5 million years ago, when dinosaurs, other large terrestrial vertebrates, and many marine organisms went extinct, opening up a vast ecological space.

“Such ecological voids can get filled quickly,” Springer explained. “We see that in mammals, even though different orders such as primates and rodents split from each other back in the Cretaceous, the orders did not diversify into their modern representations until after the Cretaceous, 65.5 million years ago. The void seems to have facilitated the radiation – that is, branching in conjunction with change – of different orders of several mammals into the adaptive zones they occupy today. After the Cretaceous, we see increased diversification, with some lineages becoming larger and more specialized.”

The researchers stress that their time tree is a work in progress. In the next two years, they expect to construct a supermatrix, also based on gene sequences, and include the majority of living mammalian species.

(Article based on press release, courtesy of the University of California Riverside. For the full release, visit http://newsroom.ucr.edu/2729).

Background information on Prof Terry Robinson:

Prof Terry Robinson holds an A2 rating from the South African National Research Foundation (NRF) which recognises him as a world leader in his field.

Prof Robinson works in the fields of evolutionary genetics and aspects of chromosome biology including molecular cytogenetics, systematics and phylogenomics. He uses technical advances in contemporary flow cytometry and data from the human and other sequenced genomes in his work.

His research group in the SU Department of Botany and Zoology explores question about the early evolution and relationship of Afrotheria, ancestral features of their genomic architecture, and the phylogenetic interpretation of chromosomal characters.

They are searching for clues about the common ancestor of mammals such the aardvark, elephant, golden mole and elephant shrew, and want to find out just how the genetic material contained within these animals have been reshuffled in the evolutionary past, how species evolved, and how these animals are related to one another.

For more information:
* Stellenbosch University News blog http://blogs.sun.ac.za/news/2009/12/15/evolutionary-geneticist-recognised-as-world-leader/
* Prof Terry Robinson’s Research Page http://academic.sun.ac.za/botzoo/robinson/index.htm