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Public release date: 11-Jan-2009
Contact: Ana Godinho
agodinho@igc.gulbenkian.pt
35-121-440-7959
Instituto Gulbenkian de Ciencia

Reverse evolution in real-time

Instituto Gulbenkian de Ciencia scientists turn back clock on
evolution in fruit fly to provide key insights into basic mechanisms
of evolution In his book, Wonderful World, Stephen Jay Gould writes
about an experiment of ‘replaying life’s tape’, wherein one could go
back in time, let the tape of life play again and see if ‘the
repetition looks at all like the original’. Evolutionary biology tells
us that it wouldn’t look the same – the outcome of evolution is
contingent on everything that came before. Now, scientists at the
Instituto Gulbenkian de Ciência (IGC) in Portugal, New York
University and the University of California Irvine, provide the first
quantitative genetic evidence of why this is so.

In this study, to be published online this week in the journal Nature
Genetics, Henrique Teotónio and his colleagues recreated natural
selection in real-time, in the laboratory (rather than based on
inferences from fossil records or from comparing existing natural
populations) and provide the first quantitative evidence for natural
selection on so-called standing genetic variation – a process long
thought to be operating in natural populations that reproduce sexually
but which, until now, had never been demonstrated.

The researchers used laboratory-grown populations of fruit fly
(Drosophila melanogaster), derived from an original group of flies,
harvested from the wild back in 1975. These ancestral flies were grown
in the laboratory, for two decades, under different environmental
conditions, (such as starvation and longer life-cycles) so that each
population was selected for specific characteristics. Henrique
Teotónio and his colleagues placed these populations back in the
ancestral environment, for 50 generations, to impose reverse evolution
on the flies, and then looked at the genetic changes in certain areas
of chromosome 3 of these flies.

Says Henrique, ‘In 2001 we showed that evolution is reversible in as
far as phenotypes are concerned, but even then, only to a
point. Indeed, not all the characteristics evolved back to the
ancestral state. Furthermore, some characteristics reverse-evolved
rapidly, while others took longer. Reverse evolution seems to stop
when the populations of flies achieve adaptation to the ancestral
environment, which may not coincide with the ancestral state. In this
study, we have shown that underlying these phenomena is the fact that,
at the genetic level, convergence to the ancestral state is on the
order of 50%, that is, on average, only half of the gene frequencies
revert to the ancestral gene frequencies – evolution is contingent
upon history at the genetic level too’.

These findings provide further insights into the basic understanding
of how evolution and diversity are generated and maintained. On the
one hand, it provides evidence for evolution happening through changes
in the distribution of alleles in a population (so-called standing
genetic variation), from generation to generation, rather than the
appearance of mutations, from one generation to the next. On the other
hand, as Henrique notes, ‘It has implications for the definition of
biodiversity: some of the ‘reversed’ flies may be phenotypically
identical to the ancestral flies, but they are genetically
different. How then do we define biodiversity?’

This study was funded by a Fundação para a Ciência e a
Tecnologia grant awarded to Henrique Teotónio, who joined the IGC
in 2003 as a group leader and currently heads the Evolutionary
Genetics group and the in-house PhD Programme in Life Sciences.

Notes for Editors:

The Instituto Gulbenkian de Ciência (IGC) is one of the leading
life science research centres in Portugal. Founded by the Calouste
Gulbenkian Foundation, the IGC´s mission is to carry out biomedical
research and training. The IGC currently acts as a host institution to
international research groups, providing state-of-the-art research
facilities in a stimulating and autonomous environment. The IGC runs
several ambitious graduate training programmes and a dedicated
outreach and public engagement in science programme. More information
is available at

Glossary:

Allele – one member of a pair or series of different forms of a gene.
Phenotype – any observable characteristic of a living organism, such
as shape, size, physical features and behaviour. An organism’s
phenotype is a result of the activities of several of its genes, the
environment or interactions between genes and the environment.