Evolutionary genetics: The evolution of evolution

G Bell*

*Biology Department, McGill University, 1205 ave Docteur Penfield, Montreal,
Quebec, Canada H3A 1B1
Correspondence to: G Bell, e-mail: Graham.bell{at}mcgill.ca

Heredity (2005) 94, 1-2. doi:10.1038/sj.hdy.6800608
Published online 3 November 2004

A recent claim, that the ability of genetic systems to evolve will itself
evolve in a predictable fashion, brings a new challenge to our studies of
evolution. It is commonly accepted that environmental change leads to
adaptation through natural selection, within the constraints set by a
particular genetic system. It is much less straightforward to enquire
whether a genetic system itself can evolve, and, if so, what the
consequences would be. Earl and Deem (2004) have recently claimed just this:
that a fundamental feature of genetic systems - their 'evolvability' - will
itself evolve predictably in response to environmental perturbation.

At a low level of genetic organization, this is a familiar phenomenon whose
mechanism is well understood. In asexual populations, severe environmental
stress creates intense selection that causes an increase of fitness in the
new conditions of growth relative to the ancestor. An indirect effect of
intense selection is the occasional fixation of mutator genes, which usually
encode defective versions of DNA polymerases, and thereby elevate rates of
point mutation by factors of 10-1000. They spread, despite the fact that
almost all the mutations they cause are deleterious, because the mutator
gene is completely linked to all the mutations it causes. Among the large
number of mutations that the mutator causes will occasionally occur a
mutation that confers increased fitness in the new environment; as this
beneficial mutation spreads through the population, it will carry the
mutator with it. Conventional population genetics theory can describe this
process and it has also been demonstrated in the laboratory (eg Sniegowski
et al, 1997).

It is now clear, however, that evolutionary change does not always proceed
smoothly through base substitution at single loci. The mosaic nature of
bacterial genomes, the mobility of plasmids and other genetic elements, the
idiosyncratic nature of mating-type genes in sexual microbes and the genetic
regulation of development in multicellular organisms all show that variation
at higher levels of genetic organization is also important in evolution.
(For a set of papers describing such phenomena, see Genetica, Vol 118,
2003). Such higher-level sources of variation are hard to incorporate in
theoretical evolutionary modelling, which usually relies on defining a
restricted range of genotypes and then finding out which remain after
selection. Individual-based computer simulation is one promising alternative
to the standard approach. In this alternative approach, types or
combinations of types with unexpected properties may appear and spread
during the course of the simulation. Systems of this sort are now being used
to investigate the fundamental features of evolutionary change that are
inaccessible to equation-driven methods (eg Yedid and Bell, 2002; Lenski et
al, 2003).

Full Text at Nature Heredity
http://www.nature.com/cgi-taf/DynaPage.taf?file=/hdy/journal/v94/n1/full/6800608a.html

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