A new type of genetic variation could strengthen natural selection. This article immediately attracted my attention and provides interesting support for a concept which does not get much attention and is even considered controversial.
The study shows that not only are individual genes and whole organisms selected for, but also gene networks. The study looked at the genomes of variations of yeast within the same species and found that gene networks had been preserved even though the genes are inactive in one variant. Balancing selection allows two variants of the same gene to exist in a population and has normally been observed with individual genes; it has now been found to work on gene networks. It is not yet known whether this is a rare case or whether selection on networks is common.
The concept which comes to mind from reading about this study is modular selection, a controversial view about the level of selection. At which level(s) selection acts often attracts heated debates and the two extremes tend to be gene selection and hierarchical selection (where natural selection occurs on many levels). Current views tend to hold that gene selection and individual selection both occur, but which is more dominant is still often the key issue (other levels are getting a resurgence in popularity, such as group selection). As genes are selected for indirectly (they are not interactors) the idea of gene complexes is used to explain how genes can be selected for in groups. The main argument in favour of gene selection is that a gene is reliably transmitted from generation to generation and so selection has time to act on it. Phenotypes are broken down at the genotype level by recombination with each new generation and so it is argued that the phenotype is not selected for (though it should be kept in mind that phenotypic variation from generation to generation is often so small that natural selection will not "ignore" subtle changes).
Modular selection bridges the gap between these two levels. Selection on a gene complex cannot explain why the gene network has been selected for in the yeast example. It would be wrong to label this modular selection for semantic reasons which should not be ignored (modular selection applies to complex multicellular organisms which form in a modular fashion; gene networks cover modular selection but are not exclusive to it).
In Lu et al (2009) modules are described as "tightly integrated complexes of characters with discrete, semi-independent and temporally persistent histories" and that these "were the principle focus of natural selection and played a leading role in evolutionary transitions". This places modular selection midway between gene selection and selection on the individual. Modules are both interactors (unlike genes) and have potential phylogenetic longevity (more so than phenotype) which circumvents the issues with each other type of selection. Schlosser (2002) states that "modules tend to be more important in delimiting actual units of selection than either organisms or genes, because they are less easily disrupted by recombination than organisms, while having less context sensitive fitness values than genes".
West-Eberhart (2003) described modular selection as a subunit of evolution. She states "[the] notion of the phenotype as a nested hierarchy of modular subunits implies both semi-independence and connectedness among subunits." This is what is found in this study using yeast, supporting the idea that modular selection is not only valid but to be found in more than just pterosaurs and other vertebrates.
This to me is exciting news and will influence our understanding of how natural selection works, particularly on which level. Sadly PhysOrg.com do not provide a link to the papers they discuss or even a reference, so I struggled to track down the paper.
Hittinger, C.T. Goncalves, P. Sampaio, J.P. Dover, J. Johnston, M. and Rokas, A. 2010. Remarkably ancient balanced polymorphisms in a multi-locus gene network. Nature. 464, 54-58.
Lu, J. Unwin, D.M. Jin, X. Liu, Y. and Ji, Q. 2009. Evidence for modular evolution in a long-tailed pterosaur with a pterodactyloid skull. Proceedings of the Royal Society B.
Schlosser, G. 2002. Modularity and the units of evolution. Theory in Biosciences. 121: 1-80.
West-Eberhard, M. J. 2003. Developmental Plasticity and Evolution. Oxford University Press, Inc. New York, pp. 56.