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The nature of the forces affecting base composition is a key question in genome evolution. There is uncertainty as to whether differences in the GC contents of non-coding sequences reflect differences in mutational bias, or in the intensity of selection or biased gene conversion. We have used a polymorphism dataset for non-coding sequences on the X chromosome ofDrosophila simulansto examine this question. The proportion of GC→AT versus AT→GC polymorphic mutations in a locus is correlated with its GC content. This implies the action of forces that favour GC over AT base pairs, which are apparently strongest in GC-rich sequences.

Gordon Lowe made major contributions to the understanding of the mechanisms of enzyme action, pioneering the use in particular of novel isotopically chiral phosphate and sulphate esters. In the latter part of his scientific work he discovered new peptide nucleic acids, probed the interaction of platinum complexes with enzymes, leading to potential new antimalarial drugs, and discovered, by combinatorial methods, effective active-site enzyme inhibitors.

Y chromosomes are genetically degenerate, having lost most of the active genes that were present in their ancestors. The causes of this degeneration have attracted much attention from evolutionary theorists. Four major theories are reviewed here: Muller's ratchet, background selection, the Hill–Robertson effect with weak selection, and the ‘hitchhiking’ of deleterious alleles by favourable mutations. All of these involve a reduction in effective population size as a result of selective events occurring in a non–recombining genome, and the consequent weakening of the efficacy of selection. We review the consequences of these processes for patterns of molecular evolution and variation at loci on Y chromosomes, and discuss the results of empirical studies of these patterns for some evolving Y–chromosome and neo–Y–chromosome systems. These results suggest that the effective population sizes of evolving Y or neo–Y chromosomes are severely reduced, as expected if some or all of the hypothesized processes leading to degeneration are operative. It is, however, currently unclear which of the various processes is most important; some directions for future work to help to resolve this question are discussed.

If selection maintains polymorphism at a site, an increase of neutral variability in the surrounding region is expected, because sites closely linked to alternative alleles at the selected site accumulate mutations as if they were in different populations. Selection thus creates a signal of increased polymorphism relative to the background `noise9 of neutral polymorphism. We show, by a theoretical study, that the effects of balanced polymorphisms on variability are highly localized in inbreeding populations, and that the overall level of variability is strongly reduced by selection against deleterious mutations. This implies that the signals of balancing selection are much stronger than in outcrossing populations.

The evolution of lethal haplotypes in the t-haplotype segregation distortion system of Mus is examined by mathematical and computer models. The models assume that there is reproductive compensation for the loss of lethal embryos, such that the net reproductive success of a females is not reduced in proportion to the frequency of lethal offspring which she produces. The initial population consists of a mixture of wild-type and homozygous male-sterile t-haplotypes. The failure of sterile males to reproduce may cause a higher fitness cost to mothers heterozygous for t-haplotypes than does elimination of a recessive lethal. Under certain conditions, a recessive lethal will spread and come to a polymorphic equilibrium. Wild-type, lethal and non-lethal haplotypes are all present at this equilibrium. If a second lethal mutation arises on a non-lethal t-haplotype in such an equilibrium population, it will increase in frequency and eventually displace the non-lethal t-haplotypes. A third lethal t-haplotype introduced at a low frequency into an equilibrium with two lethals can sometimes be selected for, although this is less likely if compensation is strong. The theoretical predictions are compared with data on natural populations.

We have used a polymorphism dataset on introns and coding sequences of X-linked loci inDrosophila americanato estimate the strength of selection on codon usage and/or biased gene conversion (BGC), taking into account a recent population expansion detected by a maximum-likelihood method.Drosophila americanawas previously thought to have a stable demographic history, so that this evidence for a recent population expansion means that previous estimates of selection need revision. There was evidence for natural selection or BGC favouring GC over AT variants in introns, which is stronger for GC-rich than GC-poor introns. By comparing introns and coding sequences, we found evidence for selection on codon usage bias, which is much stronger than the forces acting on GC versus AT basepairs in introns.

Many long-lived avian species adopt life strategies that involve a gregarious way of life at juvenile and sub-adult stages and territoriality during adulthood. However, the potential associated costs of these life styles, such as stress, are poorly understood. We examined the effects of group living, sex and parasite load on the baseline concentration of faecal stress hormone (corticosterone) metabolites in a wild population of common ravens ( Corvus corax ). Corticosterone concentrations were significantly higher in non-breeding gregarious ravens than in territorial adults. Among territorial birds, males showed higher stress levels than their mates. Parasite burdens did not affect hormone levels. Our results suggest a key role of the social context in the stress profiles of the two population fractions, and that group living may be more energetically demanding than maintaining a territory. These findings have implications for understanding hormonal mechanisms under different life styles and may inspire further research on the link between hormone levels and selective pressures modulating gregarious and territorial strategies in long-lived birds.

John Maynard Smith is one of the most influential evolutionary biologists of our time. This year he celebrates his 80th birthday, and it seemed appropriate to mark this occasion with a collection of papers by his former associates, and distinguished evolutionists whose work has been influenced by his example. John is noted for his breadth of interests and his ability to spot important but soluble problems, which he continues to attack by deceptively simple methods. He has published 13 books and collections of papers on an astonishing diversity of subjects. He has also published many semi–popular articles on biology, which are remarkable for their clarity and diversity. He is a widely sought–after lecturer, who is famous for his humorous and lucid presentations.

Population genetic studies of the evolution of breeding systems in flowering plants are reviewed. The selective advantage of a gene’s increasing the selfing rate is stressed. In the evolution of outbreeding mechanisms, some strong disadvantage to selfing must therefore be acting; it is suggested that this disadvantage is inbreeding depression. Populations with no absolute barrier to selfing, and with intermediate levels of self-fertilization, appear to be the most likely starting state for the evolution of outbreeding mechanisms. There is some evidence for inbreeding depression in such populations. The evolution of distyly and dioecy are considered in some detail. An explanation for the existence of supergenes controlling these systems is proposed. The breakdown of distyly and tristyly are also considered. The evolution of recombination rates in selfing and outcrossing species is examined briefly.

This paper reviews models of two aspects of genome evolution. The first is the problem of the conditions for establishment of chromosome rearrangements, as a result of their suppression of recombination between polymorphic genes that interact in their effects on fitness. The second is the spread of genomic elements either by their differential contributions to gametes, or by their ability to replicate and transpose themselves to new sites within the genome.