Testing the neutral theory of molecular evolution with genomic data from Drosophila

Positive selection at sites of multiple amino acid replacements since rat-mouse divergence

New alleles become fixed owing to random drift of nearly neutral mutations or to positive selection of substantially advantageous mutations. After decades of debate, the fraction of fixations driven by selection remains uncertain. Within 9,390 genes, we analysed 28,196 codons at which rat and mouse differ from each other at two nucleotide sites and 1,982 codons with three differences. At codons where rat-mouse divergence involved two non-synonymous substitutions, both of them occurred in the same lineage, either rat or mouse, in 64% of cases; however, independent substitutions would occur in the same lineage with a probability of only 50%. All three non-synonymous substitutions occurred in the same lineage for 46% of codons, instead of the 25% expected. Furthermore, comparison of 12 pairs of prokaryotic genomes also shows clumping of multiple non-synonymous substitutions in the same lineage. This pattern cannot be explained by correlated mutation or episodes of relaxed negative selection, but instead indicates that positive selection acts at many sites of rapid, successive amino acid replacement.

Purifying selection and demographic expansion affect sequence diversity of the ligand-binding domain of a glutamate-gated chloride channel gene of Haemonchus placei

Ninety-five genomic sequences of the ligand-binding domain of glutamate-gated chloride channel genes of three populations of the parasitic nematode H. placei were evaluated for patterns of diversity, demography, and selection. These genes code for subunits of ion channels, which are involved in the mode of action of the most commonly used antiparasitic drugs, the macrocyclic lactones. An extremely high frequency of unique segregating sites in exons and introns was observed, with significantly negative neutrality tests in each population for noncoding, synonymous, and nonsynonymous sites. Several tests indicated that support for balancing selection, positive selection, and hitchhiking was lacking. McDonald-Kreitman tests using H. contortus or C. elegans as an outgroup revealed an extreme excess of replacement polymorphism, consistent with weak purifying selection. Although these tests agree that negative selection may explain the excess of replacement changes, an alternative interpretation is required for the significantly negative Fu and Li's D statistics based on silent and noncoding sites. These include homogeneous forces such as background selection and demographic expansion. The lack of population subdivision and the negative values of Tajima's D for this outbreeding parasitic nematode render background selection less likely than demographic expansion. Comparison of D statistics based on different site types using neutral coalescent simulations supported this interpretation. Although this statistic was more negative for nonsynonymous sites than for synonymous sites, most comparisons of the D statistic were not significantly different between mutation classes. A few significant site comparisons were also consistent with demographic expansion, because the observed test statistic ( D(neutral) - D(selected)) were low relative to the neutral expectations. Finally, previous mitochondrial studies also identified a demographic expansion of this parasitic nematode species, which lends further support to a scenario involving both demographic and purifying forces in the ligand-binding domain of H. placei.

The genomic rate of adaptive amino acid substitution in Drosophila

The proportion of amino acid substitutions driven by adaptive evolution can potentially be estimated from polymorphism and divergence data by an extension of the McDonald-Kreitman test. We have developed a maximum-likelihood method to do this and have applied our method to several data sets from three Drosophila species: D. melanogaster, D. simulans, and D. yakuba. The estimated number of adaptive substitutions per codon is not uniformly distributed among genes, but follows a leptokurtic distribution. However, the proportion of amino acid substitutions fixed by adaptive evolution seems to be remarkably constant across the genome (i.e., the proportion of amino acid substitutions that are adaptive appears to be the same in fast-evolving and slow-evolving genes; fast-evolving genes have higher numbers of both adaptive and neutral substitutions). Our estimates do not seem to be significantly biased by selection on synonymous codon use or by the assumption of independence among sites. Nevertheless, an accurate estimate is hampered by the existence of slightly deleterious mutations and variations in effective population size. The analysis of several Drosophila data sets suggests that approximately 25% +/- 20% of amino acid substitutions were driven by positive selection in the divergence between D. simulans and D. yakuba.

Common pattern of evolution of gene expression level and protein sequence in Drosophila

Sequence divergence scaled by variation within species has been used to infer the action of selection upon individual genes. Applying this approach to expression, we compared whole-genome whole-body RNA levels in 10 heterozygous Drosophila simulans genotypes and a pooled sample of 10 D. melanogaster lines using Affymetrix Genechip. For 972 genes expressed in D. melanogaster, the transcript level was below detection threshold in D. simulans, which may be explained either by sequence divergence between the primers on the chip and the mRNA transcripts or by down-regulation of these genes. Out of 6,707 genes that were expressed in both species, transcript level was significantly different between species for 534 genes (at P < 0.001). Genes whose expression is under stabilizing selection should exhibit reduced genetic variation within species and reduced divergence between species. Expression of genes under directional selection in D. simulans should be highly divergent from D. melanogaster, while showing low genetic variation in D. simulans. Finally, the genes with large variation within species but modest divergence between species are candidates for balancing selection. Rapidly diverging, low-polymorphism genes included those involved in reproduction (e.g., Mst 3Ba, 98Cb; Acps 26Aa, 63F; and sperm-specific dynein). Genes with high variation in transcript abundance within species included metallothionein and hairless, both hypothesized to be segregating in nature because of gene-by-environment interactions. Further, we compared expression divergence and DNA substitution rate in 195 genes. Synonymous substitution rate and expression divergences were uncorrelated, whereas there was a significant positive correlation between nonsynonymous substitution rate and expression divergence. We hypothesize that as a substantial fraction of nonsynonymous divergence has been shown to be adaptive, much of the observed expression divergence is likewise adaptive.

The modern theory of biological evolution: an expanded synthesis

In 1858, two naturalists, Charles Darwin and Alfred Russel Wallace, independently proposed natural selection as the basic mechanism responsible for the origin of new phenotypic variants and, ultimately, new species. A large body of evidence for this hypothesis was published in Darwin's Origin of Species one year later, the appearance of which provoked other leading scientists like August Weismann to adopt and amplify Darwin's perspective. Weismann's neo-Darwinian theory of evolution was further elaborated, most notably in a series of books by Theodosius Dobzhansky, Ernst Mayr, Julian Huxley and others. In this article we first summarize the history of life on Earth and provide recent evidence demonstrating that Darwin's dilemma (the apparent missing Precambrian record of life) has been resolved. Next, the historical development and structure of the "modern synthesis" is described within the context of the following topics: paleobiology and rates of evolution, mass extinctions and species selection, macroevolution and punctuated equilibrium, sexual reproduction and recombination, sexual selection and altruism, endosymbiosis and eukaryotic cell evolution, evolutionary developmental biology, phenotypic plasticity, epigenetic inheritance and molecular evolution, experimental bacterial evolution, and computer simulations (in silico evolution of digital organisms). In addition, we discuss the expansion of the modern synthesis, embracing all branches of scientific disciplines. It is concluded that the basic tenets of the synthetic theory have survived, but in modified form. These sub-theories require continued elaboration, particularly in light of molecular biology, to answer open-ended questions concerning the mechanisms of evolution in all five kingdoms of life.

The Evolution of SMC Proteins: Phylogenetic Analysis and Structural Implications

The SMC proteins are found in nearly all living organisms examined, where they play crucial roles in mitotic chromosome dynamics, regulation of gene expression, and DNA repair. We have explored the phylogenetic relationships of SMC proteins from prokaryotes and eukaryotes, as well as their relationship to similar ABC ATPases, using maximum-likelihood analyses. We have also investigated the coevolution of different domains of eukaryotic SMC proteins and attempted to account for the evolutionary patterns we have observed in terms of available structural data. Based on our analyses, we propose that each of the six eukaryotic SMC subfamilies originated through a series of ancient gene duplication events, with the condensins evolving more rapidly than the cohesins. In addition, we show that the SMC5 and SMC6 subfamily members have evolved comparatively rapidly and suggest that these proteins may perform redundant functions in higher eukaryotes. Finally, we propose a possible structure for the SMC5/SMC6 heterodimer based on patterns of coevolution.

Inference of Positive and Negative Selection on the 5′ Regulatory Regions of Drosophila Genes

Both positive selection and negative selection have been shown to drive the evolution of coding regions. It is of interest to know if the corresponding 5' regions of genes may be subjected to selection of comparable intensities. For such a comparison, we chose the Accessory gland protein (Acp) genes as our test case. About 700 bp and 600 bp for the 5' and coding regions, respectively, of eight previously unstudied genes were sequenced from 21 isogenic lines of D. melanogaster and one line from D. simulans. The ratio of divergence at the amino-acid replacement sites (A) over that at the synonymous sites (S) was twice the ratio for common polymorphism. Interestingly, the 5' region shows the same trend, with the 5'/S divergence ratio being 1.8 times higher than the 5'/S ratio for common polymorphism. There are several possible explanations for the 5'/S ratios, including demography, negative selection, and positive selection. Under normal conditions, positive selection is the most likely explanation. If that is true, about 45 to 50 percent of all fixed differences at both the replacement and 5' sites were adaptive, even though the substitution rate in the former is only half that of the latter (K(A)/K(S) approximately 0.3 vs. K(5')/K(S) approximately 0.6). As previous analyses have indicated, the inclusion of slightly deleterious polymorphism confounds the inference of positive selection. The analysis of published polymorphism data covering 97 verified 5' regions of Drosophila suggests more pronounced selective constraint on the 5' untranslated region and the core promoter (together corresponding to approximately 200 bp in this data set) when compared to the more distal portion of the 5' region of genes.

Adaptive evolution of ASPM, a major determinant of cerebral cortical size in humans

A prominent trend in the evolution of humans is the progressive enlargement of the cerebral cortex. The ASPM (Abnormal spindle-like microcephaly associated) gene has the potential to play a role in this evolutionary process, because mutations in this gene cause severe reductions in the cerebral cortical size of affected humans. Here, we show that the evolution of ASPM is significantly accelerated in great apes, especially along the ape lineages leading to humans. Additionally, the lineage from the last human/chimpanzee ancestor to humans shows an excess of non-synonymous over synonymous substitutions, which is a signature of positive Darwinian selection. A comparison of polymorphism and divergence using the McDonald-Kreitman test confirms that ASPM has indeed experienced intense positive selection during recent human evolution. This test also reveals that, on average, ASPM fixed one advantageous amino acid change in every 300,000-400,000 years since the human lineage diverged from chimpanzees some 5-6 million years ago. We therefore conclude that ASPM underwent strong adaptive evolution in the descent of Homo sapiens, which is consistent with its putative role in the evolutionary enlargement of the human brain.

Perspective: detecting adaptive molecular polymorphism: lessons from the MHC

In the 1960s, when population geneticists first began to collect data on the amount of genetic variation in natural populations, balancing selection was invoked as a possible explanation for how such high levels of molecular variation are maintained. However, the predictions of the neutral theory of molecular evolution have since become the standard by which cases of balancing selection may be inferred. Here we review the evidence for balancing selection acting on the major histocompatibility complex (MHC) of vertebrates, a genetic system that defies many of the predictions of neutrality. We apply many widely used tests of neutrality to MHC data as a benchmark for assessing the power of these tests. These tests can be categorized as detecting selection in the current generation, over the history of populations, or over the histories of species. We find that selection is not detectable in MHC datasets in every generation, population, or every evolutionary lineage. This suggests either that selection on the MHC is heterogeneous or that many of the current neutrality tests lack sufficient power to detect the selection consistently. Additionally, we identify a potential inference problem associated with several tests of neutrality. We demonstrate that the signals of selection may be generated in a relatively short period of microevolutionary time, yet these signals may take exceptionally long periods of time to be erased in the absence of selection. This is especially true for the neutrality test based on the ratio of nonsynonymous to synonymous substitutions. Inference of the nature of the selection events that create such signals should be approached with caution. However, a combination of tests on different time scales may overcome such problems.

Structural diversity and functional implications of the eukaryotic TDP gene family

TDP-43 is an RNA-binding protein that functions in mammalian cells in transcriptional repression and exon skipping. The gene encoding TDP-43 (HGMW-approved gene symbol TARDBP) is conserved in human, mouse, Drosophila melanogaster, and Caenorhabditis elegans. Sequence comparison of the coding regions of the TDP genes among the four taxa reveals an extraordinarily low rate of sequence divergence, suggesting that the TDP genes carry out essential functions in these organisms. With DNA transfection assay, we have established the importance of the glycine-rich domain for the exon-skipping activity of TDP-43. Both human and mouse TDP genes belong to a gene family that also consists of a number of processed pseudogenes. Interestingly, combined database analysis and cDNA cloning have demonstrated that the primary transcript of the mammalian TDP genes undergoes alternative splicing to generate 11 mRNAs, including the one encoding TDP-43. Eight of the 11 splicing events involved the use of four each of the 5'-donor and 3'-acceptor sites, all of which reside within the last exon of the TDP-43 mRNA. The existence of multispliced isoforms of TDP-encoded proteins provides further support for the functional complexity of the eukaryotic TDP genes.

Sequence divergence, functional constraint, and selection in protein evolution

The genome sequences of multiple species has enabled functional inferences from comparative genomics. A primary objective is to infer biological functions from the conservation of homologous DNA sequences between species. A second, more difficult, objective is to understand what functional DNA sequences have changed over time and are responsible for species' phenotypic differences. The neutral theory of molecular evolution provides a theoretical framework in which both objectives can be explicitly tested. Development of statistical tests within this framework has provided insight into the evolutionary forces that constrain and in some cases change DNA sequences and the resulting patterns that emerge. In this article, we review recent work on how functional constraint and changes in protein function are inferred from protein polymorphism and divergence data. We relate these studies to our understanding of the neutral theory and adaptive evolution.

Adaptive evolution of genes and gene families

The huge influx of genomic sequence and new statistical methods is making the discovery of genes subjected to adaptive evolution increasingly common. The use of comparative genomics to identify adaptive evolution is resulting in predictions of functionally important genes and gene regions. However, the selective pressure driving the adaptive evolution of most genes remains mysterious.

Evolution of the Human ASPM Gene, a Major Determinant of Brain Size

The size of human brain tripled over a period of ∼2 million years (MY) that ended 0.2–0.4 MY ago. This evolutionary expansion is believed to be important to the emergence of human language and other high-order cognitive functions, yet its genetic basis remains unknown. An evolutionary analysis of genes controlling brain development may shed light on it. ASPM (abnormal spindle-like microcephaly associated) is one of such genes, as nonsense mutations lead to primary microcephaly, a human disease characterized by a 70% reduction in brain size. Here I provide evidence suggesting that human ASPM went through an episode of accelerated sequence evolution by positive Darwinian selection after the split of humans and chimpanzees but before the separation of modern non-Africans from Africans. Because positive selection acts on a gene only when the gene function is altered and the organismal fitness is increased, my results suggest that adaptive functional modifications occurred in human ASPM and that it may be a major genetic component underlying the evolution of the human brain.

The power and promise of population genomics: from genotyping to genome typing

Population genomics has the potential to improve studies of evolutionary genetics, molecular ecology and conservation biology, by facilitating the identification of adaptive molecular variation and by improving the estimation of important parameters such as population size, migration rates and phylogenetic relationships. There has been much excitement in the recent literature about the identification of adaptive molecular variation using the population-genomic approach. However, the most useful contribution of the genomics model to population genetics will be improving inferences about population demography and evolutionary history.

A Microsatellite Variability Screen for Positive Selection Associated With the “Out of Africa” Habitat Expansion of Drosophila melanogaster

We report a “hitchhiking mapping” study in D. melanogaster, which searches for genomic regions with reduced variability. The study's aim was to identify selective sweeps associated with the “out of Africa” habitat expansion. We scanned 103 microsatellites on chromosome 3 and 102 microsatellites on the X chromosome for reduced variability in non-African populations. When the chromosomes were analyzed separately, the number of loci with a significant reduction in variability only slightly exceeded the expectation under neutrality—six loci on the third chromosome and four loci on the X chromosome. However, non-African populations also have a more pronounced average loss in variability on the X chromosomes as compared to the third chromosome, which suggests the action of selection. Therefore, comparing the X chromosome to the autosome yields a higher number of significantly reduced loci. However, a more pronounced loss of variability on the X chromosome may be caused by demographic events rather than by natural selection. We therefore explored a range of demographic scenarios and found that some of these captured most, but not all aspects of our data. More theoretical work is needed to evaluate how demographic events might differentially affect X chromosomes and autosomes and to estimate the most likely scenario associated with the out of Africa expansion of D. melanogaster.

Estimating the distribution of fitness effects from DNA sequence data: implications for the molecular clock

We present a method for estimating the distribution of fitness effects of new amino acid mutations when those mutations can be assumed to be slightly advantageous, slightly deleterious, or strongly deleterious. We apply the method to mitochondrial data from several different species. In the majority of the data sets, the shape of the distribution is approximately exponential. Our results provide an estimate of the distribution of fitness effects of weakly selected mutations and provide a possible explanation for why the molecular clock is fairly constant across taxa and time.

Asymmetric sequence divergence of duplicate genes

Much like humans, gene duplicates may be created equal, but they do not stay that way for long. For four completely sequenced genomes we show that 20%-30% of duplicate gene pairs show asymmetric evolution in the amino acid sequence of their protein products. That is, one of the duplicates evolves much faster than the other. The greater this asymmetry, the greater the ratio Ka/Ks of amino acid substitutions (Ka) to silent substitutions (Ks) in a gene pair. This indicates that most asymmetric divergence may be caused by relaxed selective constraints on one of the duplicates. However, we also find some candidate duplicates where positive (directional) selection of beneficial mutations (Ka/Ks > 1) may play a role in asymmetric divergence. Our analysis rests on a codon-based model of molecular evolution that allows a test for asymmetric divergence in Ka. The method is also more sensitive in detecting positive selection (Ka/Ks > 1) than models relying only on pairwise gene comparisons.

Natural selection drives Drosophila immune system evolution

Evidence from disparate sources suggests that natural selection may often play a role in the evolution of host immune system proteins. However, there have been few attempts to make general population genetic inferences on the basis of analysis of several immune-system-related genes from a single species. Here we present DNA polymorphism and divergence data from 34 genes thought to function in the innate immune system of Drosophila simulans and compare these data to those from 28 nonimmunity genes sequenced from the same lines. Several statistics, including average K(A)/K(S) ratio, average silent heterozygosity, and average haplotype diversity, significantly differ between the immunity and nonimmunity genes, suggesting an important role for directional selection in immune system protein evolution. In contrast to data from mammalian immunoglobulins and other proteins, we find no strong evidence for the selective maintenance of protein diversity in Drosophila immune system proteins. This may be a consequence of Drosophila's generalized innate immune response.

Evolutionary and ecological functional genomics

A unique combination of disciplines is emerging--evolutionary and ecological functional genomics--which focuses on the genes that affect ecological success and evolutionary fitness in natural environments and populations. Already this approach has provided new insights that were not available from its disciplinary components in isolation. However, future advances will necessitate the re-engineering of scientific attitudes, training and institutions, to achieve extensive multidisciplinarity.

Effects of inbreeding on the genetic diversity of populations

The study of variability within species is important to all biologists who use genetic markers. Since the discovery of molecular variability among normal individuals, data have been collected from a wide range of organisms, and it is important to understand the major factors affecting diversity levels and patterns. Comparisons of inbreeding and outcrossing populations can contribute to this understanding, and therefore studying plant populations is important, because related species often have different breeding systems. DNA sequence data are now starting to become available from suitable plant and animal populations, to measure and compare variability levels and test predictions.

Implications of natural selection in shaping 99.4% nonsynonymous DNA identity between humans and chimpanzees: enlarging genus Homo

What do functionally important DNA sites, those scrutinized and shaped by natural selection, tell us about the place of humans in evolution? Here we compare approximately 90 kb of coding DNA nucleotide sequence from 97 human genes to their sequenced chimpanzee counterparts and to available sequenced gorilla, orangutan, and Old World monkey counterparts, and, on a more limited basis, to mouse. The nonsynonymous changes (functionally important), like synonymous changes (functionally much less important), show chimpanzees and humans to be most closely related, sharing 99.4% identity at nonsynonymous sites and 98.4% at synonymous sites. On a time scale, the coding DNA divergencies separate the human-chimpanzee clade from the gorilla clade at between 6 and 7 million years ago and place the most recent common ancestor of humans and chimpanzees at between 5 and 6 million years ago. The evolutionary rate of coding DNA in the catarrhine clade (Old World monkey and ape, including human) is much slower than in the lineage to mouse. Among the genes examined, 30 show evidence of positive selection during descent of catarrhines. Nonsynonymous substitutions by themselves, in this subset of positively selected genes, group humans and chimpanzees closest to each other and have chimpanzees diverge about as much from the common human-chimpanzee ancestor as humans do. This functional DNA evidence supports two previously offered taxonomic proposals: family Hominidae should include all extant apes; and genus Homo should include three extant species and two subgenera, Homo (Homo) sapiens (humankind), Homo (Pan) troglodytes (common chimpanzee), and Homo (Pan) paniscus (bonobo chimpanzee).

Population genetics of tandem trypsin inhibitor genes in Arabidopsis species with contrasting ecology and life history

Duplicated genes are important in the evolution and ecology of plant-defences because herbivore and pathogen attack can be countered via functional diversification at two levels: among duplicated loci and within loci. We explore molecular sequence variation for two members of a defence-related gene family, Arabidopsis thaliana trypsin inhibitors (ATTI), in A. thaliana and a closely related species, A. lyrata subspp. petraea. A worldwide sample of the inbreeding annual A. thaliana had less genetic variation at two ATTI loci (piTOTAL <or= 0.0006) than observed previously at other functional loci. A significant excess of high frequency derived alleles in the signal sequence and 5' UTR of ATTI2 was consistent with a model of positive selection. However, demographic processes such as population subdivision and expansion, both likely to have occurred in A. thaliana during the last 10 000 years, can also give rise to similar deviations from neutrality. A single population of A. lyrata subspp. petraea in Germany had up to an order of magnitude more standing genetic variation at ATTI loci than the species-wide sample of A. thaliana. Although the level of variability for ATTI1 and ATTI2 within this single population was similar to, or even greater than, observed species-wide diversity for other loci in A. lyrata, there was little evidence to reject an equilibrium neutral model. A spatially explicit sample of 87 A. lyrata subspp. petraea individuals detected outbreeding (FIS = -0.16; FIT = -0.15) but little population subdivision (FST = 0.006) in this self-incompatible perennial herb. Genetic differences between Arabidopsis species were consistent with, but not fully explained by, divergence in ecology and life history. Diversification appears to have occurred in different functional domains for the tandemly duplicated ATTI1 and ATTI2 genes; the majority of fixed replacements in ATTI1 surround the enzyme binding site of the mature protein, whereas in ATTI2 most functional evolutionary change is located in the signal peptide. This pattern is consistent with a hypothesis of subfunctionalization in trypsin inhibitory function.

Molecular evolution meets the genomics revolution

Changes in technology in the past decade have had such an impact on the way that molecular evolution research is done that it is difficult now to imagine working in a world without genomics or the Internet. In 1992, GenBank was less than a hundredth of its current size and was updated every three months on a huge spool of tape. Homology searches took 30 minutes and rarely found a hit. Now it is difficult to find sequences with only a few homologs to use as examples for teaching bioinformatics. For molecular evolution researchers, the genomics revolution has showered us with raw data and the information revolution has given us the wherewithal to analyze it. In broad terms, the most significant outcome from these changes has been our newfound ability to examine the evolution of genomes as a whole, enabling us to infer genome-wide evolutionary patterns and to identify subsets of genes whose evolution has been in some way atypical.

Polymorphism and divergence for island-model species

Estimates of the scaled selection coefficient, gamma of Sawyer and Hartl, are shown to be remarkably robust to population subdivision. Estimates of mutation parameters and divergence times, in contrast, are very sensitive to subdivision. These results follow from an analysis of natural selection and genetic drift in the island model of subdivision in the limit of a very large number of subpopulations, or demes. In particular, a diffusion process is shown to hold for the average allele frequency among demes in which the level of subdivision sets the timescale of drift and selection and determines the dynamic equilibrium of allele frequencies among demes. This provides a framework for inference about mutation, selection, divergence, and migration when data are available from a number of unlinked nucleotide sites. The effects of subdivision on parameter estimates depend on the distribution of samples among demes. If samples are taken singly from different demes, the only effect of subdivision is in the rescaling of mutation and divergence-time parameters. If multiple samples are taken from one or more demes, high levels of within-deme relatedness lead to low levels of intraspecies polymorphism and increase the number of fixed differences between samples from two species. If subdivision is ignored, mutation parameters are underestimated and the species divergence time is overestimated, sometimes quite drastically. Estimates of the strength of selection are much less strongly affected and always in a conservative direction.

Near-neutrality in evolution of genes and gene regulation

The nearly neutral theory contends that the interaction of drift and selection is important and occurs at various levels, including synonymous and nonsynonymous substitutions in protein coding regions and sequence turnover of regulatory elements. Recent progress of the theory is reviewed, and the interaction between drift and selection is suggested to differ at these different levels. Weak selective force on synonymous changes is stable, whereas its consequence on nonsynonymous changes depends on environmental factors. Selection on differentiation of regulatory elements is even more dependent on environmental factors than on amino acid changes. Of particular significance is the role of drift in the evolution of gene regulation that directly participates in morphological evolution. The range of near neutrality depends on the effective size of the population that is influenced by selected linked loci. In addition to the effective population size, molecular chaperones such as heat shock protein 90 have significant effects on the range of near neutrality.

Genomic effects of nucleotide substitutions in Drosophila simulans

Selective fixation of beneficial mutations reduces levels of linked, neutral variation. The magnitude of this "hitchhiking effect" is determined by the strength of selection and the recombination rate between selected and neutral sites. Thus, depending on the values of these parameters and the frequency with which directional selection occurs, the genomic scale over which directional selection reduces levels of linked variation may vary widely. Here we present a permutation-based analysis of nucleotide polymorphisms and fixations in Drosophila simulans. We show evidence of pervasive small-scale hitchhiking effects in this lineage. Furthermore, our results reveal that different types of fixations are associated with different levels of linked variation.

Quantifying the slightly deleterious mutation model of molecular evolution

We have attempted to quantify the frequency and effects of slightly deleterious mutations (SDMs), those that have selective effects close to the reciprocal of the effective population size of a species, by comparing the level of selective constraint in protein-coding genes of related species that have different present-day effective population sizes. In our two comparisons, the species with the smaller effective population size showed lower constraint, implying that SDMs had become fixed. The fixation of SDMs was supported by the observation of a higher fraction of radical to conservative amino acid substitutions in species with smaller effective population sizes. The fraction of strongly deleterious mutations (which rarely become fixed) is >70% in most species. Only approximately 10% or fewer of mutations seem to behave as SDMs, but SDMs could comprise a substantial fraction of mutations in protein-coding genes that have a chance of becoming fixed between species.

Changing effective population size and the McDonald-Kreitman test

Artifactual evidence of adaptive amino acid substitution can be generated within a McDonald-Kreitman test if some amino acid mutations are slightly deleterious and there has been an increase in effective population size. Here I investigate the conditions under which this occurs. I show that fairly small increases in effective population size can generate artifactual evidence of positive selection if there is no selection upon synonymous codon use. This problem is exacerbated by the removal of low-frequency polymorphisms. However, selection on synonymous codon use restricts the conditions under which artifactual evidence of adaptive evolution is produced.

Asymmetric Functional Divergence of Duplicate Genes in Yeast

Most duplicate genes are eliminated from a genome shortly after duplication, but those that remain are an important source of biochemical diversity. Here, I present evidence from genome-scale protein-protein interaction data, microarray expression data, and large-scale gene knockout data that this diversification is often asymmetrical: one duplicate usually shows significantly more molecular or genetic interactions than the other. I propose a model that can explain this divergence pattern if asymmetrically diverging duplicate gene pairs show increased robustness to deleterious mutations.

Hitchhiking mapping: a population-based fine-mapping strategy for adaptive mutations in Drosophilamelanogaster

The identification of genes contributing to the adaptation of local populations is of great biological interest. In an attempt to characterize functionally important differences among African and non-African Drosophila melanogaster populations, we surveyed neutral microsatellite variation in an 850-kb genomic sequence. Three genomic regions were identified that putatively bear an adaptive mutation associated with the habitat expansion of D. melanogaster. A further inspection of two regions by sequence analysis of multiple fragments confirmed the presence of a recent beneficial mutation in the non-African populations. Our study suggests that hitchhiking mapping is a universal approach for the identification of ecologically important mutations.

Tracking adaptive evolutionary events in genomic sequences

As more gene and genomic sequences from an increasing assortment of species become available, new pictures of evolution are emerging. Improved methods can pinpoint where positive and negative selection act in individual codons in specific genes on specific branches of phylogenetic trees. Positive selection appears to be important in the interaction between genotype, protein structure, function, and organismal phenotype.