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Speciation requires the acquisition of reproductive isolation, and the circumstances under which this could evolve are of great interest. Are new species formed after the acquisition of generalized incompatibility arising between physically separated populations, or may they arise as a result of the action of disruptive selection beginning with the divergence of a rather restricted set of gene loci? Here we apply the technique of Amplified Fragment Length Polymorphism (AFLP) analysis to an intertidal snail whose populations display a cline in shell shape across vertical gradients on rocky shores. We compare the FST values for 306 AFLP loci with the distribution of FST estimated from a simulation model using values of mutation and migration derived from the data. We find that about 4% of these loci show greater differentiation than expected, providing evidence of the effects of selection across the cline, either direct or indirect through linkage. This is consistent with expectations from non-allopatric speciation models that propose an initial divergence of a small part of the genome driven by strong disruptive selection while divergence at other loci is prevented by gene flow.

Since many years the Galapagos islands are known for the study of evolution because they are home to many radiated endemic plant and animal groups. Although the most diverse group of organisms, the insects, show several examples of radiated genera, their evolution has been studied to a much lesser extent. Here we study the evolutionary relationships within a group of endemic flightless phytophagous beetles: the Opuntia weevils (Gerstaeckeria: Coleoptera: Curculionidae). Despite a restricted dispersal power, these beetles managed to colonise several islands of the Archipelago. They live monophagously on Opuntia cacti (also a radiated group). Morphological differences between beetles from different islands are obvious, but currently the group is in need of a taxonomic revision. In order to understand the phylogeography of these beetles a 406bp region of the mitochondrial cytochrome oxidase subunit I (COI) gene was studied for 38 specimens from 15 populations. A continental Gerstaeckeria species was chosen as outgroup. Sequences were aligned by eye and phylogenetic trees constructed using a variety of distance measures. The most remarkable result is the subdivision of beetles collected on the central island of Santa Cruz. The position of the beetles from Espanola raises some intriguing questions about the colonisation of the islands.

This talk provides an overview of recent developments in speciation theory and describes how processes of ecological, sexual, and spatial differentiation can interact to create reproductively isolated lineages. - First, it is clarified why phenotypic evolution can converge on fitness minima. The resulting processes of ecological differentiation through evolutionary branching readily arise from all fundamental types of ecological interaction and can explain the diversification of clonal organisms. Second, it is shown how, in sexual populations, the evolution of assortative mating in general and of mate recognition systems in particular can be driven by those selection pressures that cause evolutionary branching in clonal populations. With the degrees of sexual and ecological differentiation increasing gradually and concomitantly, sexual populations can escape traps set by convergence stable fitness minima. Third, it is demonstrated how the introduction of spatial heterogeneity can substantially facilitate processes of evolutionary branching in clonal and sexual populations; these processes, in turn, can result in pronounced spatial segregation between ecomorphs. - The results highlighted in this talk lead to a view of speciation as a multi-layered adaptive process. The spatial patterns eventually arising from such processes may therefore bear little or no information on the underlying driving forces.

Evolutionary branching produces a protected polymorphism from an initially monomorphic population. In such polymorphic populations, evolutionary processes continue to modify the phenotypes and their abundances. The evolution of assortative mating will reduce the abundance of heterozygotes, which leads to sympatric speciation; the evolution of dominance interactions can make heterozygotes and homozygotes phenotypically identical. Sympatric speciation and dominance-recessivity are mutually exclusive evolutionary outcomes.

I investigate the effect of different factors on the odds of observing speciation vs. dominance in a resource competition model. It is a relatively simple model with one locus that represents a mating trait, and another locus for an ecological phenotype with potentially non-additive dominance interactions. The population dynamics of a resident system can have a stable point equilibrium or non-equilibrium dynamics depending on model parameters.

The effect of some factors on the probability of observing speciation is obvious a priori. For instance, when the mutational variance for the assortative mating trait is increased, it must go up. With non-equilibrium dynamics, evolving dominance interactions and assortative mating will change the density fluctuations in the population to some extent. The effect of non-equilibrium dynamics on the odds of observing speciation is investigated by means of simulated evolutionary random walks.

Wolbachia, a group of Rickettsia-like intracellular bacteria, are well-known for meddling with the sexual reproduction of their hosts. The most enigmatic effect is cytoplasmic incompatibility (CI), where males infected with Wolbachia become reproductively incompatible with uninfected females, or with females infected with a different strain of the bacteria. Given such incompatible matings, there is an obvious potential for Wolbachia to induce reproductive isolation in its host. Recently, evidence is mounting that Wolbachia-induced reproductive isolation does occur between allopatric species. In this presentation, we discuss the possibility that evolution of CI in Wolbachia allows their host to speciate via evolutionary branching (i.e., in a sympatric setting). To assess the probability of this process, we constructed a simple model of the Wolbachia-host interaction, where the host is allowed to evolve in response to its environment, and the Wolbachia symbiont is allowed to evolve its CI character. Evolution of the host would lead to evolutionary branching, if the two branches were reproductively isolated. This is provided by evolution of different Wolbachia CI types that are bidirectionally incompatible. We discuss the results with reference to studies into the effects of Wolbachia infection in different strains of spider mites.

Degree of genetic divergence is frequently used to infer that two populations belong to separate species, or that several populations belong to a single species. I explore the logical framework of this approach, including the following assumptions: 1) Speciation takes place over very long periods of time, 2) Reproductive isolation is based on the slow accumulation many genetic differences throughout the genome, 3) Genetic divergence automatically leads to reproductive isolation between species, 4) Premating and postmating reproductive isolation have a similar genetic basis. I argue that so many exceptions to these assumptions have been demonstrated that they cannot be used with any reliability to distinguish different species. In addition, genetic distance as a species criterion is mostly used within the framework of Mayrs Biological species concept and is not free of assumptions about the nature of species or of speciation. The use of genetic distance to infer separate species (or the lack of these) is not parsimonious, its theoretical foundations are not well understood and it cannot be applied over a wide range of plants and animals. I explore alternative approaches towards solving the species problems normally solved using genetic distance.

We review published records of laboratory experiments on peripatric and vicariance allopatric speciation to address the following three questions: 1) What was the true effect size of reproductive isolation? 2) Was the reproductive isolation persistent? 3) What influenced the development of isolation? Contrary to popular belief, laboratory evidence for allopatric speciation is quite weak. Assortative mating was only found among derived populations in vicariance experiments. Reproductive isolation against control populations was only intermittent, so there is reason to doubt if some cases showing significant reproductive isolation really should be attributed to speciation. The method of testing was at least as important as the speciation model. Experimental populations tested against each other were the most likely to demonstrate reproductive isolation. This study suggests that allopatric speciation experiments are more likely to yield conclusive results under divergent selection than under drift, and points to the necessity of large populations and many generations.

In populations of phytophagous insects that use the host plant as a rendezvous for mating, divergence in host preference could lead to sympatric speciation. Speciation requires the elimination of "generalist" genotypes, i.e. those with intermediate preference. This could occur due to direct selection against such genotypes, or due to indirect selection because of association of preference alleles with alleles that are oppositely selected on the two hosts. While the former mechanism has been shown to be plausible, the plausibility of the latter has been questioned. I consider a multilocus model in which one set of biallelic loci affects host preference, and a second set affects viability on the host chosen. Alleles that increase viability on one host decrease viability on the other, and all loci are assumed to be unlinked. With moderately strong selection on the viability loci, preference alleles rapidly become associated with viability alleles, and the population splits into two reproductively isolated host specialist populations in under 100 generations. Surprisingly, the conditions for speciation to occur in this model, as measured by the strength of selection on the viability loci, are only slightly more stringent than in a model where selection acts directly on the preference loci.

I will review recent attempts to model the dynamics of parapatric speciation based on the classical Bateson-Dobzhansky-Muller model of reproductive isolation and its recent multilocus generalisations unified within the framework of holey adaptive landscapes. In this framework, complete reproductive isolation evolves as a by-product of genetic divergence along ridges of high fitness genotypes in the multidimensional genotype space. The main focus will be on the waiting time to speciation, on the duration of speciation process (that is the duration of intermediate forms in the actual transition to a state of complete reproductive isolation), and on special location of the speciating population(s) within the range of the ancestral species. The evolutionary factors considered are mutation, drift, selection for local adaptation, spatial structure and genetic architecture of reproductive isolation. I will discuss some generalizations emerging from the theoretical studies.

Islands contain a disproportionately large number of species, and speciation and insularity thus seem to be associated. Furthermore, several insular species with close relatives on the mainland have lost their secondary sexual characters on islands. Finally, insular species tend to lose their dispersal ability, and reduced dispersal increases speciation. We analysed two predictions explaining the abundance of sexually dichromatic species on islands: (1) Secondary sexual characters tend to be lost on islands because of a reduction in the importance of sexual selection; and (2) sexual selection increases the rate of speciation on islands. The proportion of sexually dichromatic species in all bird families on the mainland and on large and small islands, and these values were used in comparative analyses that took the phylogenetic relationships among families into account. Bird species on small islands were less often sexually dichromatic than species in the same families on large islands or on the mainland. Bird families with a large proportion of sexually dichromatic species on islands had a large proportion of insular species, suggesting that sexual selection on islands also enhances speciation on islands. Thus, two macro-evolutionary processes seem to be antagonistically influenced by sexual selection.

Genetic polymorphism can arise in an initially monomorphic population by evolutionary branching, whereby two distinct alleles evolve from a single ancestral allele via small mutational steps. The emerging polymorphism is under disruptive selection such that heterozygotes are at a disadvantage, and assortative mating is selected for. Evolutionary branching thus may set the stage for sympatric speciation. Evolutionary branching is a common, robust outcome of evolution in soft selection models. Earlier population genetic studies showed that in Levene's soft selection model, a polymorphism of two similar alleles can be maintained only if the ecological parameters are fine-tuned. Here we reconcile these results by demonstrating that in any environment there is a small set of allele pairs that can form polymorphisms. If only two alleles are present, then fine-tuning is necessary to make sure that these are part of the polymorphism set. With mutations, however, evolution by allele substitution proceeds towards the alleles that can form polymorphisms. Depending on the ecological parameters, the emerging polymorphism is either resolved at a single evolutionarily stable allele or evolutionary branching gives rise to a persistent polymorphism of distinctively different alleles.

Specialisation is considered to be an important step leading to speciation. Due to the close association between parasites and hosts, many generalist parasites have a high potential to become specialized on different host species. We investigated this hypothesis for a common ectoparasite of seabirds, the tick Ixodes uriae. We examined patterns of neutral genetic variation between ticks collected from three host species in sympatry, the Black-legged kittiwake (Rissa tridactyla), the Atlantic puffin (Fratercula arctica), and the Common guillemot (Uria aalge), at different independent sites. We found higher genetic differentiation between ticks from different sympatric host species than between ticks from nearby allopatric populations of the same host species. Patterns suggesting isolation by distance were found among tick populations of each host group, but no such patterns existed between tick populations of different hosts. An experimental study of local adaptation in this parasite suggests that selection during tick feeding may have resulted in the rapid formation of these specialized groups in sympatry. Our findings reinforce the idea that host race formation, or speciation after a host shift, could be an important mechanism explaining the rich diversity of parasites and merits further examination in other host-parasite systems.

Disruptive selection can arise from evolution toward a convergence stable fitness minimum and thus allows for evolutionary branching in clonally reproducing populations. The concept of adaptive speciation is based on the expectation that, in sexually reproducing populations, such an ecological setting also selects for reproductive isolation between the incipient branches. At the same time, however, disruptive selection can increase the genetic variance of the selected trait, thus diminishing the strength of disruptive selection and, potentially, preventing speciation. Here we investigate the interplay between these two competing implications of disruptive selection in sexually reproducing populations. The relative speed of these processes is of crucial importance. In one extreme the emergence of reproductive isolation occurs much more slowly than the equilibration of genetic variance: it then becomes important to predict the equilibrium genetic variance of a randomly mating population. The genetic variance of a selected trait that directly determines the type of resources that organisms can utilize is expected to equilibrate at the variance of the underlying resource distribution, thus removing the disruptiveness of selection. However, this outcome only applies unless two constraints interfere:

• The additive genetic variance of the population cannot exceed a maximal value that is determined by the genetic make-up of the selected trait.
• Under certain assumptions, the shape of the trait’s frequency distribution cannot depart from normality.

Selection may therefore remain disruptive and promote adaptive speciation after genetic variance has equilibrated, even if the evolution of reproductive isolation is slow. Chances for adaptive speciation are further enhanced if the evolution of reproductive isolation occurs on the same timescale or faster than the equilibration of genetic variance.

A model gene system causing pre-mating isolation is shown. A gene causing an 8h variation in the period of circadian rhythm in the adult locomotor activity was found in Bactrocera cucurbitae (Diptera: Tephritidae) by artificial selection for development time from egg to adult eclosion. The periods of free-running rhythm in constant darkness were about 22h in lines selected for short development time, and about 30h in lines selected for long development time. Cross experiments between lines indicated that the development time was controlled by polygene, but the circadian rhythm was controlled by a major gene. The major gene, which pleiotropically controls development and mating times, inadvertently caused significant reproductive isolation in laboratory by a variety of means including shifts in mating activity patterns. Fly lines selected for short and long developmental periods differ in their preferred times of mating during the evening. This difference translates into significant pre-zygotic isolation, as measured by mate choice tests. The pre-mating isolation was caused only by the difference in time of mating. In this talk, the quantitative and molecular genetic bases of the gene system are focused. Also, the adaptive behaviour of clock genes is discussed in relation to speciation.

We have tested Kaneshiro's peripatric speciation model, which predicts that ancestral females will discriminate against bottlenecked males but not vice versa. Our quantitative analysis of mating asymmetries in published laboratory experiments showed that bottlenecked males indeed have lost part of their attractiveness, but females have only lowered their acceptance thresholds and not lost their ancestral preferences. Reproductive isolation is unlikely to evolve under such premises. Contrary to the model, the mating asymmetries were not limited to bottlenecks, but regularly occurred between derived and ancestral populations. The simplest way to explain the observed mating asymmetries is by loss of genetic variation and inbreeding in the derived populations. We conclude that the Kaneshiro model is unlikely to isolate small daughter populations from their ancestor. With slight modifications, however, it becomes a strong candidate for speciation in allopatry manifested in isolation between daughter populations in secondary contact, challenging reinforcement and ring speciation as an explanation for reproductive character displacement.

Different approaches (karyotypic analysis of F2 specimens and of second meiotic divisions of F1 specimens) and contrasting results (Mendelian, random segregation or cosegregation of isomorphic chromosomes) have been reported up to now in current literature on the segregation pattern of robertsonian metacentric chromosomes of the mouse in multiple heterozygotes. In the present contribution data are presented based on the FISH (Fluorescent In Situ Hybridization) analysis with telometric probes of DAPI stained metaphases of spermatocytes II of mice heterozygous for 2, 3 or 4 robertsonian metacentrics in an all-telocentric background, the karyotype of which has been reconstructed starting from lab strains. The use of telometric probes allowed to distinguish metacentric chromosomes from pairs of acrocentric ones, with their centromeric regions close to each other. Data showed that isomorphic chromosomes tend to cosegregate (metacentrics with metacentrics, acrocentrics with acrocentrics) at an extent which depends on the karyotype of the specimens; the values found of cosegregation have a limited effect on the efficiency of underdominant chromosome rearrangements as reproductive isolation factors.

We evaluate the adaptive dynamics of a predator–prey model including both ecological population dynamics and evolutionary strategy dynamics. Evolutionary dynamics can include evolutionary changes in the resident strategy values, invasion of novel species, and adaptive speciation by strategy splitting at a minimum of the adaptive landscape. The number of niches in the community depends upon the predator niche breadth and is set by the ESS, whether or not the resident population has the same number of species, or strategy values, as the ESS. When the community is under-saturated in terms of species number, invasions are relatively easy and the adaptive landscape is changeable. The evolutionary dynamics may then include evolutionary diet switching of predator strategies. Some resident strategies may also evolve towards evolutionary minima, where new species easily can invade by immigration or adaptive speciation. Approaching the ESS community, the landscape becomes more rigid and the resident strategies become less affected by invasions. The model illustrates how ecological and evolutionary dynamics can produce consumer–resource communities varying in diversity and level of consumer specialisation. Evolutionary game theory and adaptive dynamics provide fresh perspectives on coevolution, species diversity, complexity versus stability, and the consequences of species deletions.

Most biologists believe that most speciation events occur in allopatry. This is intuitively reasonable and there is even some evidence for it. Natural selection and genetic drift are the most commonly invoked processes. Recently there has been an upsurge of interest in modelling processes of speciation in sympatry, and also speciation driven by sexual selection. These studies have persuaded a few people that sympatric speciation is not as improbable was once thought, and a larger number of people that sexual selection might well be important in many speciation events. Most of the evolutionary biology community will probably need more empirical evidence to change their minds, particularly about non-allopatric speciation. Is there any point in more modelling studies? I think that there may be, and suggest that a major focus should be on the factors responsible for differences in the rates of speciation in different lineages. This work will need to be informed by a close interplay between theory and empirical studies. Some preliminary findings will be discussed in relation to the cichlid fishes of Lakes Malawi and Victoria.

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