Has adaptive dynamics contributed to the understanding of adaptive and sympatric speciation?

Competition, virulence, host body mass and the diversification of macro-parasites

Adaptive speciation has been much debated in recent years, with a strong emphasis on how competition can lead to the diversification of ecological and sexual traits. Surprisingly, little attention has been paid to this evolutionary process to explain intrahost diversification of parasites. We expanded the theory of competitive speciation to look at the effect of key features of the parasite lifestyle, namely fragmentation, aggregation and virulence, on the conditions and rate of sympatric speciation under the standard 'pleiotropic scenario'. The conditions for competitive speciation were found similar to those for non-parasite species, but not the rate of diversification. Adaptive evolution proceeds faster in highly fragmented parasite populations and for weakly aggregated and virulent parasites. Combining these theoretical results with standard empirical allometric relationships, we showed that parasite diversification can be faster in host species of intermediate body mass. The increase in parasite load with body mass, indeed, fuels evolution by increasing mutants production, but because of the deleterious effect of virulence, it simultaneously weakens selection for resource specialization. Those two antagonistic effects lead to optimal parasite burden and host body mass for diversification. Data on the diversity of fishes' gills parasites were found consistent with the existence of such optimum.

Biased sex-ratio and sex-biased heterozygote disadvantage affect the maintenance of a genetic polymorphism and the properties of hybrid zones

The evolution of biodiversity is a major issue of modern biology, and it is becoming increasingly topical as the ongoing erosion of diversity puts serious threats on human well-being. An elementary mechanism that allows maintaining diversity is the interplay between dispersal and heterozygote selective disadvantage, which can lead to self-sustainable spatial genetic structures and is central to the stability of hybrid zones. Theoretical studies supporting the importance of this mechanism assume a balanced sex-ratio and a heterozygote disadvantage equally affecting both sexes, despite the multiplicity of empirical evidence that (i) adult sex-ratio is usually biased towards either male or female and that (ii) heterozygote disadvantage often affects a single sex. We expanded the existing theory by weighting the strength of selection against heterozygote according to the biased in sex-ratio and in heterozygote disadvantage. The range of conditions allowing for the maintenance of polymorphism can then either double or vanish. We discuss the implications of such finding for birds, mammals and insects diversity. Finally, we provide simple analytical predictions about the effect of those biased on the width and speed of hybrid zones and on the time for the spread of beneficial mutations through such zones.

Within-host competition and diversification of macro-parasites

Although competitive speciation is more and more regarded as a plausible mechanism for sympatric speciation of non-parasite species, virtually no empirical or theoretical study has considered this evolutionary process to explain intra-host diversification of parasites. We expanded the theory of competitive speciation to parasite species looking at the effect of macro-parasite life history on the conditions for sympatric speciation under the so-called pleiotropic scenario. We included within-host competition in the classical Anderson and May framework assuming that individuals exploit within-host resources according to a quantitative trait. We derived the invasion fitness function of mutants considering different distributions of individuals among hosts. Although the mutant fitness depends on parameters describing the key features of macro-parasite life history, and on the relative distributions of mutant and residents in hosts, the conditions for competitive speciation of macro-parasites are exactly the same as those previously established for free-living species. As an interesting by-product, within-host competitive speciation is expected not to depend on the aggregation level of the parasites. This theoretical pattern is confirmed by comparing the speciation rate of weakly and strongly aggregated monogenean parasites.

Sympatric speciation under incompatibility selection

The existing theory of sympatric speciation assumes that a local population splits into two species under one-dimensional disruptive selection, which favors both of the opposite extreme values of a quantitative trait. Here we model sympatric speciation under selection that favors high values of either of the two independently inherited traits, each required to efficiently consume one of the two available resources, but acts, because of a tradeoff, against those possessing high values of both traits. Such two-dimensional incompatibility selection is similar to that involved in allopatric speciation. Using a hypergeometric phenotypic model, we show that incompatibility selection readily leads to sympatric speciation. In contrast to disruptive selection, two distinct modes of sympatric speciation exist under incompatibility selection: under strong tradeoffs both of the new species are specialists, each consuming its own resource, but under moderate tradeoffs speciation may be asymmetric and involve the origin of a specialist and a generalist species. Also, incompatibility selection may lead to irreversible specialization: under strong tradeoffs, the population speciates if it consists mostly of unspecialized individuals, but remains undivided if most of the individuals are specialized to consume one of the resources. Incompatibility selection appears to be more realistic than disruptive selection, implying that incompatibility between individually adaptive alleles or trait states drives both allopatric and sympatric speciation.