Rapid evolution of interspecific interactions (during a timespan of about 100 years) has the potential to be an important influence on the ecological dynamics of communities. However, despite the growing number of examples, rapid evolution is still not a standard working hypothesis for many ecological studies on the dynamics of population structure or the organization of communities. Analysis of rapid evolution as an ecological process has the potential to make evolutionary ecology one of the most central of applied biological sciences.

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Rapid evolution as an ecological process.

One of the greatest challenges for evolutionary biology is explaining the widespread occurrence of sexual reproduction and the associated process of genetic recombination. A large number of theories have been developed that provide a sufficient short-term advantage for sex to offset its two-fold cost. These theories can be broadly classified into environmental (or ecological) and mutation-based models. Traditionally, the different theories have been viewed as competing, and empirical work has attempted to distinguish between them. Here we highlight the advantages that may be gained from considering that multiple mechanisms (environmental and mutational) may be at work, and that interactions between the theories may be very important.

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A pluralist approach to sex and recombination

Reciprocal selection is the underlying mechanism for host-parasite coevolutionary arms races. Its driving force is the reduction of host lifespan or fecundity that is caused by a parasite. Parasites evolve to optimize host exploitation, while hosts evolve to minimize the ‘parasite-induced' loss of fitness (virulence). Research on the evolution of virulence has mostly emphasized the role of parasite evolution in determining virulence. However, host evolution, accelerated by sexual recombination, contributes to the evolution and expression of virulence as well. The Red Queen hypothesis predicts that genetic variation among host offspring facilitates selection for reduced virulence. Here, we outline a synthesis between current thinking about the evolution of virulence and the evolution of sex.

Sex against virulence: the coevolution of parasitic diseases

Molluscs are numerically the second largest phylum in the animal kingdom with more than 1, 20,000 living species, which are divided into eight classes. Many species are promiscuous and there are different forms of sperm storage, providing the potential for sperm competition. This chapter presents different lines of evidence which suggest that sperm competition might be important in the evolution of the reproductive behavior in some groups of molluscs. It also shows that there are huge gaps in our understanding of the reproductive behavior of molluscs. However, the significance of molluscs' structure with respect to sexual selection and sperm competition is, in most cases, unclear. Several mollusc species are well-suited for studies on sperm competition. Moreover, studies on sperm competition in gastropods and cephalopods proved to be particularly rewarding. Both gastropods and cephalopods are unique because of their several important features. Their elaborate mating behavior may rival the complexity of those of various vertebrates. Sperm selection occurs in both groups. This provides a unique possibility of female choice in sperm competition. Furthermore, the adaptive significance of sperm polymorphism is still not known and needs to be assessed.

8 – Sperm Competition in Molluscs

Larval digenetic trematodes commonly castrate their first intermediate hosts, and should therefore impose strong selection on the timing and mode of host reproduction. Here we examine spatial variation in infection by trematodes in the freshwater snail Potamopyrgus antipodarum. Snails were collected at 11 different sites at Lake Alexandrina on the South Island of New Zealand from transects that ran perpendicular to the shore and across several different habitat types (from 0 to 8 m deep). Logistic regression was used to analyze the relationships between the frequency of trematode infection and snail size, habitat type, and transect location. On average, the probability of infection increased 3.3 times with each 1 mm increase in shell length. Prevalence of infection by the most common species of trematode, Microphallus sp., was highest in the shallow-water habitats where its final hosts (waterflow) spend most of their time. Prevalence of infection by another parasite, Telogaster ophistorchis (final host: eels) increased with depth, but because Microphallus was much more common, total infection by all trematodes decreased with depth. The effects of transect location were minor for Telogaster, but there was significant variation in Microphallus prevalence among transects, especially in the shore-bank habitat. Taken together, these results suggest that the risk of infection is spatially variable, but generally higher in shallow-water habitats, which may explain the greater frequency of sexual individuals as well as earlier reproduction among individuals near shore.

Spatial variation in infection by digenetic trematodes in a population of freshwater snails (Potamopyrgus antipodarum).

Abstr-act.-Phally, a genital dimorphism found in some species of self-compatible simultaneous hermaphrodites, presents an opportunity to examine factors maintaining outcrossing within an animal species that can self-fertilize. Both aphallics and euphallics can self-fertilize, but aphallics cannot donate sperm because they do not develop a functional prostate and penis. In this field study of Nigerian populations of the freshwater gastropod, Bulinus truncatus (Mollusca: Pulmonata), we evaluate ecological correlates of euphally to test hypotheses predicting a selective advantage of outcrossing due to the production of genetically variable offspring. The prevalence of euphally across 49 populations ranged from 0% to 81%. We found no association between prevalence of euphally and any of the following: population density, snail density, mollusk species abundance, water pH, oxidation-reduction potential, dissolved oxygen content, habitat instability (as estimated by habitat type), human activity, vegetation density, desiccation rate, and water chemistry variation. There was a significant but weak correlation between conductivity and proportion of euphallics (r2 = 10%), with lower ion concentrations favoring higher levels of euphally. The prevalence of the most abundant trematode taxon (Xiphidiocercariae) correlated positively with proportion of euphallics, explaining 10% of the variation in proportion of euphallics after we controlled for mean snail age and time of year. Trematode richness (number of taxa per population) was not associated with proportion of euphallics across sites when we controlled for time of year. However, indexes of trematode diversity that incorporated both prevalence and richness did correlate significantly with proportion of euphallics. These results are consistent with hypotheses predicting an advantage of outcrossing due to temporal fluctuations in the biotic environment.

S. J. Schrag

Papers

Ecological correlates of male outcrossing ability in a simultaneous hermaphrodite snail