A considerable fraction of the world's biodiversity is of recent evolutionary origin and has evolved as a by-product of, and is maintained by, divergent adaptation in heterogeneous environments. Conservationists have paid attention to genetic homogenization caused by human-induced translocations (e.g. biological invasions and stocking), and to the importance of environmental heterogeneity for the ecological coexistence of species. However, far less attention has been paid to the consequences of loss of environmental heterogeneity to the genetic coexistence of sympatric species. Our review of empirical observations and our theoretical considerations on the causes and consequences of interspecific hybridization suggest that a loss of environmental heterogeneity causes a loss of biodiversity through increased genetic admixture, effectively reversing speciation. Loss of heterogeneity relaxes divergent selection and removes ecological barriers to gene flow between divergently adapted species, promoting interspecific introgressive hybridization. Since heterogeneity of natural environments is rapidly deteriorating in most biomes, the evolutionary ecology of speciation reversal ought to be fully integrated into conservation biology.

Self-rated health is a frequently used health indicator, but there is little data on its comparability across cultures. We employed samples from Tampere, Finland, and Florence, Italy, of the European Longitudinal Study on Aging to examine the cultural and gender differences in self-rated health. Personal interview data was used and vital status ascertained after 7 years. After adjusting for several health-related variables, we found no substantial difference in self-rated health between genders, although women in Florence were three times and men in Florence four times more likely to report good self-rated health than men in Tampere. The correlational structure of self-rated health was similar in both areas. The significant graded association between self-rated health and mortality in both areas was mostly explained by other health indicators included in a multivariate model. Results suggest that self-rated health is a useful summary of physical health, but it may predict mortality better in men than in women and be sensitive to cultural environment. Therefore, direct gender and cultural comparisons of self-rated health should be made with caution.

Sexual populations should be vulnerable to invasion and replacement by ecologically similar asexual females because asexual lineages have higher per capita growth rates. However, as asexual genotypes become common, they may also become disproportionately infected by parasites. The Red Queen hypothesis postulates that high infection rates in the common asexual clones could periodically favor the genetically diverse sexual individuals and promote the short-term coexistence of sexual and asexual populations. Testing this idea requires comparison of competing sexual and asexual lineages that are attacked by natural parasites. To date no such data have been available. Here, we report on long-term dynamics and parasite coevolution in a "mixed" (sexual and asexual) population of snails (Potamopyrgus antipodarum). We found that, within 7-10 years, the most common clones were almost completely replaced by initially rare clones in two different habitats, while sexuals persisted throughout the study period. The common clones, which were initially more resistant to infection, also became more susceptible to infection by sympatric (but not allopatric) parasites over the course of the study. These results are consistent with the Red Queen hypothesis and show that the coevolutionary dynamics predicted by the theory may also favor sexual reproduction in natural populations.

2000. Dr. Pangloss restrained by the Red Queen -steps towards a unified defence theory. -Oikos 89: 267 -274.Animals and plants defend themselves against a variable community of biological enemies. We argue that the effectiveness of allocation to defence (the success of defence per unit allocation) may be expected to decrease as the diversity of attack types increases, and asked how the optimal allocation to defence covaries with the effectiveness of defence. Variation in effectiveness links optimal defence to coevolutionary processes; the prime characteristic of coevolutionary interactions is that they promote and maintain genetic variation in both hosts and their enemies, leading to variation in the effectiveness of defence. We present a simple model suggesting that as effectiveness decreases, the fitness benefit of defence disappears. In other words, when effectiveness is low, the optimal strategy is to tolerate damage. As effectiveness increases, the optimal allocation flips rapidly from no-defence (tolerance) to high allocation to defence, and then decreases at a decelerating pace as effectiveness increases. We conclude that diversifying coevolution, as it covaries with the effectiveness of defence, constrains the evolution of optimal defence strategies and may be a very important component in determining the optimal allocation to defence and variation in the success of defence as it is seen in the wild.

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.

We examined clonal diversity and the distribution of both clonal and sexual genotypes in a single population of freshwater snails (Potamopyrgus antipodarum) in which diploid sexual individuals and triploid parthenogens coexist. A genetic analysis of individuals from three habitat zones in Lake Alexandrina, New Zealand revealed extremely high clonal diversity: 165 genotypes among 605 clonal individuals. The frequency of triploid clonal individuals increased with increasing depth in the lake, and most of the individual clones were habitat specific, suggesting that differences among habitats are important in structuring the clonal subpopulation. There were also high levels of clonal diversity within habitats, suggesting frequent origins of habitat-specific clones. In contrast, diploid sexual individuals were proportionately more common in the shallow regions of the lake (where infection by trematode larvae is highest), and there was no significant spatial structure in the sexual subpopulation. We suggest that habitat specialization by clones, as well as parasite-mediated selection against common clones, are important factors affecting the structure of this mixed population of sexual and clonal snails.

We studied the effects of re-oligotrophication and climate warming on plankton richness and community stability over a period of 30 years in the deep mesotrophic Lake Zurich (Switzerland). We assembled monthly time-series of phytoplankton and zooplankton taxonomic richness, phytoplankton functional groups (species with similar functional traits) and physico-chemical environmental descriptors (temperature, conductivity, pH, P-PO43-, N-NO3-, light absorption). We used multiple linear regression to test: 1) the effect of environmental variability over time and depth on the accrual of plankton richness; and 2) the relative effect of richness and environmental variability on the stability of plankton. Environmental change was characterised by increase in temperature, decrease in phosphorus levels, reduced temporal variability of both, and higher heterogeneity of phosphorus over depth (spatial heterogeneity). These conditions occurred concurrently with accrual in plankton taxonomic and functional richness. Increase in temperature and spatial heterogeneity were the best predictors of phytoplankton richness, while phytoplankton richness and spatial heterogeneity had the strongest effects on zooplankton richness. Temporal stability in phytoplankton biovolume was mainly affected by variability in phosphorus and temperature, while zooplankton abundance levels were more strongly linked to fluctuations in nitrogen, temperature and phytoplankton biovolumes. Our analysis highlights that climate warming and re-oligotrophication may favour an increase in spatial (depth) heterogeneity in the water column of deep lakes, enhancing the potential for phytoplankton species co-existence and an increase in plankton richness. Our analysis also suggests that the intensity of fluctuations in key environmental variables can be a better predictor of plankton community stability then average richness

The maintenance of sexual reproduction by natural selection poses a paradox for evolutionary biology. Assuming ''all else equal,'' a sexual lineage producing both males and females has a slower intrinsic growth rate than a clonal lineage producing only females. Hence, clones should rapidly outcompete their sexual counterparts, unless there are ecological or mutation-clearing advantages to sex, or unless the all-else-equal assumption is incorrect. Here we examined the crucial all-else-equal assumption using a lake population of freshwater snails (Potamopyrgus antipodarum) in which both diploid sexual and triploid parthenogenetic individuals coexist. We contrasted the variation in life-history traits between sexual and clonal forms to the variation in life histories among habitats and sampling locations. We also conducted a laboratory experiment in which sexual snails competed with a single clone for 1 yr. We found that size at maturity varied among habitats within the lake, but this difference was not attributable to reproductive mode. There were also no differences among clonal and sexual individuals with respect either to brood size or to the frequency of developmental errors in brooded embryos. Finally, in the laboratory competition experiment, the growth rate of sexual individuals was significantly lower than the growth rate of snails from a single clone. Taken together, these results suggest that the allelse-equal assumption is correct, and that a cost of males exists in natural sexual populations of these snails.