Kinship ecology of competition: size hierarchies in kin and nonkin laboratory cohorts of tadpoles.

This study provides an experimental test of the ecological significance of kinship in competitive interactions among individuals. Tadpoles of the fire-bellied toad, Bombina variegata (Anura, Discoglossidae) reared in the laboratory in a high density of siblings grew better and developed faster than when they were reared with similar densities of nonrelatives. Kinship-based genotypic uniformity may therefore be advantageous: it can lead to faster and more uniform growth. In nature it may well be kin recognition that permits such advantage to be realised. The largest tadpoles from pure-sibship cohorts accounted, on average, for the smaller fraction of the cohort total biomass than the largest tadpoles reared with cohorts of nonrelatives, although they attained the same absolute weight and developmental stage. The smallest individuals from pure-sibship cohorts had a much larger absolute body weight than the smallest tadpoles from mixed-sibship cohorts, and were also much more advanced developmentally. The results could be an effect of an altruistic restraint in growth rate on the part of the largest tadpoles in sibling cohorts and certainly not of the smallest ones. However, the results could also be due to the altruism-independent facilitation of living conditions in pure-sibship groups, resulting solely from potential advantages of homogeneous water conditioning.

Size-dependent variation of gender in high density stands of the monoecious annual, Ambrosia artemisiifolia (Asteraceae).

We examined the relationship between size variability and the distribution of functional gender in stands of the monoecious, wind-pollinated annual Ambrosia artemisiifolia. Populations of 60 individuals were grown in the greenhouse at a density of 372 m-2 and at two nutrient levels. Among the surviving plants, after self-thinning, variability in above-ground biomass and gender was higher in the high nutrient treatment. Among individuals there was a significant positive correlation between maleness and both height and biomass. Fecundity was also positively correlated with both measures of size. Based on the pattern of distribution of male and female flowers within the plant, it appears that the increase in maleness in larger plants is due to increased branching and axis elongation. These results demonstrate that competitive interactions, which lead to increased variability in biomass and fecundity, can also generate variability in gender within populations.

Does fitness erode in the absence of selection? An experimental test with Tribolium.

In the absence of natural selection, average fitness in the population is expected to decline due to the accumulation of deleterious mutations. Replicate populations of flour beetles (Tribolium confusum) were maintained for 22 generations in the virtual absence of selection (random mating, favorable environment, excess of food, and mortality of only 3%). Larva-to-adult survivorship rates were similar in the stock population and selection-free populations. In contrast, starvation resistance of adult beetles from selection-free populations was significantly reduced (by more than 2% per generation). When tested in the favorable environment, beetles in one selection-free population had significantly slower development and smaller sizes of females than beetles from the stock population. Since such changes in these fitness components are usually maladaptive, they indicate possible erosion of fitness under relaxed selection at the rate of 0.1-0.2% per generation. No fitness erosion was detectable in the second selection-free population.

Phenotypic plasticity and similarity of DNA among genotypes of an annual plant.

When measured directly, rather than inferred from pedigree analyses, the relationship between similarity in phenotype and similarity in DNA sequence was detectable at the level of members of a single population and strongly depended on the environmental context. Genetic divergence among 27 co-occurring genotypes of Abutilon theophrasti, a common annual plant, was less than 5 per cent as revealed by RAPD-PCR analysis based on over 400 bands per genotype. Nevertheless, within this narrow range, there was a positive correlation between genetic similarity and similarity in the performance of genotypes on temperature and moisture gradients, suggesting that plasticity itself has a genetic basis. No relationship was detected, however, when the phenotypic plasticity was expressed in response to gradients of light intensity or soil fertilization, indicating a weaker genetic basis, or suggesting possible involvement of a few genes of major effect.

Microevolutionary responses in experimental populations of plants to CO2-enriched environments: parallel results from two model systems.

Despite the critical role that terrestrial vegetation plays in the Earth's carbon cycle, very little is known about the potential evolutionary responses of plants to anthropogenically induced increases in concentrations of atmospheric CO2. We present experimental evidence that rising CO2 concentration may have a direct impact on the genetic composition and diversity of plant populations but is unlikely to result in selection favoring genotypes that exhibit increased productivity in a CO2-enriched atmosphere. Experimental populations of an annual plant (Abutilon theophrasti, velvetleaf) and a temperate forest tree (Betula alleghaniensis, yellow birch) displayed responses to increased CO2 that were both strongly density-dependent and genotype-specific. In competitive stands, a higher concentration of CO2 resulted in pronounced shifts in genetic composition, even though overall CO2-induced productivity enhancements were small. For the annual species, quantitative estimates of response to selection under competition were 3 times higher at the elevated CO2 level. However, genotypes that displayed the highest growth responses to CO2 when grown in the absence of competition did not have the highest fitness in competitive stands. We suggest that increased CO2 intensified interplant competition and that selection favored genotypes with a greater ability to compete for resources other than CO2. Thus, while increased CO2 may enhance rates of selection in populations of competing plants, it is unlikely to result in the evolution of increased CO2 responsiveness or to operate as an important feedback in the global carbon cycle. However, the increased intensity of selection and drift driven by rising CO2 levels may have an impact on the genetic diversity in plant populations.