Ionic regulatory strategies of crabs: the transition from water to land

Adaptation to a sudden extreme change in environment, beyond the usual range of background environmental fluctuations, is analysed using a quantitative genetic model of phenotypic plasticity. Generations are discrete, with time lag tau between a critical period for environmental influence on individual development and natural selection on adult phenotypes. The optimum phenotype, and genotypic norms of reaction, are linear functions of the environment. Reaction norm elevation and slope (plasticity) vary among genotypes. Initially, in the average background environment, the character is canalized with minimum genetic and phenotypic variance, and no correlation between reaction norm elevation and slope. The optimal plasticity is proportional to the predictability of environmental fluctuations over time lag tau. During the first generation in the new environment the mean fitness suddenly drops and the mean phenotype jumps towards the new optimum phenotype by plasticity. Subsequent adaptation occurs in two phases. Rapid evolution of increased plasticity allows the mean phenotype to closely approach the new optimum. The new phenotype then undergoes slow genetic assimilation, with reduction in plasticity compensated by genetic evolution of reaction norm elevation in the original environment.

If large genomes are truly saturated with unnecessary 'junk' DNA, it would seem natural that there would be costs associated ith accumulation and replication of this excess DNA. Here we examine the available evidence to support this hypothesis, which we term the 'large genome constraint'. We examine the large genome constraint at three scales: evolution, ecology, and the plant phenotype. In evolution, we tested the hypothesis that plant lineages with large genomes are diversifying more slowly. We found that genera with large genomes are less likely to be highly specious -- suggesting a large genome constraint on speciation. In ecology, we found that species with large genomes are under-represented in extreme environments -- again suggesting a large genome constraint for the distribution and abundance of species. Ultimately, if these ecological and evolutionary constraints are real, the genome size effect must be expressed in the phenotype and confer selective disadvantages. Therefore, in phenotype, we review data on the physiological correlates of genome size, and present new analyses involving maximum photosynthetic rate and specific leaf area. Most notably, we found that species with large genomes have reduced maximum photosynthetic rates - again suggesting a large genome constraint on plant performance. Finally, we discuss whether these phenotypic correlations may help explain why species with large genomes are trimmed from the evolutionary tree and have restricted ecological distributions. Our review tentatively supports the large genome constraint hypothesis.

This theme issue pursues an exploration of the potential of taking into account the environmental sensitivity of development to explaining the evolution of metazoan life cycles, with special focus on complex life cycles and the role of developmental plasticity. The evolution of switches between alternative phenotypes as a response to different environmental cues and the evolution of the control of the temporal expression of alternative phenotypes within an organism's life cycle are here treated together as different dimensions of the complex relationships between genotype and phenotype, fostering the emergence of a more general and comprehensive picture of phenotypic evolution through a quite diverse sample of case studies. This introductory article reviews fundamental facts and concepts about phenotypic plasticity, adopting the most authoritative terminology in use in the current literature. The main topics are types and components of phenotypic variation, the evolution of organismal traits through plasticity, the origin and evolution of phenotypic plasticity and its adaptive value.