Evolutionary Change In Response Research Articles

Experimental social evolution with Myxococcus xanthus.

Genetically-based social behaviors are subject to evolutionary change in response to natural selection. Numerous microbial systems provide not only the opportunity to understand the genetic mechanisms underlying specific social interactions, but also to observe evolutionary changes in sociality over short time periods. Here we summarize experiments in which behaviors of the social bacterium Myxococcus xanthus changed extensively during evolutionary adaptation to two relatively asocial laboratory environments. M. xanthus moves cooperatively, exhibits cooperative multicellular development upon starvation and also appears to prey cooperatively on other bacteria. Replicate populations of M. xanthus were evolved in both structured (agar plate) and unstructured (liquid) environments that contained abundant resources. The importance of social cooperation for evolutionary fitness in these habitats was limited by the absence of positive selection for starvation-induced spore production or predatory efficiency. Evolved populations showed major losses in all measured categories of social proficiency- motility, predation, fruiting ability, and sporulation. Moreover, several evolved genotypes were observed to exploit the social behavior of their ancestral parent when mixed together during the developmental process. These experiments that resulted in both socially defective and socially exploitative genotypes demonstrate the power of laboratory selection experiments for studying social evolution at the microbial level. Results from additional selection experiments that place positive selection pressure on social phenotypes can be integrated with direct study of natural populations to increase our understanding of principles that underlie the evolution of microbial social behavior.

The structure and function of RuBisCO and their Implications for Systematic Studies

As "the most abundant protein in the world,'' ribulose-1,5-bisphosphate carboxylase (RuBisCO) attracts the attention of genetic engineers and plant phylogeneticists. The active site, which is responsible for almost all carbon fixation on earth, is in the large subunit (LSU). Over 30% of the 476 amino acids in the LSU are involved in intermolecular associations. Using available sequence data, we find that 105 (22%) of the residues are absolutely conserved across 499 seed plants, with an additional 110 demonstrating only one change. Our analyses show that conserved domains are not fully explained by current structural data. This has several implications for systematic studies. First, the number of potentially variable sites is likely to be slightly over 1000, rather than 1428. Second, rates of change can vary greatly across the molecule; functional constraints on amino acids and codon biases greatly increase the potential for homoplasy. Third, some changes are correlated, and thus might be down-weighted accordingly. Fourth, some of the variation in RuBisCO may be adaptive and present insights into the nature of evolutionary change in response to the environment.

RESPONSE TO NATURAL ENVIRONMENTAL HETEROGENEITY: MATERNAL EFFECTS AND SELECTION ON LIFE-HISTORY CHARACTERS AND PLASTICITIES IN MIMULUS GUTTATUS.

Recent studies in plant populations have found that environmental heterogeneity and phenotypic selection vary at local spatial scales. In this study, I ask if there is evolutionary change in response to environmental heterogeneity and, if so, whether the response occurs for characters or character plasticities. I used vegetative clones of Mimulus guttatus to create replicate populations of 75 genotypes. These populations were planted into the natural habitat where they differed in mean growth, flowering phenology, and life span. This phenotypic variation was used to define selective environments. There was variation in fitness (flower production) among genotypes across all planting sites and in genotype response to the selective environment. Offspring from each site were grown in the greenhouse in two water treatments. Because each population initially had the same genetic composition, variation in the progeny between selective environments reveals either evolutionary change in response to environmental heterogeneity or environmental maternal effects. Plants from experimental sites that flowered earlier, had shorter life spans and were less productive, produced offspring that had more flowers, on average, and were less plastic in vegetative allocation than offspring of longer-lived plants from high-productivity areas. However, environmental maternal effects masked phenotypic differences in flower production. Therefore, although there was evidence of genetic differentiation in both life-history characters and their plasticities in response to small-scale environmental heterogeneity, environmental maternal effects may slow evolutionary change. Response to local-scale selective regimes suggests that environmental heterogeneity and local variation in phenotypic selection may act to maintain genetic variation.

Speciation, extinction and climatic change in hominid evolution

Abstract The temporal association of hominid evolution with a period of global climatic instability and cooling is suggestive of a causal relationship. A number of authors have proposed a climatic forcing model for the timing and nature of evolutionary changes in human evolution during the Pliocene and Pleistocene (5-3 Mya), such as the appearance of new taxa, and by inference only a limited role for continuous evolutionary change in response to inter- and intra-specific competition at the local level. A major problem with such models is that both climatic change and hominid evolution are now recognised as complex with numerous events. By splitting climatic change as revealed in deep sea cores into a number of distinct attributes (temperature, stability and variability) and examining the relationship of each to the appearance, diversity and disappearance of hominid taxa it is possible to investigate more closely the effect of climatic change on hominid evolution. It is shown that the effect of climatic change can be observed in relation to extinction events, but that there is no significant relationship with the first appearance of hominid taxa. This implies that the mechanism by which climate influences evolution is primarily through extinction, and that further factors dependent upon local competitive conditions play a significant part in the appearance of new taxa.

Evolutionary responses of stomatal density to global CO2 change

Stomatal density is known to respond to CO2 levels during leaf development. Current interest in the increasing concentration of atmospheric CO2 has stimulated much experimentation on the responses of plants to relatively short-term exposure in artificially high CO2 levels. Attempts to extrapolate from short-term to long-term responses raise fundamental questions concerning evolutionary change in response to rising global CO2 levels. We consider the improved water use efficiency observed under elevated CO2 levels to be the main driving force of natural selection affecting the genotypic component controlling stomatal density. Whether a response is merely phenotypic or becomes incorporated into the genotype depends on two factors: (i) the time scale of exposure and (ii) the generation time of a species. Measurements of stomatal density on fossil leaves of Salix herbacea through a glacial cycle covering the last 140 000 years have shown a decrease in stomatal density in response to the rising CO2 levels of this period. This accords with the shorter-term observations on leaves of trees seen in herbarium specimens where the stomatal density has decreased in response to the rising CO2 levels of the last 200 years. The results indicate that natural selection over the 140000-year period may have favoured a similar response to that shown by trees phenotypically over the last 200 years. Since there is now some evidence for the genetic control of stomatal density, the role of natural selection affecting it must be considered when translating responses from short-term experiments to predict how stomatal density will be affected by long-term climatic and atmospheric change.

Heritability of Brain Size and Surface Features in Rhesus Macaques (Macaca mulatta)

The extent of heritability for overall brain size and regional cortical surface features such as sulcus lengths is important for demonstrating a genetic component to the observed phenotypic differences among individuals and for evaluating the potential for evolutionary change in response to selection. Although the genetics of brain size has been extensively considered, the detailed morphology of the cortical surface has not previously been subjected to genetic analysis. We estimated the heritability of brain size and cortical sulcus lengths using 438 endocranial casts taken from skeletons of rhesus macaques (Macaca mulatta) from the Cayo Santiago population. Estimates were obtained both by mother-offspring regression and symmetric-differences-squared (SDS) methods. Brain size, measured as cranial capacity, was highly and significantly heritable in this population, confirming results of previous studies with laboratory mice. Overall, cortical sulcus lengths were also heritable, with 35% of the sulci significantly heritable at the 5% level in the mother-offspring analysis. The average mother-offspring heritability estimate, 0.31, was the same as the average heritability obtained previously from a series of 56 cranial metric characters. The SDS analyses generally corresponded to the findings based on mother-offspring regressions, although the significance test appeared more conservative. Both gross and detailed morphology of the brain are heritable.

Population characteristics of Australian leafrollers (Epiphyas spp., Lepidoptera) infesting orchards

Abstract Light‐brown apple moths (LBAMs) are Australian leafrollers of the genus Epiphyas, capable of infesting orchards and other horticultural crops. Currently, five species are recognized, two of which, E. postvittana and E. pulla, can be serious economic pests. By means of principal‐components and group‐average cluster analyses applied to six parameters of demographic performance, two forms suspected of being interspecific hybrids were tested comparatively with possible parent species, and were found to be indistinguishable from E. postvittana. The frequency distribution of cornuti, a structural feature of the male genitalia used in identification, was investigated experimentally in the two economic species and their hybrids. The numerical variability of this character was shown to be much greater than had been recognized previously. It was found also that, in E. postvittana, cornuti frequencies may change in response to environmental influences. Hybrids tended to have cornuti numbers intermediate between those of their progenitors.LBAMs seem to be undergoing rapid evolutionary change in response to man‐induced alterations in the environment, and represent a hazard to horticulture not only in Australia, but possibly also in any temperate or sub‐tropical region to which they might gain access overseas. The situation would justify undertaking a systematic, Australia‐wide, survey of these and related leafrollers.