Gene flow and genetic drift in urban environments

Evidence is growing that human modification of landscapes has dramatically altered evolutionary processes. In urban population genetic studies, urbanization is typically predicted to act as a barrier that isolates populations of species, leading to increased genetic drift within populations and reduced gene flow between populations. However, urbanization may also facilitate dispersal among populations, leading to higher genetic diversity within, and lower differentiation between, urban populations. We reviewed the literature on nonadaptive urban evolution to evaluate the support for each of these urban fragmentation and facilitation models. In a review of the literature with supporting quantitative analyses of 167 published urban population genetics studies, we found a weak signature of reduced within‐population genetic diversity and no evidence of consistently increased between‐population genetic differentiation associated with urbanization. In addition, we found that urban landscape features act as barriers or conduits to gene flow, depending on the species and city in question. Thus, we speculate that dispersal ability of species and environmental heterogeneity between cities contributes to the variation exhibited in our results. However, >90% of published studies reviewed here showed an association of urbanization with genetic drift or gene flow, highlighting the strong impact of urbanization on nonadaptive evolution. It is clear that species biology and city heterogeneity obscure patterns of genetic drift and gene flow in a quantitative analysis. Thus, we suggest that future research makes comparisons of multiple cities and nonurban habitats, and takes into consideration species' natural history, environmental variation, spatial modelling and marker selection.     

On the structure of retina of a fresh-water mullet, Rhinomugil corsula (Mugilidae, Pisces).

1. The structure of the eye-ball of Rhimomugil corsula has been described. 2. A mid-horizontal ridge (band) along the vitreal surface of the retina in each eye of R. corsula has been observed which divides the retina into a superior and an inferior hemispheres probably adapted for aquatic (dim-light vision) and aerial (bright-light vision) vision. 3. The superior hemisphere has been provided with poorly developed choroid gland and scarce pigment in the pigment epithelial cells. 4. The visual cell layer in the retina of the superior hemisphere contains both single and twin cones. They are alternately arranged in parallel rows. 5. The external and internal limiting membranes have been observed in both the hemispheres under light microscope. 6. The outer plexiform layer of the superior hemisphere contains many horizontal cells arranged into four distinct rows. 7. The inferior hemisphere has a well developed choroid gland, and dense pigment in the pigment epithelial cells.     

Changes in uterine phosphatase levels in mice deprived of LH during early pregnancy.

Mice injected with normal rabbit serum on Day 4 of gestation showed a progressive increase in specific alkaline and acid phosphatase activities on Days 6 and 7. Mice injected with antiserum to LH on Day 4 showed a significant decrease in specific alkaline phosphatase activity estimated biochemically on Days 6, 7 and 8, but this decrease was histochemically evident only on Day 8. Similarly, acid phosphatase activity in antiserum-treated mice was significantly increased on Days 6, 7 and 8 when estimated biochemically, but only on Day 8 when studied histochemically.     

Effect of environmental factors on the activity of thyroid gland in an air breathing siluroid fish Heteropneustes fossilis Bloch.

1. Studies on cyclic activity of thyroid in relation to environmental factors were made in an air breathing siluroid fish, H. fossilis. 2. The activity of thyroid gland showed parallelism with water temperature and photoperiod but from the regression analysis it was evident that the effect of photoperiod on thyroid activity is more pronounced than the effect of water temperature. 3. The maximum thyroid activity coincides well with the breeding phase of this animal.