Because of topographical barriers, however, it was difficult for the central government to gain complete control over the entire country, except when an unusually strong dynasty was in power.
Aqueducts transport water across topographical barriers to their destination, taking various forms and traveling at different levels with respect to the ground such as pipelines and canals.
In these arrangements roads funnel traffic through neighborhoods (e.g., Tel Sheva) and no clear boundaries such as additional roads or topographical barriers are found between them (e.g., Arara and Kseifa).
When topographical barriers were associated with divergence patterns, the estimated divergence date for the clade coincided with the estimated time of barrier formation.
Despite the linear proximity between the former ranges of P. t. virgata and P. t. tigris, significant topographical barriers to dispersal exist.
The geographic distribution and topographical barriers can lead to difficulties in seed dispersal resulting in limited gene flow among populations (Hamrick and Godt 1996).
We conclude that the populations from the southeastern Qinghai-Tibet Plateau suffered severe isolation owing to orogenesis, which significantly reduced seed dispersal, so that gene flow through the seeds was confined between neighboring populations within areas without strict topographical barriers.
This suggests that the ridges between the M. stellata populations are topographical barriers that impede pollen flow, possibly because different environmental conditions on the ridges hinder movements of beetle pollinators between the valleys.
Based on the comparison of the indices of genetic diversity and differentiation between two lineages, we consider the topographical barriers in gene flow through seed dispersal to be the most likely factors leading to the higher divergence in the southern clade.
The topographical barriers combined with limited expenditure on public health, poor quality of care, a high turnover of service providers, a lack of drugs and supplies and a lack of ownership of health programmes by communities all contribute to issues of demand and supply.
