A stroke can result in a number of visual problems, including visual neglect, decreased visual acuity, diplopia (double-vision), squint, and homonymous hemianopsia (visual field defects).
Natural Selection. Natural selection is the process by which individuals with characteristics that are advantageous for reproduction in a specific environment leave more offspring in the next generation, thereby increasing the proportion of their genes in the population gene pool over time.
Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined on average for the individuals within a population. The fitness of a particular genotype corresponds to the average effect on all individuals with that genotype.