1) In populations, genetic variation must be present before natural selection can act upon the population. It is created by the direct action of directional natural selection. It tends to be reduced in a population when diploid organisms produce gametes. It arises in response to changes in the environment.
On the Galapagos islands, small iguanas can sprint faster than large iguanas. If predators (for example, cats) that preferably catch and eat slower iguanas are introduced to the island, then iguana body size is likely to decrease in the presence of cats; the introduction of cats would put the iguanas under disruptive selection.
2) If genetic drift occurs in a population, any of the statements can be true. Genetic drift may decrease the population's fitness, cause a decrease in genetic variation, or occur in a relatively small population.
3) A farmer uses triazine herbicide to control pigweed in his field. For the first few years, the triazine works well and almost all the pigweed dies. But after several years, the farmer sees more and more pigweed. The best explanation for this is that triazine-resistant weeds were more likely to survive and reproduce, leading to an increase in the population of pigweed.
4) Small populations suffer from higher rates of homozygosity due to all of the above factors: Crossing Over, Gene duplication, Mutations, and Genetic Drift.