Natural selection is the most important way that evolution can take place -- but it's not the only way. Another important mechanism of evolution is what biologists call genetic drift, when random events eliminate genes from a population. Two important examples of genetic drift are founder events and the bottleneck effect.
Imagine you have a jar containing three different colors of marbles: red, yellow and green. If you pick just two or three marbles out of the jar, it's possible you might pick all yellow and red just by chance. If the different colors of marbles were different genes and the three marbles you picked were a new population, the new population would have only red and yellow genes but no green ones -- and that's very similar to the way founder events affect genetic variation. When a small group separates from a larger population and strikes out on its own, that small group might be carrying genes that are rare in the original population. These rare genes will now be common among the new group's descendants. Other genes present in the original population, however, may be absent from the new group altogether. Huntington's Disease, for example, is more common among the Afrikaner or Dutch-descent population of South Africa than in most other populations, because a gene for Huntington's happened to be unusually common among the small group of original Dutch colonists.
Bottleneck effects happen when some catastrophe, like an earthquake or a tsunami, kills off most of a population at random and leaves only a handful of survivors. The catastrophe has to be something that strikes at random, however, and kills individuals irrespective of the genes they carry. A plague that only killed individuals lacking a particular gene would be an example of natural selection, and not a bottleneck effect, because it kills individuals with a specific genetic makeup, rather than striking at random. Bottleneck effects dramatically reduce genetic diversity because most of the population dies and the genes carried by diverse individuals perish with them. Northern elephant seals, for example, were hunted almost to extinction in the late 19th century; at one point there were as few as 20 left alive. Their population rebounded to more than 30,000 over the following century, but there is much less genetic variation among northern elephant seals than among southern populations, which did not undergo such intense hunting.
Both population bottlenecks and founder events have similar effects: they reduce the amount of genetic diversity in a population. Some genes are eliminated from the population, while others that may originally have been rare now become common. The important similarity between founder events and population bottlenecks is their randomness. In natural selection, the genes with the best survival qualities are the ones that get passed on to the next generation. In a founder event or a population bottleneck, the genes that get passed on aren't necessarily any better than the ones that got eliminated -- they were just favored by chance.
The difference between founder events and population bottlenecks is the type of event that causes them. A founder event occurs when a small group of individuals is separated from the rest of the population, whereas a bottleneck effect occurs when most of the population is destroyed. The end result is very similar -- genetic diversity is reduced. But the type of event leading to that result is very different, and that's why these two types of genetic drift are classified separately.