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A bottleneck population rsults when the population size is drastically reduced for some reason, such as if most of the population is destroyed by a natural disaster, like a flood. This usually ends up severely reducing the gene pool.

The founder effect refers to a gene pool that originates from a small population moving into a new area; the few organisms that begin the population bring only a few alleles with them to start the new gene pool.

Genetic drift is a random change in allele frequencies in a gene pool. Frequently, a bottleneck population is considered to be an example of genetic drift because it is a chance occurrence...

The Genetic Drift is the random changes that occur in the gene frequency of small isolated populations, resulting in the loss or preservation of genes over the generations.

The hypothetical flood created what is called a population bottleneck. It reduced the genetic variation in the smaller population such that, even if the group’s number again reached 12 members, its genetic diversity might very well be lower than the genetic diversity of the original population. All of the descendants came from just a few surviving individuals, who carried just a fraction of the alleles present in the former population. Likewise, when a few individuals leave a large population and establish a new one, they bring only a fraction of the genetic diversity of the original population with them. Any descendants of the founding members face the possibility of a drastically reduced genetic diversity. An example of this principle, known as the founder effect, is evident in the Amish community in Pennsylvania. All of the people in this community are descendants of about 200 individuals who established the community after leaving Europe in the early 1700s. One of these founders carried an unusual allele that causes a rare kind of dwarfism. As a result, in the Pennsylvania Amish community today the frequency of this rare allele is 1 in 14 individuals. In the general population this allele appears in 1 in 1,000 individuals.

The chances of a tossed coin coming up heads is .5. The chances of tails is also .5. And if you throw thousands of tosses, you'll find that the proportion of heads and tails is very close to .5 : .5. But if you throw only, say, four tosses, it's very possible that you might come up with four heads and no tails, or four tails and no heads, or one heads and three tails, or one tails and three heads. It works the same with genes. A gene in a large population will stay at the same frequency, but if there are only a few reproducing individuals, there's a good chance that the gene will either get lost in the next generation or will increase in frequency. The tendency of gene frequency to drift either up or down in small populations is called "genetic drift."

Let's say we have red birds, yellow birds, and blue birds in an island. They are all equal in number.

A) Bottleneck

There was a forest fire and most of the birds died. By chance, many red birds survived and almost all blue birds and yellow birds died. Red birds reproduced. In few years, if you look at the island, you will see only or mostly red birds.

(Bottleneck is when a group of population died out from some causes, often natural disaster, and the group of population thrived.)

B) Founder's effect

In US, you see so many people with different hair colors. Let's say a group of red haired friends went sailing and got lost in a deserted island. They decided to just live there. They reproduce and make red haired babies. Their babies reproduce and make more red hair babies. After many many years, if someone found the island, you see the people in the island have red hair.

Darwin said survival of the fittest. Whoever is superior(healthy people, good looking people, successful people, etch...) tend to reproduce more, so their offspring will carry on the genes.

However, genetic drift is an exception. Red birds didn't survive because they were better than blue or yellow birds. They just happened to survive. It could have been blue birds or yellow birds. The red hair people didn't survive because they are fit/superior. The founders of the lands happened to have all red hair.

Hope this helps,