How can geographical isolation change a populations gene pool?

Why is rapid evolutionary change more likely to occur in small populations?How can geographical isolation change a populations gene pool?
One reason is that for very small pools of individuals, initial-state variations become very important. Imagine, for instance, that a certain fish normally comes in two colors, black or red, red being slightly dominant evolutionarily. Suppose in the general population 10% of the fish are red. It will take a large number of generations before the population will all become predominantly red. Now imagine you select a random sample of only 10 fish and isolate them. There is a significant probability that you will have picked more than 1 red fish. So the switchover will happen faster.


Another reason is so-called bottle-necking which happens with very small gene pools and much interbreeding. This can create some ';wild swings'; in the population's gene pool.How can geographical isolation change a populations gene pool?
My example for these questions is the polar bear vs. brown bear. Consider a population of bears living in the North American continent. In the Artic where with snow the environment is very white. To be successful as a hunter, a more successful bear in the artic will be one that is camoflaged within its surroundings i.e white. Similarly a bear in the forested regions will be more successful if it is brown.





Therefore, in the artic bears that have the genes for white fur (polar bears) will be more successful and geographically this gene will predominate in the artic region.





Conversely, in lower latitudes and forested regions a polar bear would stick out like a sore thumb and not be a successful hunter. But brown fur bears will be successful. Hence in these geographical regions the genes that make brown fur will be more successful.





As the generations progress these genes become localized to the geographical regions.
Geographical location cannot change a gene pool. That is what Jean Lemark (French biologist, circa 1801) would have said. Lemark said that an organsim could change to fit an environment (his theory of need). Darwin (1860) said, due to variation, an organism is born adapted. DeVries (1901) identified mutation (he didn't know about DNA) as the cause of variation. So, geographical location cannot change, an organism is born 'changed'/adapted.





However, as species migrate to a common geographical location, the location has certain ';demands'; for survival. Thus, due to genetic variation and mutation, the traits of a species will change. Those changes will result with different species expressing similar phenotypes.





For example, in a cold environment, in some species it is an advantage to have dark areas on their bodies so as to absorb more light thus helping to maintain body temperature. This is found in snow rabbits and Siamese cats.





This kind of evolution is called convergent evolution.





Rapid evolution is more likely to occur in small populations simply due to the small size of the gene pool. If there is any change in the gene pool, due to its size, the change is spread rapidly relative to the size of the population. If a gene pool is large, any change would take longer to spread.