Why can isolation cause in differences in species and possibly lead to a new species?

I can't explain it to fit for your college thesis but take an example - a two families get stuck on an island. One family has only all fair haired children and one family has only dark haired children. For simplicities sake let's assume all the fair haired children are girls and all the dark haired children are boys. As new generations are born everyone on the island will eventually be mostly dark haired where initially the ratio was 50/50. This is because fair hair is a recessive genetic trait where dark hair is a dominant genetic trait. In order for a person to have the fair hair coloration they have to receive the fair hair gene from both parents and this would be impossible for the first generation of children born to these two families on the island. The second generation COULD however have fair haired children because every one of the first generation carried the recessive gene.

And this is only the most basic generalization.

How is this involved in making a new species? More genes are involved than just genes with hair - for example Japan is a small, and very isolated country (compared to other countries in the world). Japanese people all tend to have similar physical characteristics in facial structure, hair color, and body type. Same for Eskimos, but go to a place like Africa and you see a lot more variety. Some Africans are tall and lanky, some are more heavily built, and Africa has also had a lot of influence from Europe and Middle Asia especially around the Mediterranean and some in South Africa. Although different races of people aren't exactly different species it makes the same point.

And I haven't read it but I think Darwin's Origin of the Species addresses this better than any answer you'll find on Answers.com