How does natural selection cause sympatric speciation? The geographical location is the same, I don't see?

Sympatric speciation is based on methods of isolation other than geographical isolation.  For example, a species may become separated into two by way of some behavioural difference.

As good example would be a species of butterfly that lays its eggs on a particular plant.  The adults meet and mate on this type of plant.  The caterpillars feed on the plant so that they become imprinted with that plant - when these caterpillars become butterflies, they seek out the same type of plant.

Now lets say that a female buterfly makes a mistake and lays her eggs on the wrong plant.  Now her offspring become imprinted on the wrong plant - they only meet and mate with other butterflies whose parents have made the same mistake (possibly their own siblings, at least initially).  Now the two 'branches' of the same species are separated by their choice of plant - and each will never meet the other and thus each will start to evolve and speciate separately.

well basically, allopatric speciation usually leads to sympatric speciation. let me explain, as you know for allopatric there is a barrier (e.g. river, mountain...)  between two groups of the same animals. as a result of the separation, they grow in a different condition. and now let's say they meet or the barrier was corrupted. even though, they are the same species..they grew apart and developed features that are very different from each other, as a result they can't  reproduce. so they can be classified as two different types of species of the same animal, but have different features.