What's the difference between allopatric and sympatric speciation? What are some evolutionary implications of this?

For speciation to occur, a population must split into at least two isolated parts to avoid the spread of genes between them; the nature of this isolation is what defines sympatry and allopatry. In allopatric speciation, the population is divided geographically, such as by a new river or mountain range forming, or through the colonisation of a new island. Geographical separation like this often limits the spread of genes between the two populations by so much that there is no selection pressure to evolve any traits to help maintain or increase this separation. However, in sympatric speciation, the population is not divided geographically and therefore must be separated by other means. As the separation is necessary to limit the spread of genes between the two parts, they only really need to prevent mating between them; this is called reproductive isolation. This reproductive isolation can occur in many different ways, for example: changes in ecological preferences can create small scale spatial isolation, such as an insect evolving a preference for a new plant species to live and feed on; and changes in colouration or body size may reduce the attractiveness of one population to the other, reducing mating between the two. As opposed to allopatric speciation, sympatric separation processes often do not fully prevent the spread of genes, meaning that there is a selection pressure to evolve traits to maintain or increase this reproductive isolation. These changes can include the loss or gain of sexually attractive traits to reduce the occurrence of mating between the populations, changes in genital morphology to reduce the success of mating between them, or genetic and embryological changes to reduce the viability of any offspring created by parents from separate populations.