Speciation is about how species form. It is a major part of evolutionary biology. Darwin thought most species arose directly from pre-existing species. This is called anagenesis: species by changing, or 'phyletic evolution'.
https://simple.wikipedia.org/wiki/Speciation
Scientists think that geographic isolation is a common way for the process of speciation to begin: rivers change course, mountains rise, continents drift, organisms migrate, and what was once a continuous population is divided into two or more smaller populations.
https://www.quora.com/What-are-the-Causes-of-speciation
Sympatric speciation occurs when populations of a species that share the same habitat become reproductively isolated from each other. This speciation phenomenon most commonly occurs through polyploidy, in which an offspring or group of offspring will be produced with twice the normal number of chromosomes. Where a normal individual has two copies of each chromosome (diploidy), these offspring may have four copies (tetraploidy). A tetraploid individual cannot mate with a diploid individual, creating reproductive isolation. Sympatric speciation is rare.
http://www.sparknotes.com/biology/evolution/speciation/section2.rhtml
Causes of speciation. Geographic isolation. In the fruit fly example, some fruit fly larvae were washed up on an island, and speciation started because populations were prevented from interbreeding by geographic isolation.
https://www.quora.com/What-are-the-Causes-of-speciation
Allopatric speciation, the most common form of speciation, occurs when populations of a species become geographically isolated. When populations become separated, gene flow between them ceases.
http://www.sparknotes.com/biology/evolution/speciation/section2.rhtml
Factors which prevent individuals from mating. 1  Geographic isolation: Species occur in different areas, and are often separated by barriers.  Temporal isolation: Individuals do not mate because they are active at different times. This may be different times of the day or different seasons.
https://simple.wikipedia.org/wiki/Speciation
Individuals in the far west would have zero chance of mating with individuals in the far eastern end of the range. So we have reduced gene flow, but not total isolation. This may or may not be sufficient to cause speciation. Speciation would probably also require different selective pressures at opposite ends of the range, which would alter gene frequencies in groups at different ends of the range so much that they would not be able to mate if they were reunited.
https://www.quora.com/What-are-the-Causes-of-speciation
This is thought to be the most comon cause of speciation. The first person to think of it was Moritz Wagner, a German explorer and natural historian. Wagner's early career was as a geographer, and he published a number of geographical books about North Africa, the Middle East, and Tropical America.
https://simple.wikipedia.org/wiki/Speciation
Reduction of gene flow. However, speciation might also happen in a population with no specific extrinsic barrier to gene flow. Imagine a situation in which a population extends over a broad geographic range, and mating throughout the population is not random.
https://www.quora.com/What-are-the-Causes-of-speciation
Parapatric speciation is extremely rare. It occurs when populations are separated not by a geographical barrier, such as a body of water, but by an extreme change in habitat. While populations in these areas may interbreed, they often develop distinct characteristics and lifestyles.
http://www.sparknotes.com/biology/evolution/speciation/section2.rhtml
what are the causes of speciation
Geographic isolation are the causes of speciation. One of the causes of speciation is geographic isolation.