Causes of speciation <span>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. 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.
</span>Reduction of gene flow <span>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. 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.</span>
He is wrong because speciation can occur by reproductive isolation as well which does not require geographical isolation.
Explanation:
Speciation occurs through reproductive and geographical isolation. Separation of two populations by ant geographical barriers followed by accumulation of genetic variations in response to local climatic conditions leads to reproductive isolation, that is, speciation.
However, sympatric speciation occurs when two population living together in the same habitat. These populations are not allowed to interbreed due to the structural or behavioral mechanism of reproductive isolation.
