how does speciation occur

A change in the environment occurs. Through random mutations already present in the population (this is known as natural variation) certain individuals will be able to cope with the change more so than others. These individuals are more likely to reach sexual maturity and so are more likely to pass down these advantageous alleles (a form of a gene) to their offspring. This makes the next population express the beneficial phenotype at a higher frequency in the population.This keeps occurring where the phenotype is more heavily expressed and exaggerated in each generation until the new population is so much more different than the ancestral population the two populations can no longer mate to produce fertile offspring.