A fundamental question in ecology is the question of how the vast diversity of species that we find on this planet originate.
Key to this is the idea of reproductive isolation, or the group of mechanisms which prevent two groups of living organisms from breeding with each other. Reproductive isolation can be subdivided into two majour groups: prezygotic mechanisms whereby species physically do not mate or have fertilization occur, such as in species living in different habitats or in species choosing not to mate with one another, and postzygotic mechanisms, where fertilization and conception occur, but the embryo either has very low fitness, may be sterile (they are unable to have offspring), or the offspring of the new young are sterile. An example of the latter are the offspring of donkeys and horses, known as mules which, while able to survive on their own, are unable to have viable offspring. Postzygotic mechanisms can occur due to changes in the chromosome number of the offspring, rearrangement of the chromosomes themselves, Haldane’s rule (the sex which has only one of each sex chromosome, such as male humans with their single X and single Y chromosome compared to the females with two X chromosomes, is more susceptible to harmful mutations since they only have one copy of each chromosome), and the Dobzhansky-Muller incompatibility (when random mutations in the genes of two different populations that have divided due to some, potentially geographic, barrier cause the hybrids to not be viable when the populations are mixed again).
