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).
Regarding speciation at the landscape level, we can describe three forms. Allopatric speciation occurs when populations are geographically isolated from one another. Parapatric speciation occurs when two populations diverge while physically located side-by-side, caused by a spatial gradient of selective conditions known as a cline. It is important to note that a hybrid zone, or a zone between two diverging populations where the individuals are able to mate and produce hybrid offspring, is an essential component of parapatric speciation. Lastly, sympatric speciation occurs when populations diverge into new species within the same geographical area, usually due to resource competition.
Variation in the adaptations of populations is critical to the divergence of populations into new species. Some examples of adaptations in plants include photosynthetic and metabolic rates in plants in response to high and low sunlight, protective antifreeze compounds in cold-tolerant species, and differences in the rate of nutrient uptake and water regulation in response to heat. Some examples of adaptations in animals include smaller bodies for better heat regulation, differences in the types of foods that are eaten, differences in nutrient requirements, differences in ways to transport oxygen (such as through lungs or gills), ways to regulate temperature, and ways to regulate the amount of water leaving the body.