In the classification of living organisms (taxonomy), you probably remember that a species is the most specific taxon:
Apart from being the most basic taxonomical category, species also refers to a group of closely related organisms that are able to interbreed and produce fertile offspring. Thus, a species is reproductively isolated from other species.
You may be familiar with certain hybrids- offspring produced from different species- such as the mule (female horse+male donkey), the yakalo (bison + yak) and the liger (lion + tiger). However, these hybrids are infertile and unable to reproduce.
New species can be formed as well from existing species due to differentiation within that species for a variety of reasons. This is known as speciation. Groups within a certain species may become isolated, reducing mating with the rest of the population. In this time, genetic differences develop that result in the rise of a new species. There are two types of speciation: allopatric speciation, caused by physical/geographic separation, and sympatric speciation, caused by ecological and behavioural differences within the species in the same geographical area.
In allopatric speciation, there arises some sort of physical or geographical feature that separates one population of a species into two different areas. For example, the formation of a new branch of a river, the creation of a valley through erosion, the rise of a new mountain range and the colonization of a new island could all be situations where the population is split up physically.
When this split occurs, the gene pool of both populations is now split as well. Gene flow, that is, the movement of genetic material (alleles and associated traits) between two populations is hindered by a physical barrier. As a result, the allele frequencies (frequencies of certain alleles and genotypic traits) of both populations become different.
If and when some genetic mutation occurs in one of the populations, the lack of gene flow between the populations means that this mutation will not make its way to the other population and spread. New alleles arise independently in each population via mutation and the allele frequencies between the two populations gradually become more and more different. (If you are unfamiliar with alleles and the concept of genetic variation and inheritance, please look at our previous post on the topic to get more information)
When these populations are split, they are exposed to new climates, predators, resources and competitors than before. Think of it like going to a new school- you are introduced to a new group of people and social situation than you were used to before. Just like how a new high school can be a place where only the strongest survive, the new environment of the split population will favour members of the species who have particular characteristics and adaptations. Natural selection, the survival and reproduction of individuals of a species due a difference in their genetic information, would play a part here. Now, both populations would be significantly differentiated, because members of the new population would be modified due to natural selection favouring useful adaptations in the new environment, while the old population would be undergoing its own changes.
In sympatric speciation, the original species is not split up by a physical barrier into geographically separate locations. Instead, the population is eventually differentiated while being in the same place.
Sympatric speciation may occur when two groups within the same species and in the same region become accustomed to two different habitats within that region. For example, one group of crickets of a certain species might begin to prefer loamy soil rather than the sandy soil preferred by other in the species. Thus, because they prefer two different environments, the two groups have a limited gene flow. This is known as habitat isolation.
Behavioural isolation occurs when the presence or absence of a specific courtship behaviour (sometimes an action, call, marking or colour) prevents reproduction from taking place. For example, male fireflies use specific light patterns to attract females. Various species of fireflies display their lights differently. If a small difference were to occur in the light pattern used or accepted by a certain group of fireflies, mating would be hindered, reducing gene flow once more.