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There are multiple ways to defining the species concept. The choice of which concept to use is dependent on the particularities of the species concerned.[207] For example, some species concepts apply more readily toward sexually reproducing organisms while others lend themselves better toward asexual organisms.[link widoczny dla zalogowanych] Despite the diversity of various species concepts, these various concepts can be placed into one of three broad philosophical approaches: interbreeding, ecological, and phylogenetic.[208] The biological species concept (BSC) is a classic example of the interbreeding approach. Defined by Ernst Mayr in 1942, the BSC states that "species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups"[209]:120. Despite its wide and long-term use, the BSC like others is not without controversy, particularly in prokaryotes,[210] and this is called the species problem.[207] Some researchers have attempted a unifying monistic definition of species, while others adopt a pluralistic approach and suggest that there may be a different ways to logically interpret the definition of a species.[207][208]"
Barriers to reproduction between two diverging populations are required for the populations to become new species. Gene flow may slow this process by spreading the new genetic variants also to the other populations. Depending on how far two species have diverged since their most recent common ancestor,[link widoczny dla zalogowanych], it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules.[211] Such hybridsare generally infertile. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species,[link widoczny dla zalogowanych] or possess a totally new phenotype.[212] The importance of hybridisation in creating new species of animals is unclear, although cases have been seen in many types of animals,[213] with the gray tree frog being a particularly well-studied example.[214]
Speciation has been observed multiple times under both controlled laboratory conditions and in nature.[215] In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms.[216][217] As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed.
The third mechanism of speciation is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations.[206] Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grassAnthoxanthum odoratum, which can undergo parapatric speciation in response to localised metal pollution from mines.[220] Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produced a gradual change in the flowering time of the metal-resistant plants,[link widoczny dla zalogowanych] which eventually produced complete reproductive isolation. Selection against hybrids between the two populations may cause reinforcement, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.