Condition for the emergence of species
In reproductive isolation, there is no more gene flow through one or more reproductive barriers. Reproductive barriers can have an intrinsic but also a strange effect and permanently lead to the emergence of a new species (exception: isolation through sterility).
Time isolation: Species can not reproduce because they reproduce during different seasons / times of the day (for example, frog species whose mating season is in different months).
Genetic isolation: By accidental mutations, individuals can no longer be reproductive with the original population.
Physiological isolation: Due to the different shape of the copulation organs, certain species can not reproduce with each other (e.g., in closely related insect species).
Ecological isolation: By exploiting different ecological niches in the same area, there is reproductive isolation (for example, Darwin's finches occupying different ecological niches, some individuals feeding on insects, others on grains).
Geographic isolation: Because of geographic barriers, sub-populations of one species can not reproduce among themselves (for example, by continental drift or an increase in sea level).
Behavior insulation: Different behavior during the mating season isolates species from each other (for example, closely related species of birds that mate at the same time have a different courtship call).
Sterilitдt: Crossing two unrelated species may produce bastards with an odd set of chromosomes that are not reproductive. (For example, in a donkey-and-horse cross, the offspring are all infertile because they have an odd set of chromosomes that makes gametes impossible to form).
polyploidy: By polyploidy is meant the presence of more than two chromosome strains. Reproduction is possible only among individuals with identical numbers of chromosome consorts. (For example, plants often form triploid or tetraploid chromosomes and are thus genetically isolated from their native haploid chromosome set).