Evolution depends upon the origin of new populations and species. There are two distinct ways in which new species arise from pre-existing ones.
II. Transformation in time.
This causes increase in the existing number of species. Multiplication of species by major mutations was supported by De Vries and recently by Goldschmidt.
Mutational speciation is not supported by modern geneticists because observed drastic mutations are hightly deleterious, the second, method of speciation in sympetric speciation. The sympetric speciation is the establishment of a new reproductive isolated species population within the normal cruising range of reproductive individuals of the parental species.
According to May this method does not explain the genetic repeating of species. There are a number of evidences that speciation is usually done by geographic speciation.
A single population in a homogenous environment spreads to adjoining areas. This migration to new environment produces racial differentiation. In this way a polytypic species is evolved consisting of many races and subspecies. The subspecies may diverge from the parental population in such a way which decrease or prevent their ability to exchange of genes have been strongly developed, the subspecies rank as distinct species. Changes in the environment again permit geographically isolated population to exist together in the same region as sympartic species.
Sometimes it has been seen that two differently adapted populations or subspecies of a polytypic species can come together before reproductive isolation between them. In such case they can exchange to a limited degree results in hybrid.
The hybrids are adapted to environments different from those occupied by either of the parental populations. In animals the hybrids are most often unsuccessful because of their weakness or sterility. But some of them may give rise to a new population. The process of speciation occurs only in allopetric populations.
It cannot act on sympatric population inhibiting the same area. The sympatric species can arise either due to change in the chromosome number or due to introgressive hybridization and polyploidy. By hybridization and polyploidy a number of species can be formed in plants and a few in animals.
For examples: Diploid old world cotton (Gossypium herbaceum) having 13 pairs of chromosomes (26). Tetraploid New world cotton (Gossypium hirsutum) having 26 pairs of chromosomes (52). Similarly different species of Drosophila have different number of chromosomes. Drosophila melanogaster have 4 pairs of chromosomes (8): Drosophila willisotoni have 3 pairs of chromosomes (6): Drosophila uirilis having 6 pairs of chromosomes (12). Drosophils persimilis possesses 5 pairs of chromosomes (10).
Introgressive hybridization may form hybrid swarm which is a population in which F1 and F2 and late generations of hybrids segregation are intermingled with backcross progeny of various degrees. A backcross progeny will contain genes derived principally from one of the parental species and thus, it is most likely that it is well adapted to a habitat already present.
The best documented example of introgression is found in Raphanobrassica. In 1927, G.D. Karpechenko performed a cross between radish (Raphanus sativum) and cabbage (Brassica oleracea). and got Fj sterila hybrid as well as few fertile plants. Radish and cabbage each have 18 chromosomes (2n) while the Raphenobrassica has 36 chromosomes (4n). These hybrid tetraploids shared characters of both the parents; and were unable to interbreed with either of parent. Thus naturally formed a new species.
Generally hybridization can give rise only to populations which are intermediate between and share characteristics of the parental populations.