Synthesis theory of evolution provides explanation for transformation of a species by natural selection and for splitting of a species into reproductively isolated subgroups. The latter process is called speciation.
Modern synthetic theory recognizes five basic process:
1. Gene mutation
2. Changes in chromosome structure and number
3. Genetic recombination
4. Natural selection
5. Reproductive isolation.
First three provide genetic variability without which changes cannot take place. Natural selection and reproductive isolation guide population into adaptive channels. Four basic processes (i) mutation, (ii) genetic recombination (which results from inter-crossing between individuals of population as well as between them and occasional new genotypes which enter by migration), (iii) structural changes in chromosomes (which affects linkage and epistatic interaction of genes), and (iv) natural selection interact to produce a progressive change in population, which keeps it adopted to changing environment.
Three accessory processes affect the working of these five basic processes. Migration of individuals from one population to another, as well as hybridization between races or closely related species both increases the amount of genetic variability available to a population.
Effects of a chance, acting on small populations, may alter the way in which natural selection guides the course of evolution. Mutation, genetic recombination and natural selection are equally indispensable for evolutionary change to take place.
An evolutionary line of organisms can be compared to an automobile being driven along highway. Mutation corresponds to gasoline in tank. Since it is the only possible source of new genetic variation, it is essential for continued progress, but it is not the source of motive power. This source is genetic recombination through shuffling of genes and chromosomes, which goes on during sexual cycle.
Since this process provides immediate source of variability upon which selection exerts its primary action, it can be compared to engine of an automobile. Natural selection, which directs genetic variability towards adaptation to environment, can be compared to driver of vehicle. Structural changes in chromosome can have profound effects upon interrelationships between genetic recombination and natural selection and so can be compared to transmission and accelerator of automobile.
Finally, reproductive isolation, which includes all of the barriers to gene exchange between populations, has a canalizing effect similar to that which the highway, with its limits and directive signs, exerts upon driver of automobile, thus permitting several vehicles to drive in same direction at same time.