The following points highlight the six main factors that cause genetic divergence in species. The factors are: 1. Mutation 2. Recombination 3. Genetic Drift 4. Natural Selection 5. Hybridization 6. Polyploidy.
Genetic Divergence: Factor # 1. Mutation:
Gene mutations bring about genetic divergence. It is due to the changes in the chemical composition of gene by substitution, insertions, deletions and inversion of nitrogenous bases. Mutations are mostly harmful. But sometimes beneficial mutations too takes place which serves as raw material for speciation. Selection will operate either for or against the mutated gene. In either case gene frequencies will change.
In haploid organisms, mutated character, whether dominant or recessive, will be expressed immediately. But in a diploid organism only the dominant mutation will show its effect even in heterozygous condition immediately. Nevertheless, recessive mutations may spread through a population. Once the recessive mutation has established in homozygous state it is exposed to natural selection.
Genetic Divergence: Factor # 2. Recombination:
Reshuffing of genes promote genetic variability and plays a limited role in evolution. Crossing over at the time of meiosis, independent segregation or assortment of genes at the time of gamete formation and even chromosomal aberrations especially inversions and translocations, all result in the new combination of genes.
Recombination may be advantageous or dis-advantageous for the survival of the organism. The advantageous combinations are favoured by natural selection.
Genetic Divergence: Factor # 3. Genetic drift:
The frequency of any given allele in a population, relative to other alleles at the same locus, is known as the gene frequency. It was proved mathematically by G.H. Hardy in Britain and W. Weinberg in Germany in 1908 that, provided there are no disruptive influences such as mutation or selection, the frequency of alleles in a population remain constant generation after generation. This is known as the Hardy-Weinberg principle.
There will be a continual movement of genes or gene flow, within the population as a result of breeding, but overall gene frequencies remain constant This stability is referred to as genetic equilibrium. But due to loss of an allele or isolation, the gene frequencies exhibit great fluctuations generation after generation in a population.
This results in genetic drift. The genetic drift may be directed “steady drift” or undirected “random drift” in gene-frequency in a population.
The random drift in a population depends upon following factors:
1. The number of breeding individuals.
2. The selective value of the allele.
3. Mutation pressure.
4. Gene flow.
Genetic Divergence: Factor # 4. Natural Selection:
Natural selection is important in the perpetuation or elimination of the genes carried by individuals in a population. There are two times of natural selections operating in the evolution. The first one is stabilizing selection. It normally occurs only when the environment remain constant. It is most destructive against those individuals that deviates from normal.
The second one is the progressive or directional selection. In a changing environment, a deviated (new) character may be beneficial to the organism. Selection favours a new character, so help in emergence of new forms. From evolutionary point, progressive or directional selection play a more important role in the origin of species.
Still there is a third type of natural selection known as disruptive selection. This acts to break up a previously homogeneous population into several different adaptive forms. So it is the only type of natural selection that tends to increase the amount of variance within population.
Genetic Divergence: Factor # 5. Hybridization:
It is the crossing of two genetically different individuals. Interbreeding among the individuals of different populations of a species results in the introduction of new genes in a gene pool. Hybridization can also take place between the individuals of different species. This generally takes place in plants than the animals.
The hybrids of different species naturally have a gene complement or chromosomal complement of it two parental species. They exhibit characteristics somewhat different from both the parents. These are reproductively isolated from both species and therefore, form a new species.
Genetic Divergence: Factor # 6. Polyploidy:
It is the change in the number of chromosomes of the individuals. The individuals with increased number of chromosomes sets are polyploids. Interspecific hybridization combined with polyploidy results in the origin of new species.
Some of the examples are the evolution of Townsend-grass, new world cotton, etc. in nature and production of Raphanobrassica artificially. All these are allotetraploids. The polyploidy adds no new genes to a gene pool, but results in new combinations especially in allopolyploids.