Synthetic Theory of Evolution - Basic factors involve in the process of organic evolution.
Synthetic Theory of Evolution
1. The modern concept of evolution is the synthesis of Darwin's and Hugo de Vries's theories. It is the result of the Work of a number of scientists namely T. Dobzhansky, R.A. Fisher, J.B.S. Haldane, Sewall Wright, Ernst Mayr, and G.L. Stebbins.
2. According to the synthetic theory, there are five basic factors involved in the process of organic evolution.
|. Genetic variations: The evolution of species occurs through the accumulation of genetic variations in the gene pool of a population over a long period of time. Variations occur in many ways —
(i) Mutations: Gene mutation is the permanent random alteration of the nucleotide sequence of a gene. Gene pool is the
sum total of all the different genes and their alleles present in a population. Variations occur in the gene pool due to the change of gene frequency by mutation.
(i) Gene recombination: It occurs due to the dual percentage, an independent assortment of chromosomes, crossing over during meiosis, random fusion of gametes and formation of new alleles.
(iii) Gene migration (gene flow): It is the movement of alleles among populations by the migration of breeding individuals. Constant gene flow can occur between adjacent animal populations due to the migration of organisms.
(iv) Genetic drift: It is the drastic change in allele frequency when the population size becomes very small. it was proposed by Sewall Wright in 1930 and hence, also called Sewall Wright Effect. it occurs when a section of the population migrates or dies of natural calamity and thus the gene frequency of the remaining population gets altered. Two important examples of genetic drift are the founder effect and the bottleneck effect.
- Founder effect: When a few individuals or a small group of individuals called founders move from some large population and invade a new geographical region, the population in the new settlement may have different genotype frequency than that of the parent population. The descendants of the founder isolate in new areas and tend to have an allele ratio similar to the founders rather than to the source population.
- Bottleneck effect: The yearly or seasonal cycle phenomenon of decrease or increase in the size of a population causing
periodic squeezing of some of the genes in the gene pool is called the bottleneck phenomenon.
(v) Hybridization: it is the crossing of organisms that are genetically different in one or more characters. It helps in
the intermingling of genes of different groups of the same variety, species and sometimes of different species.
il. Inheritance of Variations: Transmission of variations or characteristics from parent to offspring is an important mechanism in evolution and is called heredity.
Organisms possessing hereditary characters that are helpful in the changing environment are favored in the struggle for existence.
iii. Natural Selection: It results from differential reproduction which means some members of the population have traits that enable them to grow up and reproduce at a higher rate and leave more surviving offspring in the next generation than others. They are selected by nature.
There are different kinds of natural selections based upon different organism-environment relationship.
Stabilizing selection or balancing selection: It favors average-sized individuals and eliminates extreme variants. It
reduces variations and does not promote evolutionary change.
Directional selection or progressive selection: It favors the phenotype which is non-average or extreme and then pushes the phenotype of the population in that direction. In this selection, the population changes in one particular direction.
Disruptive selection or diversifying selection: It is just the opposite of stabilizing selection, i.e., it favors individuals at both extremes and mostly eliminates members with mean expressions, thus, produces two peaks in trait distribution may lead to the development of two different populations.
if disruptive selection results information of many new species then it is termed as adaptive radiation.