MECHANISM OF EVOLUTION
It is the population that evolves and not its individual members. The individual's role in the evolutionary process is to pass its genetic variation to its offspring. Mechanism of evolution involves three steps: variations ,their inheritance and natural selection.
Variations
• Evolution occurs through the accumulation of genetic variations in the gene pool of population over long periods of time
• The change in genes occurs in many ways such as mutations, genetic drift, gene migration, non-random mating, gene recombination and hybridization.
Mutations
• Gene mutation is a random change in the base sequence of a gene. It occurs by substitution, addition or deletion of one or more bases. The mutated gene may give rise to a new protein or may fail to produce any. This may change the phenotype (trait).
• Mutations can be of two types: Chromosomal aberrations and genetic variations Aberrations occur by deletion, inversion, translocation and duplication of a chromosome segment. The chromosomal aberrations result in loss of genes, new positions of genes or addition of genes. Gene variation results in change in gene frequency.
Genetic drift
• The theory of genetic drift was developed by geneticist Sewall Wright in 1930. It is also called as Sewall Wright Effect or scattering of variability
• The term genetic drift refers to chance elimination of the genes of certain traits independent of gene's useful or harmful effect when a section of population migrates or dies of natural calamity. It alters the gene frequency of the remaining population. Hence genetic drift is a mechanism of evolution that acts in concern with natural selection to change species characteristics over time. Two important examples of genetic drift are bottleneck effect and founder effect.
• Bottleneck phenomenon: A population may be formed of thousands of individuals but in the next season or next year only a relatively few individuals may survive. The few individuals form the progenitors for the future generation of the population which may multiply sporadically in the next generation and may decline after one or two generations. This yearly or seasonal phenomenon of cyclic fluctuation in population density causing periodic squeezing of some of the genes in a gene pool in random fashion is called Bottleneck phenomenon. The term was used by Stebbins.
• Founder effect: When a few individuals or a small group of individuals from some large population invades a new or isolated geographical region, they become the founders. These founders carry on a limited portion of the parental gene. Their gene pool may contain certain alleles in a very low frequency or may lack a few alleles. The descendants of the founder i e, the founder population or marginal isolates in new areas will tend to have allele ratios similar to the founders rather than to the source population.
The resemblance of the descendents to the founders is called founder effect or founder principle by Mayr.
Gene migration (Gene flow)
• If the migrating individuals interbreed with the members of the local population, these may bring many new alleles into the local gene pool of the host population. This is called gene migration.
• This addition or removal of alleles when individuals enter or leave a population from another locality is called gene flow.
Non-random mating
• Repeated mating between individuals of certain selected traits changes the gene frequency. The selection of more brightly coloured male bird by a female bird may increase the gene frequency of bright colour in the next generation
Gene recombination
It occurs due to the following reasons:
- Dual parentage.
- Independent assortment of chromosomes.
- Crossing over during meiosis.
- Random fusion of gametes.
- Formation of new alleles.
Hybridization
• It is the crossing of organisms which are genetically different in one or more traits (characters). It helps in intermingling of genes of different groups of the same variety, species and sometimes different species.
• All the above factors produce genetic variation in parental reproduction. In monoparental reproduction, three small aberrations and gene mutations are the sly serves of genetic variations.
Inheritance of variation
• The transmission of characteristics or variations from parent to offspring is called heredity which is an important mechanism of evolution.
• Organisms possessing hereditary characteristics that are helpful, either in the animal's environment or in some other environment, are favoured in the struggle for existence. Thus, the offspring are able to benefit from the advantageous characteristics of their parents.
Natural selection
It is differential reproduction which means some members of population have traits (genes) that enable them to grow up and reproduce at a higher rate and leave more surviving offsprings in the next generation than others, i.e, they are selected by nature. If differential reproduction continues for many generations, genes of the individuals which produce more offspring will become predominant in the gene pool (total gene content of a whole species) of the population.
Types of natural selection
Based upon different organism-environment relationships, recognized following different kinds of natural selections have been-
(i) Stabilizing selection
(ii) Directional selection
(iii) Disruptive selection
• Stabilizing selection or Balancing selection : It acts in the absence of large scale environmental change or directional changes for a long period. It favors an intermediate form and eliminates the extreme variants.
• Directional selection or Progressive selection: It produces a regular change within a population in one direction in respect to certain characteristics. This is due to change in the environment in a particular direction. It favours individuals that change in response to the environmental
change and become best adapted to new environmental or ecological conditions. It favours the phenotype which is non-average or extreme and then pushes the phenotype of the population in that direction.
• Disruptive selection or Diversifying selection : It is just the opposite of stabilizing selection i.e., the extremes have more adaptable phenotypes than the average ones. Consequently, the original population is disrupted into two more separate groups that later evolve into new species. If disruptive selection results in many new species then it is termed as adaptive radiation. This kind of selection is rare.