05 Origin and Evolution of life - part 07 - Modern Synthetic Theory of Evolution

05 Origin and Evolution of life - part 07 - Modern Synthetic Theory of Evolution

Modern Synthetic Theory of Evolution : 

• It is the result of true synthesis of all biological discipline. 
• Studies pertaining to genetical, ecological, anatomical, geographical, palaeontological etc. were persued to explain mechanism of evolution. 
• Also due importance was given to both mutations and natural selection. 
• R. Fischer, J. B. S. Haldane, T. Dobzhansky J. Huxley, E. Mayr, Simpson, Stebbins, Fisher, Sewall Wright, Medel, T. H. Morgan etc. are the main contributors of modern theory of evolution. 
• Stebbins in his book discussed five key factors such as - 

1. gene mutations
2. mutations in the chromosome structure and number
3. genetic recombinations 
4. natural selection and 
5. reproductive isolation contributed in the evolution of new species.

• All individuals of the some species constitute apopulation. 
• The populations occur in small groups of `interbreeding populations'. Such small interbreeding group of a population is referred as `Mendelian population'. 
• The total genetic information encoded in sum total of genes in a Mendelian population is called gene pool. 
• Simply, gene pool means the total number of genes of all individuals in a population. 
• The gametes produced by individual furnish a pool of genes of next generation from which the genes will be selected. 
• The migration of population effectively alter the gene pool. 
• The gene pool also changes due to replacement of one generation by another in the Mendelian population. 
• Thus any change in the gene pool affects population. 
• Genes are arranged linearly on the chromosome having definite positions. According to Mendel, every gene that influences a trait has two alleles. 
• The proportion of an allele in the gene pool, to the total number of alleles at a given locus, is called gene frequency. 
• Modern synthetic theory comprises five main factors that are broadly divided into three main concepts like - 

1. genetic variations caused due to various aspects of mutation, recombination and migration. 
2. natural selection and
3. isolation, for explaining the evolution of species.

1. Genetic variations : 

• The change in gene and gene frequencies, is known as genetic variation. 
• Genetic variations are caused by following factors : 

I. Gene Mutation : 

• Sudden permanent heritable change is called mutation. 
• Mutation can occur 

1. in the gene, 
2. in the chromosome and 
3. in chromosome number. 

• Mutation that occurs within the single gene, is called point mutation or gene mutation. 
• This leads to the change in the phenotype of the organism, causing what is called variation. 

II. Genetic recombination :

•  In sexually reproducing organisms, during gamete formation, exchange of genetic material occurs between non-sister chromatids of homologous chromosomes. This is called crossing over. 
• It produces new genetic combinations which result in variation.
• Fertilization between opposite mating gametes leads to various recombinations resulting into the phenotypic variations causing change in the frequencies of alleles. 

III. Gene flow : 

• Gene flow is movement of genes into or out of a population. 
• Gene movement may be in the form of - 

1. migration of organism or 
2. gametes (dispersal of pollens) or 
3. segments of DNA (transformation). 

• Gene flow also alters gene frequency causing evolutionary changes. 

IV. Genetic drift : 

• Any random fluctuation (alteration) in allele frequency, occurring in the natural population by pure chance, is called genetic drift. 
• For example, when the size of a population is severely reduced due to natural disasters like earthquakes, floods, fires, etc. cause elimination of particular alleles from a population. 
• Smaller populations have greater chances for genetic drift. It will result in the change in the gene frequency. 
• Genetic drift is also an important factor forevolutionary change. 

V. Chromosomal aberrations : 

• The structural, morphological change in chromosome due torearrangement,is called chromosomal aberrations. 
• It changes the genes arrangement (order or sequence) that results in the variation. 
• Chromosomal aberrations occur due to - 

a. Deletion : 

• Loss of genes from chromosome.

b. Duplication : 

• Genes are repeated or doubled in number on chromosome. 

c. Inversion :

• A particular segment of chromosome is broken and gets reattached to the same chromosome in an inverted position due to 1800 twist. 
• There is no loss or gain of gene complement of the chromosome. 

d. Translocation :

• Transfer (transposition) of a part of chromosome or a set of genes to a non-homologous chromosome is called translocation. 
• It is effected naturally by the transposonspresent in the cell.

2. Natural selection :

• According to Darwin, natural selection is the main driving force behind the evolution. 
• This holds that genetic variations rise within the population. 
• The `fittest' will be at the selective advantage and will be more likely to produce offsprings than the rest, as the `fit' continues to enjoy greater survival and reproductivity, new species will eventually evolve. 
• Alternatively, natural selection is the process by which better adapted organisms grow and produce more number of offsprings in the population. 
• It brings about evolutionary changes by favouring differential reproduction of genes that bring about changes in gene frequency from one generation to next generation.
• Selection against harmful mutations leads to amutation balance in which allele frequency of harmful recessives remain constant generation after generation.
• Natural selection encourages those genes or traits that assure highest degree of adaptive efficiency between population and its environment. 
• Industrial melanism is one of the best example for natural selection.
  
• Thus natural selection has resulted in the establishment of a phenotypic traits in changing the environmental conditions.

3. Isolation : 

• Isolation is the separation of the population of a particular species into smaller units which prevents interbreeding between them. 
• Some barrier which prevents gene flow or exchange of genes between isolated populations, is called isolating mechanism. 
• Number of isolating mechanisms are operated in nature and therefore divergence and speciation may occur. 
• The isolating mechanisms are of two types namely - 

1. Geographical isolation and 
2. Reproductive isolation. 

I. Geographical Isolation : 

• It is also called as physical isolation. 
• It occurs when an original population is divided into two or more groups by geographical barriers such as river, ocean, mountain, glacier etc. These barriers prevent interbreeding between isolated groups. 
• The separated groups are exposed to different kinds of environmental factors and they acquired new traits by mutations. 
• The separated populations develop distinct gene pool and they do not interbreed. Thus, new species have been formed by geographical isolation. 
• E.g. Darwin's Finches. 

II. Reproductive Isolation : 

• Reproductive isolations occurs due to change in - 

1. genetic material
2. gene pool and 
3. structure of genital organs. 

• It prevents interbreeding between population. 

Types of Isolating Mechanisms :
A. Pre-mating or pre-zygotic isolating mechanism : 

• This mechanism prevent fertilization and zygote formation. 

i. Habitat isolation or (Ecological isolation) : 

• Members of a population living in the same geographic region but occupy separate habitatsso that potential mates do not meet.

ii. Seasonal or temporal isolation : 

• Members of a population living in the same geographic region but are sexually mature at different years or different times of the year. 

iii. Ethological isolation : 

• Due to specific mating behaviour the members of population do not mate. 

iv. Mechanical Isolation :

• Members of two population have difference in the structure of reproductive organs. 

B. Post-mating or Post-zygotic barriers :
i. Gamete mortality - 

• Gametes have a limited life span. 
• Due to one or the other reasons, if union of the two gametes does not occur in the given time, it results in the gamete mortality. 

ii. Zygote mortality - 

• Here, egg is fertilized but zygote dies due to one or the other reasons. 

iii. Hybrid sterility - 

• Hybrids develop to maturity but become steriledue to failure of proper gametogenesis (meiosis). e.g. Mule is an intergeneric hybrid which is sterile.