Evolution - Synthetic Theory of Evolution| NCERT concept | Neet Concept | Biology

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Synthetic Theory of Evolution 

• Modern concept of evolution is the synthesis of Darwin's and Hugo de Vries 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 .
• According to synthetic theory there are five basic factors involved in the process of organic evolution .

Genetic variations :

• Evolution of species occurs through the accumulation of genetic variations in the gene pool of population over 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 gene pool due to the change of gene frequency by mutation .
• ( ii ) Gene recombination : It occurs due to the dual percentage , 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 . A 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 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 founder effect and 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 new settlement may have different genotype frequency than that of parent population . The descendants of the founder isolate in new area and tend to have 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 size of a population causing periodic squeezing of some of the genes in gene pool is called bottleneck phenomenon .
• ( v ) Hybridisation : It is the crossing of organisms which are genetically different in one or more characters . It helps in intermingling of genes of different groups of the same variety , species and sometimes of different species . 

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 favoured in the struggle for existence .

Natural Selection :

• It results from differential reproduction which means some members of 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 .
• Stabilising selection or balancing selection :
• It favours average sized individuals and eliminates the extreme variants . It reduces variations and does not promote evolutionary change .
• Directional selection or progressive selection : It favours the phenotype which is non - average or extreme and then pushes the phenotype of the population in that direction . In this selection , population changes towards one particular direction .
• Disruptive selection or diversifying selection : It is just the opposite of stabilising selection , i.e. , it favours individuals at both extremes and mostly eliminates members with mean expressions , thus , produces two peaks in trait distribution that may lead to development of two different populations .
• If disruptive selection results in formation of many new species then it is termed as adaptive radiation .

Industrial melanism is an adaptation where the moths living in an industrial area developed melanin pigments to match their body to the tree trunks . It can be explained briefly as follows :

• Before industrialisation , the peppered moth ( Biston betularia ) existed in two forms - light colour ( white winged ) and melanised ( dark winged ) .
• Bark of the trees in the past , were covered by whitish lichens , so , white moths were unnoticed by predator birds and dark coloured moths were eaten .
• After industrialisation , tree trunks became dark due to industrial smoke and soot , so the white moths were selectively picked up by birds and black moths remained unnoticed . This resulted in increase in population of black moths and decrease in population of white moths . 
• Thus , industrial melanism supports evolution by natural selection

Isolation :

• it is the prevention of mating amongst interbreeding groups due to physical , ( i.e. , geographical , ecological ) and biotic , ( i.e. , physiological , behavioural , genetic , mechanical ) barriers .
• Reproductive isolation is the prevention of interbreeding between the two populations of two different species . Thus , it preserves the integrity of a species by checking hybridisation although it may lead to the origin of a new species by accumulating variations in a population .

Speciation :

• The populations of species separated by geographical and physiological barriers , accumulate different genetic differences due to mutations , recombination , hybridisation , genetic drift and natural selection . These populations undergo morphological and genetic differentiation , become reproductively isolated and form new species . Speciation is of following types :
• Allopatric speciation : A part of the population becomes geographically separated from the main population and gradually forms a new species . E.g. , Darwin's finches in Galapagos Islands .
• Sympatric speciation : It occurs without geographical isolation . A small segment of the original population becomes isolated reproductively , a new subspecies emerges and gradually a new species is formed .
• Parapatric speciation : It occurs when a population of a species enters a new niche or habitat . It results in barrier of gene flow between the populations of new niche and two species are formed due to reproductive isolation .
• Quantum speciation : The budding off of a new and very different daughter species from a semi - isolated peripheral population of the ancestral species .

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MANISH MEVADA

M.Sc, M.Phil, B.Ed

GUJARAT BIOLOGY NEET

NEET MATERIAL IN GUJARATI 

KNOWLEDGE ON THE WAY.