Theories of Evolution of Class 12

Defined as “dissimilarities of features among members of the same species”. Offsprings of same parent are different and also differ from their parent. This is due to sexual reproduction. Variation occuring at gene level are inheritable hence are the raw material for evolution. The genetic constitution which governs such traits is called genotype. Phenotype is the physical expression of genes of an organism. Natural selection can act upon genetic variation only when it is expressed in the phenotype.

Somatogenic and Blastogenic Variation

Changes in somatic parts of the body are somatic variations e.g. wrestler’s muscle, circumcised penis of muslims, academic or cultural achievements etc., not inheritable, hence are no factor for evolution. Blastogenic variations are the changes at germinal cell level or in the genotype or gene patterns, these are inheritable and hence form the basis for evolution.

Continuous and Discountinuous Variation

Continuous variations are small, gradual changes e.g. changes in color, size, shape, body weight etc. The discontinuous variation are the discrete and sudden, without any intermediate stage or gradations. This is qualitative (substantive) when related to color, shape and size e.g., Ancon sheep in Seth Wright’s farm quantitative (meristic) when related to the number of parts e.g. - hexadactyli, single kidney, single horn etc.

Determinate and Indeterminate variation

Determinate variation are adaptive, selective and of definite evolutionary line, also called as orthogenic variation; controlled by strong genes, sometimes, it may become deleterious beyond the limit of its usefulness e.g. extra large antlers of Irish Elk and tooth of Smedolon. Indeterminate variations have no evolutionary line, it is sudden and nondirectional.

Causes of variation

Effect of environmental conditions

Change in the gene pattern like :

  • Random distribution of homologous genes in meiosis
  • Crossing over
  • Dual parentage
  • Random union of gametes during fertilization

Evolutionary agents are the forces that change the gene frequency of the population or deviation from Hardy-Weinberg equilibrium.

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