Revision: Heredity and Evolution Science and Technology 2 SSC (English Medium) 10th Standard Maharashtra State Board

The transmission of characters from the parents to their offsprings is called heredity. 

The term heredity may be defined as "transmission of genetically based characteristics from parents to offspring".

or

A sequence of three nucleotides on mRNA that codes for a specific amino acid is called a triplet codon.

Mutation is a sudden change in one or more genes, or in the number or in the structure of chromosomes.

or

Mutation is a phenomenon which results in alteration of DNA sequences and consequently results in changes in the genotype and the phenotype of an organism.

Evolution is a slow and continuous process whereby complex forms of life have emerged from simpler forms through millions of years.

or

The origin of new species by gradual modification is called speciation.

The fittest individuals in nature are most likely to reproduce and pass on their good qualities to their offspring. It is called natural selection.

The modern version of Darwin’s theory of natural selection, which incorporates genetics as the source of variations, is called Neo-Darwinism.

Differences between individuals of the same species that arise naturally and can be passed to offspring are called variations.

The process of formation of a new species from the pre-existing species is called speciation.

According to A.E. Emerson, speciation is the evolutionary process by which new species arise due to genetic divergence and isolation.

• Mendel is the father of modern genetics; he discovered the basic principles of heredity.
• Hugo de Vries (1901) proposed the mutation theory explaining sudden genetic changes.
• Walter Sutton (1902) linked chromosomes with heredity through his study on grasshoppers.
• Avery, McCarty & McLeod (1944) proved DNA is the genetic material in living organisms.
• Jacob & Monod (1961) developed a model of protein synthesis, leading to recombinant DNA technology.

• Transcription is the process in which genetic information from one strand of DNA (template strand) is copied into RNA with the help of RNA polymerase.
• It occurs in the nucleoid in prokaryotes and in the nucleus in eukaryotes; mRNA then moves to the cytoplasm for translation.
• A transcription unit has three parts: promoter (start site), structural gene, and terminator (stop site).
• The process occurs in three stages: initiation (RNA polymerase binds promoter), elongation (RNA chain is formed), and termination (RNA polymerase detaches).
• Only one DNA strand acts as a template (3′→5′), while the other is the coding strand (5′→3′).
• In eukaryotes, primary RNA (hnRNA) is processed by capping, tailing, and splicing to form mature mRNA.

• Translation is the process in which mRNA codons are read to form a sequence of amino acids, producing a polypeptide (protein) on ribosomes.
• It requires amino acids, mRNA, tRNA, ribosomes, ATP, Mg²⁺ ions, enzymes, and release factors.
• Ribosomes are the site of protein synthesis and have three tRNA binding sites: A site, P site, and E site.
• Initiation begins with the start codon AUG, where the initiator tRNA binds at the P site, and ribosomal subunits join.
• During elongation, amino acids are added one by one, peptide bonds are formed, and tRNA shifts from A site to P site.
• Termination occurs when a stop codon (UAA, UAG, UGA) is reached, the release factor binds, and the polypeptide chain is released.

• Evolution is the gradual and progressive development of living organisms over a long period due to natural selection.
• Life originated around 3.5 billion years ago from simple inorganic and organic compounds, eventually forming primitive cells.
• The diversity of life on Earth today, from unicellular organisms to complex plants and animals, is a result of continuous evolutionary processes.
• According to the theory of evolution, all complex organisms evolved slowly from unicellular ancestors over 300 crore (3 billion) years.
• Evolution is considered an organizational and multi-dimensional process, reflecting structural and functional advancement in living organisms.

• Structures like human hand, ox foreleg, bat’s patagium, and whale flipper differ in function but have similar bone structure.
• This similarity in bones and joints across different animals suggests a common ancestry.
• Such internal structural similarities serve as strong anatomical evidence for evolution.

1. Darwinism - Proposed by Charles Darwin after extensive travels; published in the book "On the Origin of Species by Means of Natural Selection"; the essence of the theory is natural selection.

2. Galapagos Islands - Darwin observed variations in tortoises and finches across the islands of the Archipelago, which became key evidence for his theory.

3. Natural Selection - Nature selects only those organisms that are fit to survive; the rest perish — this is the concept of "Survival of the Fittest".

4. Competition & Modification - All organisms reproduce prolifically and compete for survival; only those with useful modifications win the competition and sustain.

5. Evidences for Darwinism -

• Long-necked Giraffe - Giraffes developed long necks to reach leaves; this adaptation became fixed and was passed to offspring.
• Black Peppered Moths - Gradually became more common due to industrial melanism.
• DDT Resistance in Mosquitoes - Some mosquitoes developed resistance to DDT, survived, and produced resistant offspring.

• Jean-Baptiste Lamarck proposed that morphological changes in organisms occur due to their activities or laziness.
• He introduced the principle of use or disuse of organs, leading to development or degeneration of body parts.
• He explained evolution through acquired characters, such as the long neck of giraffes or strong shoulders of blacksmiths.
• These acquired characters, according to Lamarck, are passed on to the next generation.
• Lamarck’s theory was later disproved, as acquired characters are not inherited genetically.

• Speciation - Process of formation of a new species from pre-existing species; a species is a group of similar organisms that can interbreed and produce fertile offspring.
• Intraspecific Speciation - Formation of new species within the same population; divided into allopatric and sympatric speciation.
• Allopatric Speciation - New species formed due to geographical isolation (mountains, glaciers, migration); mode of evolution is adaptive radiation; e.g. 14 species of finches in the Galapagos Islands, marsupials in Australia.
• Sympatric Speciation - New species formed within a single population without geographical isolation; occurs due to reproductive isolation and mutations; e.g. Cichlid fishes in Lake Victoria.
• Interspecific Speciation - New species formed by hybridisation of two different species; e.g. Triticum aestivum (hexaploid) and Gossypium herbaceum (tetraploid).

• Modern humans (Homo sapiens sapiens) evolved from Cro-Magnons around 50,000 to 11,000 years ago, mainly near the Caspian and Mediterranean regions.
• They have a fully upright posture, binocular vision, a mobile neck, and spinal curves that help with walking and balance.
• Their brain capacity is about 1450–1600 cm³, with a large cerebrum, well-developed forehead and chin, and the ability to speak and think clearly.
• They have proportionate limbs, flat nails, less body hair, and a broad chest, which are all features that support advanced physical and mental tasks.
• Modern humans made metal tools, started agriculture, formed permanent homes, and developed writing, science, and industries, leading to today's civilizations.