(a) What is organic evolution? Discuss in detail the indirect evidences of organic evolution with suitable examples. (20…

(a) Organic Evolution and Its Indirect Evidences

Organic evolution is the gradual change in the heritable characters of populations over generations, by which new species arise from pre-existing ones and all life shares common ancestry. Darwin called it "descent with modification". It is driven by mutation, recombination, natural selection, genetic drift and gene flow. Because large changes take very long to occur, evolution is inferred mainly from indirect evidence.

1. Comparative morphology and anatomy: Homologous organs have the same origin and basic plan but may serve different functions. Examples are the forelimbs of man, whale, bat and horse (divergent evolution); in plants, the thorns of Bougainvillea and the tendrils of Passiflora, which are both modified axillary branches, and the phylloclades of Opuntia and the stem tubers of potato, which are both modified stems. Analogous organs differ in origin but perform similar functions, showing convergent evolution. Examples are the wings of insects and birds; the tendrils of pea (leaflets) and Passiflora (axillary branch); and the tuberous root of sweet potato and the stem tuber of potato.
2. Vestigial organs: reduced, functionless remnants of organs that were useful in ancestors, such as the human vermiform appendix, coccyx and nictitating membrane; the vestigial hind-limb bones of whales and pythons; and, in plants, the scale leaves on rhizomes and the staminodes of Scrophularia and Cassia.
3. Comparative embryology: as von Baer observed, early embryos of related animals resemble one another more closely than their adults do; for example, all vertebrate embryos have pharyngeal pouches and a tail. Haeckel's biogenetic law ("ontogeny recapitulates phylogeny") is not accepted in its literal form, because embryos repeat ancestral embryonic stages, not adult ancestors. In plants, the filamentous moss protonema resembles a green alga, and fern prothalli resemble thalloid bryophytes.
4. Palaeontology (fossils): fossils in dated rock strata show a sequence from simple to complex forms and record extinct transitional forms. Archaeopteryx combines reptilian and bird characters; Tiktaalik links fish and tetrapods; and the horse series (Hyracotherium to Equus) shows a branching radiation with increasing size and fewer toes. Among plants, Rhynia and other Rhyniophytes are among the earliest vascular plants, and **seed ferns (Pteridospermales, e.g., Lyginopteris)** combine fern-like foliage with seeds, linking ferns and gymnosperms.
5. Connecting links: living organisms with characters of two groups, such as Peripatus (annelids and arthropods), the platypus (reptiles and mammals), Euglena (plants and animals), and Gnetum (gymnosperms with some angiosperm-like features).
6. Biogeography: related species are distributed according to past land connections and barriers. Darwin's finches of the Galápagos radiated from a mainland ancestor, and Australia's marsupials evolved in isolation. The Glossopteris flora, found in India, Africa, South America, Antarctica and Australia and studied in India by Birbal Sahni, reflects the former union of these lands in Gondwana.
7. Comparative biochemistry and molecular biology: all organisms share DNA, a nearly universal genetic code, ATP and similar enzymes. Sequence similarity in cytochrome c, haemoglobin, rRNA and whole genomes (human and chimpanzee DNA differ by only about 1–2%) follows the same branching pattern as morphology and allows molecular clocks to date divergences. Serological (precipitin) tests show similar gradations.
8. Evidence from domestication and observed change: artificial selection produced cabbage, cauliflower, kohlrabi, broccoli and kale from wild Brassica oleracea, and maize from teosinte. Changes have also been observed in nature, such as industrial melanism in the peppered moth (Biston betularia), heavy-metal tolerance in grasses on mine spoils, and herbicide resistance in weeds.

(b) Structure and Functions of the Chloroplast

Chloroplasts are lens-shaped plastids, about 4–10 µm long, with 20–100 per mesophyll cell in higher plants.

1. Envelope: two membranes separated by an intermembrane space. The outer membrane is freely permeable to small molecules (porins); the inner membrane is selectively permeable, carries transporters (e.g., the triose-phosphate translocator), and has the TOC–TIC complexes that import nuclear-encoded proteins.
2. Stroma: the matrix, containing the enzymes of the Calvin cycle (RuBisCO is the most abundant protein on Earth), 70S ribosomes, several copies of a circular chloroplast DNA (about 120–160 kb) in nucleoids, starch grains and plastoglobuli (lipid droplets).
3. Thylakoids: a continuous internal membrane system enclosing the thylakoid lumen. Stacked disc-like thylakoids form grana (10–100 per chloroplast), which are joined by unstacked stroma lamellae (frets). The pigments (chlorophyll a, chlorophyll b, carotenoids) and electron carriers are embedded in the membrane. There is lateral heterogeneity: PSII and its light-harvesting complex LHCII lie mainly in the appressed grana; PSI and ATP synthase (CF₀–CF₁) lie in the stroma lamellae and grana margins; and the cytochrome b₆f complex is distributed throughout.

Functions:

1. Light reactions (thylakoids): light is absorbed by the antenna pigments; water is split at PSII (the oxygen-evolving complex), releasing O₂; electrons pass through PSII, plastoquinone, cytochrome b₆f, plastocyanin, PSI and ferredoxin to reduce NADP⁺ to NADPH (the Z-scheme). The proton gradient across the thylakoid drives ATP synthesis (photophosphorylation, both non-cyclic and cyclic).
2. Carbon fixation (stroma): the Calvin–Benson cycle uses ATP and NADPH to fix CO₂ into triose phosphates, which are exported to make sucrose or kept to make starch. In C₄ plants, bundle-sheath chloroplasts (often agranal in maize) carry out the Calvin cycle.
3. Photorespiration: the chloroplast shares this pathway with the peroxisome and mitochondrion.
4. Biosynthesis: fatty acids, many amino acids, purines and pyrimidines, chlorophyll, carotenoids, and precursors of hormones (ABA, gibberellins, jasmonate); reduction of nitrite to ammonium and assimilation of sulphate.
5. Genetic functions and signalling: the chloroplast is semi-autonomous, with its own DNA, RNA polymerases and ribosomes, supporting its endosymbiotic origin from a cyanobacterium. It shows maternal inheritance in most angiosperms, and sends retrograde signals to the nucleus about stress and developmental state.

(c) Role of Mutations in Plant Breeding and Crop Improvement

A mutation is a sudden heritable change in the genetic material. It may be a gene (point) mutation (base substitution, insertion or deletion), a chromosomal mutation (deletion, duplication, inversion, translocation), or a genomic mutation (aneuploidy, polyploidy). Mutations are the ultimate source of all genetic variation. Mutation breeding uses induced mutations to create variation where it is lacking. It began with Stadler's X-ray work on barley and maize (1928), following Muller's work on Drosophila (1927).

Mutagens:

1. Physical: gamma rays (e.g., from ⁶⁰Co in gamma gardens), X-rays, fast and thermal neutrons, UV light and ion beams.
2. Chemical: alkylating agents such as ethyl methanesulphonate (EMS), methyl methanesulphonate and diethyl sulphate; sodium azide; base analogues (5-bromouracil); and nitrous acid. Seeds, pollen, budwood or tissue cultures are treated. M₁ plants are raised, and desirable recessive mutants are selected in M₂ and later generations. Chimeras in vegetatively propagated crops are separated by repeated cutting back or by in vitro culture.

Role in crop improvement:

1. Improving one or two traits of an adapted variety without disturbing the rest of its genotype, which is faster than hybridisation and backcrossing.
2. Creating new alleles not present in the germplasm, e.g., semi-dwarfing, earliness, male sterility, altered oil or starch quality.
3. Vegetatively propagated and apomictic crops, where hybridisation is difficult: new flower colours in chrysanthemum, rose and bougainvillea; seedless and improved fruits; sugarcane and banana clones.
4. Breaking undesirable linkages and inducing translocations to transfer alien chromosome segments (e.g., Sears' transfer of leaf-rust resistance from Aegilops umbellulata to wheat by irradiation).
5. Polyploidy for seedless triploid watermelon, triploid sugar beet and synthetic amphidiploids.

Examples:

• India: the amber-grained wheat Sharbati Sonora (IARI, 1967), selected after gamma irradiation of Sonora 64; rice Jagannath (X-ray mutant of T-141, CRRI) and PNR-381; cotton MCU-7; BARC's Trombay groundnut varieties (e.g., TAG-24, TG-37A), mungbean TARM-1 (powdery mildew resistant) and other pulses; and many ornamental mutants from NBRI, Lucknow.
• World: the semi-dwarf rice Calrose 76 (gamma-ray sd1 allele), the malting barley 'Golden Promise', 'Rio Red' and 'Star Ruby' grapefruit, and Verticillium-wilt-resistant peppermint 'Todd's Mitcham'. The FAO/IAEA Mutant Variety Database lists over 3,300 officially released mutant varieties.
• Modern tools: TILLING combines chemical mutagenesis with high-throughput mutation detection; genome editing (CRISPR–Cas) creates precise, targeted mutations. India exempted SDN-1 and SDN-2 genome-edited plants without foreign DNA from GM regulations (2022), and in 2025 ICAR released two genome-edited rice varieties, DRR Dhan 100 (Kamala) and Pusa DST Rice 1.

Limitations:

1. Most induced mutations are recessive and harmful, so large populations must be screened.
2. Mutations are random, and the desired change is rare.
3. Pleiotropy, linked undesirable mutations and chromosome damage reduce fertility and vigour.
4. Chimeras form in vegetatively propagated material.
5. Mutation breeding is less effective for polygenic traits such as yield, drought or salinity tolerance.
6. Some mutagens are hazardous and need special handling facilities.