Samacheer Kalvi 12th Biology Chapter 16 Exercise Solutions

Homologous organs have the same basic structure and embryonic origin but perform different functions, indicating divergent evolution from a common ancestor. Example: Forelimbs of human (grasping), whale (swimming), and bat (flying) — same bone arrangement but different functions. Analogous organs have different origins and structures but perform similar functions, indicating convergent evolution. Example: Wings of birds and insects — both used for flight but structurally different and evolved independently.

Genetic drift refers to random changes in allele frequencies that occur particularly in small populations. Unlike natural selection, genetic drift is not directed by environmental factors — it is a chance event. The founder effect occurs when a small group of individuals separates from a larger population and colonises a new area. The new population's allele frequencies may differ from the parent population simply due to the small sample. The bottleneck effect occurs when a population undergoes a drastic reduction in size due to a natural disaster or other catastrophe. The surviving population has reduced genetic variation compared to the original population.

The five conditions for Hardy-Weinberg equilibrium are: (i) Large population size — minimises the effect of genetic drift, (ii) Random mating — no sexual selection or assortative mating, (iii) No mutation — no new alleles being introduced, (iv) No migration (gene flow) — no immigration or emigration, (v) No natural selection — all genotypes have equal fitness. When any of these conditions is violated, allele frequencies change, indicating that evolution is occurring.

Stabilising selection favours the intermediate phenotype and eliminates extreme variants. Example: Human birth weight — very low and very high birth weights have higher infant mortality. Directional selection favours one extreme phenotype, shifting the population mean in that direction. Example: Industrial melanism in peppered moths — dark-coloured moths were favoured in polluted areas. Disruptive selection favours both extremes simultaneously while eliminating the intermediate. Example: Beak size in finches — birds with either large or small beaks are favoured over medium beaks when only large and small seeds are available.

Fossils are the preserved remains or traces of organisms that lived in the past. They provide the most direct evidence for evolution because: (i) They show that organisms different from present-day species existed in the past, (ii) They reveal transitional forms between groups (e.g. Archaeopteryx between reptiles and birds), (iii) They demonstrate that life has changed over time — older rock layers contain simpler organisms, (iv) They help establish evolutionary lineages and phylogenetic relationships, (v) They provide information about extinct species and ancient environmental conditions.

The Modern Synthetic Theory (Neo-Darwinism) is an updated version of Darwin's theory that integrates natural selection with modern genetics. It was developed by Dobzhansky, Mayr, Simpson, and others in the 1930s–40s. Key features: (i) Mutation is the ultimate source of genetic variation, (ii) Natural selection acts on this variation, favouring advantageous traits, (iii) Genetic drift causes random changes in allele frequency, especially in small populations, (iv) Gene flow (migration) introduces new alleles between populations, (v) Recombination during meiosis creates new gene combinations. This theory explains evolution as a change in allele frequencies over generations due to these combined factors.

Lamarckism (Jean-Baptiste Lamarck, 1809):

• Organisms evolve through the use and disuse of organs — organs that are used develop further, while unused organs degenerate.
• Acquired characters are inherited by offspring. Example: Giraffe stretched its neck to reach leaves, and this longer neck was passed to the next generation.
• Internal vital force drives organisms toward greater complexity.
• Criticism: Acquired characters are NOT inherited. Weismann's experiment (cutting off mouse tails for 22 generations — offspring still had tails) disproved this. Lamarckism is not accepted in modern biology.

Darwinism (Charles Darwin, 1859):

• Overproduction: Organisms produce more offspring than can survive.
• Variation: Individuals show heritable variations; no two individuals are exactly alike.
• Struggle for existence: Organisms compete for limited resources.
• Natural selection (Survival of the fittest): Individuals with favourable variations survive and reproduce more successfully.
• Origin of species: Over many generations, favourable variations accumulate, leading to the formation of new species.
• Example: Peppered moth — dark-coloured moths survived better in polluted areas (industrial melanism).
• Limitations: Darwin could not explain the source of variations or the mechanism of inheritance.

Mutation Theory (Hugo De Vries, 1900):

• Evolution occurs through sudden, large mutations (saltations), not gradual accumulation of small variations.
• Mutations are the raw material for evolution and can produce new species in a single generation.
• Based on observations of evening primrose (Oenothera lamarckiana) showing sudden variant forms.
• Criticism: Most mutations are harmful or neutral; very few are beneficial. Large mutations rarely produce viable new species. The "mutations" De Vries observed in Oenothera were later found to be due to polyploidy and hybridisation, not true mutations.

Neo-Darwinism (Modern Synthetic Theory) integrates Darwin's natural selection with genetics (mutation, recombination, genetic drift, gene flow) and is the currently accepted theory.

The Hardy-Weinberg law states that in a large, randomly mating population with no mutation, no migration, and no natural selection, allele and genotype frequencies remain constant from generation to generation. The population is said to be in Hardy-Weinberg equilibrium.

Equations: p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies), where p = frequency of dominant allele A, q = frequency of recessive allele a, p² = frequency of AA (homozygous dominant), 2pq = frequency of Aa (heterozygous), q² = frequency of aa (homozygous recessive).

Conditions for equilibrium: (i) Large population, (ii) Random mating, (iii) No mutation, (iv) No migration (gene flow), (v) No natural selection. When any condition is violated, allele frequencies change and evolution occurs.

Problem 1: In a population, 16% show a recessive trait (aa). Find all allele and genotype frequencies.

• q² = 0.16 → q = 0.4 (frequency of allele a)
• p = 1 − q = 1 − 0.4 = 0.6 (frequency of allele A)
• p² = (0.6)² = 0.36 (frequency of AA = 36%)
• 2pq = 2 × 0.6 × 0.4 = 0.48 (frequency of Aa = 48%)
• Check: 0.36 + 0.48 + 0.16 = 1.00 ✓

Problem 2: In a population of 1000 individuals, 360 are AA, 480 are Aa, and 160 are aa. Verify Hardy-Weinberg equilibrium.

• Allele frequencies: p = (2×360 + 480)/2000 = 1200/2000 = 0.6; q = (2×160 + 480)/2000 = 800/2000 = 0.4
• Expected: p² = 0.36 (360), 2pq = 0.48 (480), q² = 0.16 (160) — matches observed. Population is in equilibrium.

Significance: Deviations from Hardy-Weinberg equilibrium indicate that evolutionary forces (selection, drift, migration, or mutation) are acting on the population.

Morphological evidence:

• Homologous organs: Organs with the same basic structure and embryonic origin but different functions. Example: Forelimbs of human (grasping), whale (swimming), bat (flying), and cheetah (running) — all have humerus, radius, ulna, carpals, metacarpals, and phalanges in the same arrangement. This indicates common ancestry with divergent evolution (adaptive radiation).
• Analogous organs: Organs with different origins and structures but similar functions. Example: Wings of bird (modified forelimb with feathers) and wings of insect (outgrowth of exoskeleton) — both enable flight but evolved independently. This indicates convergent evolution.
• Vestigial organs: Reduced, non-functional organs that were functional in ancestors. Examples: Appendix in humans (functional in herbivorous ancestors for cellulose digestion), coccyx (tail vertebrae), wisdom teeth, muscles of ear pinna.

Palaeontological evidence:

• Fossils are preserved remains or traces of past organisms found in sedimentary rocks. Older rock layers (strata) contain simpler organisms; newer layers contain more complex ones. Transitional fossils show links between groups: Archaeopteryx (between reptiles and birds — has feathers and teeth), Tiktaalik (between fish and amphibians), horse evolution series (Eohippus → Mesohippus → Merychippus → Equus).

Embryological evidence:

• Early embryos of all vertebrates (fish, amphibians, reptiles, birds, mammals) show striking similarities — gill slits, notochord, tail. This indicates common ancestry. Ernst Haeckel proposed "Ontogeny recapitulates phylogeny" (biogenetic law), meaning the development of an embryo repeats the evolutionary history of its species (though this is now considered an oversimplification).

Molecular evidence:

• Similarities in DNA sequences and protein structures across species indicate common ancestry. Cytochrome c (a respiratory protein) is found in all eukaryotes with nearly identical amino acid sequences. Humans share ~98.7% DNA with chimpanzees, ~85% with mice. The degree of molecular similarity reflects evolutionary relatedness — more closely related species share more DNA/protein sequence similarity.

Natural selection is the process by which organisms with favourable variations survive and reproduce more successfully, passing those traits to the next generation. There are three types:

Stabilising selection: FAVOURS the intermediate phenotype and ELIMINATES both extremes. This reduces variation and maintains the status quo. Example: Human birth weight — babies with very low or very high birth weights have higher mortality rates; intermediate weights (~3.2 kg) have the highest survival. Another example: Body size in many species where medium size is optimal for survival. Diagram: The distribution curve becomes narrower and taller over time, concentrating around the mean.

Directional selection: FAVOURS one extreme phenotype and SHIFTS the population mean in that direction. This occurs when environmental conditions change and one extreme is better adapted. Example: Industrial melanism in the peppered moth (Biston betularia) — before industrialisation, light-coloured moths were camouflaged on lichen-covered trees; after industrialisation, dark-coloured moths (carbonaria) were better camouflaged on soot-covered trees, so their frequency increased. Another example: Antibiotic resistance in bacteria — bacteria with resistance genes survive and reproduce when antibiotics are used. Diagram: The entire distribution curve shifts to one side.

Disruptive selection: FAVOURS both extremes simultaneously and ELIMINATES the intermediate phenotype. This can lead to the formation of two distinct groups within a population, potentially leading to speciation. Example: Beak size in Darwin's finches on islands where only large seeds and small seeds are available — birds with large beaks eat large seeds, birds with small beaks eat small seeds, but birds with medium beaks are less efficient at both. Another example: Black-bellied seedcracker finch — birds with either very large or very small beaks survive better. Diagram: The single distribution curve develops two peaks at the extremes, with a dip in the middle.