Heredity and Evolution — Maharashtra (SSC) Class 10 Science Solutions (Free)

Law of Segregation: the two alleles an organism carries for a single trait separate during gamete formation, so each gamete carries only one allele of that pair; the pair is restored when gametes combine at fertilisation. Law of Independent Assortment: when considering TWO OR MORE different genes at once, the alleles of one gene are distributed into gametes independently of how the alleles of a different gene are distributed — inheriting a particular version of one trait does not influence which version of an unrelated trait is inherited alongside it. What a dihybrid cross demonstrates: crossing two individuals heterozygous for two different genes (AaBb × AaBb) produces offspring in a 9:3:3:1 phenotypic ratio across the four possible trait combinations. This specific ratio only emerges because the A/a gene and B/b gene assort into gametes completely independently of each other — if the two genes were instead physically linked and inherited together, the ratio would look very different. Mendel's dihybrid cross results were exactly the evidence that led him to propose independent assortment as a second, separate law alongside segregation.

A dominant allele produces its associated trait even when only one copy is present, masking the effect of any recessive allele paired with it. A recessive allele's trait is only expressed when TWO copies of it are present, with no dominant allele to mask it. Example — seed colour in pea plants: yellow seed colour (A) is dominant over green seed colour (a). Cross AA (yellow) × aa (green): every offspring inherits one A and one a, giving Aa — and since A is dominant, ALL of this first generation (F1) appears yellow, even though every individual is actually carrying a hidden green allele. Cross F1 × F1 (Aa × Aa): this reveals the hidden recessive allele. The resulting second generation (F2) shows a 3:1 ratio — 3 yellow (from AA and Aa genotypes) to 1 green (from aa) — demonstrating that the green trait was never lost in the F1 generation, merely masked, and reappears once two recessive alleles happen to combine again.

Evolution is the change in the genetic makeup (allele frequencies) of a population over successive generations, which over long timescales can lead to the formation of entirely new species. Darwin's theory of natural selection rests on a few connected observations: 1. Variation already exists naturally within any population. 2. Organisms compete for limited resources (food, space, mates). 3. Individuals whose traits happen to suit their environment better survive and reproduce more successfully than others. 4. Because advantageous traits are heritable, they become more common in the population over successive generations. Key supporting evidence: - Fossil records show a gradual, ordered sequence of anatomical change within lineages over geological time. - Homologous structures — the shared bone arrangement of a bat's wing, a human arm, and a whale's flipper, despite their very different functions — point to a common ancestor whose limb structure was later adapted differently in each lineage. - Antibiotic resistance in bacteria is natural selection observable within a human lifetime: bacteria with a chance mutation conferring resistance survive antibiotic treatment and reproduce, rapidly making resistant strains common. - The peppered moth in industrial England: as soot darkened tree bark, darker moths (better camouflaged) survived predation at a higher rate than lighter moths, visibly shifting the population's colouring within a few generations.

Speciation is the evolutionary process by which one species gives rise to one or more new, distinct species, ultimately reaching a point where the resulting populations can no longer interbreed with each other. Allopatric speciation: occurs when a GEOGRAPHIC barrier (a mountain range, a river, an ocean) physically separates a population into two groups, preventing gene flow between them. Isolated from each other, the two groups accumulate different genetic changes over time — through mutation, drift, and adaptation to their own local environment — until they have diverged enough to become reproductively isolated, even if the original geographic barrier were later removed. The classic example is Darwin's finches: an ancestral finch population that reached the separate islands of the Galápagos diversified into distinct species on each island, isolated from one another by open ocean. Peripatric speciation: occurs when a SMALL group breaks away from the main population and colonises a new location (a "founder" population). Because this small founding group carries only a limited, non-representative sample of the original population's genetic variation, genetic drift acts especially strongly and quickly on it, alongside adaptation to the new environment — together driving relatively rapid speciation compared to allopatric speciation acting on a larger, more stable population. Both mechanisms ultimately require reproductive isolation to complete the process of forming a genuinely distinct new species.

Genetic drift is a RANDOM change in a population's allele frequencies from one generation to the next, arising purely from chance in which individuals happen to survive and reproduce — not from any advantage the alleles themselves confer. Why drift matters more in small populations: in a large population, random chance events tend to average out, leaving allele frequencies relatively stable. In a SMALL population, the same random chance events can shift allele frequencies dramatically, since each individual represents a much larger share of the total gene pool — an allele can even be lost entirely, or become the only version present ("fixed"), purely by chance. The founder effect is a specific case of this: when a small group breaks off to found a new population, the alleles present in that small founding group (which may not represent the full genetic diversity of the original population) become the entire genetic starting point for the new population. Gene flow is the movement of alleles between separate populations, typically through migration of individuals who then interbreed with the new population. Gene flow tends to work AGAINST drift's effects — by introducing alleles from outside, it can restore genetic diversity that drift had reduced, and it tends to make separate populations more genetically similar to each other over time, rather than allowing them to diverge. Together, drift (which can cause populations to diverge, especially when small) and gene flow (which can counteract that divergence by mixing populations) are two of the key forces — alongside natural selection and mutation — that jointly determine how a population's genetics changes over time.

Adaptive radiation is the relatively rapid evolutionary process in which a single ancestral species diversifies into many descendant species, each adapted to exploit a different ecological niche — typically occurring when a species reaches a new environment with multiple unoccupied niches and little competition. Darwin's finches: a single ancestral finch species that colonised the Galápagos Islands diversified into 13 distinct species, each with a beak shape specifically adapted to a different food source available on the islands — from crushing hard seeds, to picking out insects, to (in one unusual species) piercing skin to feed on blood. Each beak shape represents adaptation to a specific niche that was available and unoccupied when the ancestral population arrived. Hawaiian honeycreepers: a single ancestral bird species that reached the isolated Hawaiian islands diversified into more than 50 species, showing an even more dramatic range of bill shapes and feeding behaviours — from nectar-feeding curved bills resembling a hummingbird's, to thick, finch-like seed-crushing bills — again each adapted to a different available food source. Both cases occurred in geographically isolated island environments with many unoccupied ecological niches and few competing species already established, which is precisely the set of conditions that allows adaptive radiation to proceed unusually quickly.

Vestigial organs are structures in a present-day organism that are reduced in size or function compared to a corresponding, FULLY functional structure found in that organism's evolutionary ancestors or in closely related living species. Human examples: - The appendix — believed to have helped ancestral hominids digest a more plant-heavy, fibrous diet, but of greatly reduced digestive use in modern humans. - Wisdom teeth (third molars) — likely useful when ancestral human diets required more chewing surface for tougher, less-processed food, but now often too large for the modern, smaller human jaw. - The coccyx (tailbone) — the remnant of a tail present in our primate ancestors. In other species: whales and some snakes retain small, non-functional internal hip bones and limb remnants — direct physical evidence that their evolutionary ancestors were four-legged land animals, structures which serve no walking function today. Why vestigial organs matter as evidence: there is no reason a structure with no current function would exist at all UNLESS it was inherited from an ancestor in which that same structure WAS fully functional — vestigial organs are essentially anatomical leftovers, and their presence is difficult to explain under any framework other than shared evolutionary descent from ancestors that once needed them.