(a) Biodiversity hot spots (b) Navigation in birds (c) Sickle cell anaemia (d) Circadian rhythms (e) Biome

(a) Biodiversity hot spots

Biodiversity hot spots are regions that are exceptionally rich in endemic species and also severely threatened. The concept was proposed by Norman Myers (1988) and formalised with Conservation International (Myers et al., 2000) to decide where limited conservation funds would save the most species.

• Criteria: a region must contain at least 1,500 endemic vascular plant species (over 0.5% of the world total) and must have lost at least 70% of its original natural vegetation.
• Extent: 36 hot spots are now recognised. Their remaining intact habitat covers only about 2.5% of the Earth's land surface, yet they hold over half of the world's endemic plant species and nearly 43% of endemic bird, mammal, reptile and amphibian species.
• India: four hot spots lie wholly or partly in India: the Himalaya, Indo-Burma (north-east India), the Western Ghats and Sri Lanka, and Sundaland (the Nicobar Islands). The Western Ghats have very high endemism, for example most of their amphibians, including the purple frog (Nasikabatrachus sahyadrensis), and the lion-tailed macaque (Macaca silenus, Endangered) and Nilgiri tahr (Nilgiritragus hylocrius, Endangered).
• Significance: protecting these small areas can prevent a large share of expected extinctions. In India this is done through protected areas, biosphere reserves, the Biological Diversity Act, 2002 (amended 2023), ex-situ conservation and community conservation; critics note that the approach neglects less endemic but important areas and ecosystem services.

(b) Navigation in birds

Migratory and homing birds need both a compass (to know direction) and a map (to know their position relative to the goal).

• Sun compass: birds use the sun's azimuth, corrected for its movement by their circadian clock. Kramer showed that caged starlings orient using the sun, and shifting their clock by six hours shifts their orientation by about 90°.
• Star compass: night migrants learn the centre of rotation of the night sky (near Polaris); Emlen's planetarium experiments with indigo buntings showed this.
• Magnetic compass: European robins use an inclination compass, sensing the angle of the field lines to the Earth rather than polarity (Wiltschko). A light-dependent radical-pair mechanism in cryptochrome (Cry4) in the retina is the leading explanation; the earlier idea of iron-rich magnetite receptors in the beak is now doubtful. Magnetic intensity and inclination may also give map information.
• Olfactory map: homing pigeons deprived of smell or of wind-borne odours home poorly (Papi's experiments).
• Landmarks and memory: the hippocampus stores spatial maps; pigeons follow roads and coastlines near home.
• Innate versus learned routes: Perdeck's displacement of starlings showed that first-year birds fly a fixed inherited direction and distance (vector navigation), while adults correct for displacement using a learned map; hand-reared birds such as blackcaps show inherited migratory directions.
• Examples: the Arctic tern, which migrates between Arctic and Antarctic, about 70,000 km a year; the bar-headed goose, which crosses the Himalaya at over 6,000 m; and the Amur falcon, which gathers in Nagaland and flies non-stop across the Arabian Sea to southern Africa, tracked by satellite.

(c) Sickle cell anaemia

Sickle cell anaemia is an autosomal recessive haemoglobinopathy caused by a point mutation in codon 6 of the β-globin gene (HBB) on chromosome 11 (GAG → GTG), replacing glutamic acid with valine (Glu6Val).

• Pathology: in deoxygenated HbS, the hydrophobic valine binds a pocket on a neighbouring molecule, so HbS polymerises into long, rigid fibres. Red cells become sickle-shaped, rigid and fragile; they block small vessels and are destroyed early (life about 10–20 days instead of 120).
• Symptoms: chronic haemolytic anaemia and jaundice; painful vaso-occlusive crises in bones, chest (acute chest syndrome) and abdomen; dactylitis (hand-foot syndrome) in infants; stroke; splenic sequestration and later autosplenectomy, with a high risk of infection; priapism, leg ulcers and kidney damage. Heterozygotes (HbAS, sickle cell trait) are usually healthy.
• Heterozygote advantage: HbAS carriers are protected against severe Plasmodium falciparum malaria, because infected cells sickle and are removed and parasite growth is impaired. This balancing selection keeps the allele frequent in malarious regions of Africa, the Middle East and India.
• India: the allele is common among tribal communities of central and western India (Madhya Pradesh, Chhattisgarh, Odisha, Maharashtra, Gujarat, Jharkhand) and in the Nilgiris. The National Sickle Cell Anaemia Elimination Mission (launched July 2023) aims to eliminate it as a public health problem by 2047 through screening of about 7 crore people aged 0–40 years and genetic counselling.
• Diagnosis and treatment: solubility test, haemoglobin electrophoresis or HPLC, point-of-care tests and DNA testing; management with hydration, pain relief, folic acid, penicillin prophylaxis and vaccination, hydroxyurea (raises HbF), transfusion, and stem cell transplantation. Gene therapies (exa-cel/Casgevy and lovo-cel/Lyfgenia) were approved in 2023.

(d) Circadian rhythms

Circadian rhythms (Latin circa diem, "about a day") are endogenous rhythms of about 24 hours in physiology, behaviour and gene expression, such as sleep–wake cycles, body temperature, cortisol and melatonin secretion. They persist in constant conditions with a free-running period close to, but not exactly, 24 hours (about 24.2 hours in humans), and they are temperature-compensated.

• Master clock: in mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus; destroying it abolishes rhythms, and transplanting it restores rhythms with the donor's period. It synchronises peripheral clocks in the liver, heart and other organs.
• Entrainment: rhythms are set to the 24-hour day by zeitgebers, chiefly light. Light is detected by intrinsically photosensitive retinal ganglion cells containing melanopsin and reaches the SCN through the retinohypothalamic tract. Feeding time and temperature entrain peripheral clocks. The SCN controls the pineal gland, which secretes melatonin at night. In birds the pineal gland and retina also act as clocks.
• Molecular clock: the transcription factors CLOCK and BMAL1 activate the Period (Per) and Cryptochrome (Cry) genes; PER and CRY proteins accumulate, enter the nucleus and inhibit CLOCK–BMAL1, shutting down their own synthesis; their breakdown (regulated by casein kinase phosphorylation) restarts the cycle, taking about 24 hours. The period gene was first identified in Drosophila by Konopka and Benzer (1971), and Hall, Rosbash and Young received the 2017 Nobel Prize for this mechanism.
• Significance: rhythms anticipate daily changes in environment; they underlie photoperiodic timing of breeding and migration; and their disruption by jet lag, shift work or night light is linked to sleep disorders, metabolic disease and some cancers. Chronotherapy times drugs to the body clock.

(e) Biome

A biome is a large regional community of plants and animals, recognised by its dominant vegetation (life forms) and shaped mainly by climate, particularly temperature and rainfall. It is broader than an ecosystem: one biome includes many similar ecosystems on different continents, such as the tropical rainforests of the Amazon, Congo and South-East Asia.

• Climate control: Whittaker's scheme places biomes on a graph of mean annual temperature against annual precipitation. Soil, fire, seasonality and grazing modify the boundaries, and on mountains similar zonation appears with altitude.
• Major terrestrial biomes: tropical rainforest (hot and wet, multi-layered canopy, highest diversity); tropical seasonal (deciduous) forest; savanna (grasses with scattered trees, seasonal rain, fire); desert (under about 250 mm rain, succulents and nocturnal, water-saving animals); temperate grassland; temperate deciduous forest; boreal forest or taiga (conifers, long cold winters); and tundra (treeless, permafrost, mosses, lichens, caribou). Aquatic biomes are classified by salinity, depth and flow.
• Structural features: the rainforest has emergent, canopy and understorey layers with lianas and epiphytes; the taiga has needle-leaved evergreen conifers; the tundra has low, cushion plants.
• India: tropical wet evergreen forests in the Western Ghats and north-east; tropical moist and dry deciduous forests (sal, teak) across the peninsula; thorn forests and desert in Rajasthan and Gujarat; grasslands such as the terai and Banni; mangroves in the Sundarbans; and an altitudinal sequence in the Himalaya from subtropical to temperate, subalpine, alpine and cold desert (Ladakh).

Biomes are used in macroecology and conservation planning, and their predicted shifts under climate change help forecast changes in species distributions.