(a) What are ecotones? Explain by giving examples. Also write a note on 'edge effect'. (10 marks) (b) Comment on 'measures of…

(a) Ecotones and edge effect

An ecotone is a transition zone where two adjacent communities or ecosystems meet and merge. It may be narrow or broad, shows a gradient of environmental conditions, and holds species of both neighbours plus a few edge species largely confined to it.

Examples: forest–grassland (the shola–grassland mosaic of the Nilgiris), land–sea mangroves (Sundarbans), river–sea estuary, marsh or lake margin, riparian zone along a river, and the Himalayan tree line between forest and alpine meadow.

Edge effect is the tendency of an ecotone to show higher species diversity and population density than either adjoining community, because it offers the resources and cover of both and its own niches. Many birds and game animals feed, nest and take cover along edges. Man-made edges can also harm interior species through predation, invasive species and drying winds.

(b) Measures of central tendency of data

Central tendency is the single typical value around which data cluster.

• Arithmetic mean, x̄ = Σx/n, uses every value, but extreme values distort it.
• Median is the middle value of ordered data; for grouped data L + [(N/2 − cf)/f] × h. It resists outliers and suits skewed data.
• Mode is the most frequent value; for grouped data L + [(f₁ − f₀)/(2f₁ − f₀ − f₂)] × h. It suits nominal data but may be absent or multiple.
• Geometric mean (nth root of the product) suits growth rates and ratios; harmonic mean n/Σ(1/x) suits rates.

For 12, 14, 14, 15, 45 the mean is 20 but the median and mode are 14, so the outlier makes the median the better choice. In symmetrical data all three coincide; in moderately skewed data Mode ≈ 3 Median − 2 Mean. Biological uses: mean clutch size, median lethal dose, modal size class.

(c) Gel electrophoresis

Principle: charged molecules move through a porous gel in an electric field, and the gel acts as a molecular sieve. DNA carries uniform negative charge (phosphate groups), so fragments separate by length; shorter ones travel farther, distance being roughly proportional to the negative logarithm of size.

Working:

1. An agarose gel (0.5–2%) is cast with a comb to form wells; polyacrylamide is used for proteins and very small fragments.
2. The gel is submerged in buffer (TAE or TBE).
3. Samples mixed with loading dye and glycerol are loaded, with a size-marker ladder.
4. Current is applied; DNA runs towards the anode (+).
5. Bands are stained (ethidium bromide) and seen under UV. In SDS-PAGE, SDS gives proteins uniform negative charge so they separate by mass.

Applications: checking PCR products and restriction digests, DNA fingerprinting in forensics and paternity, protein size and purity, diagnosis (haemoglobin variants in sickle-cell disease), and Southern and Western blotting.

(d) Conditioned learning in animals

Conditioned learning is associative learning in which an animal links a stimulus, or its own act, with a consequence.

• Classical (Pavlovian) conditioning: a neutral stimulus (bell) repeatedly paired with an unconditioned stimulus (food) that evokes an unconditioned response (salivation) becomes a conditioned stimulus and alone evokes the conditioned response. Pavlov demonstrated it in dogs; honeybees likewise learn to extend the proboscis to an odour paired with sugar.
• Operant (instrumental) conditioning: behaviour is shaped by its consequences. Thorndike's cats learned to escape a puzzle box by trial and error (law of effect); in Skinner's box a rat learns to press a lever for food (positive reinforcement) or to stop a shock (negative reinforcement).

Related processes are acquisition, extinction (the response fades when the conditioned stimulus is no longer reinforced), spontaneous recovery, generalisation and discrimination. Such learning helps animals predict food and danger and is the basis of animal training.

(e) Klinefelter's and Turner's syndromes

Klinefelter's syndrome: karyotype 47,XXY (44 + XXY), about 1 in 500–1000 males, caused by nondisjunction of sex chromosomes in a parent's meiosis. Males are tall with long limbs, have small firm testes, low testosterone, azoospermia (sterility), gynaecomastia and sparse body hair, and often mild learning difficulty. One Barr body is present. Testosterone replacement is the treatment.

Turner's syndrome: karyotype 45,X (44 + X), about 1 in 2000–2500 female births; most 45,X conceptions abort. Females are short (about 143 cm untreated), with webbed neck, shield-like chest, low hairline, cubitus valgus, streak gonads (ovarian dysgenesis), primary amenorrhoea, sterility and poor secondary sexual characters. Heart (coarctation of the aorta) and kidney (horseshoe kidney) defects are common; intelligence is usually normal. Barr body is absent. Growth hormone and oestrogen therapy are given.