Describe types of selection (directional, stabilizing, disruptive)

5.4.7 · Biology › Evolution & Natural Selection

WHAT are we selecting on?

Trait distribution ko do cheezein describe karti hain:

Iska mean ( $μ$ ) — jahan peak hoti hai.
Iska variance ( $σ^{2}$ ) — curve kitni wide/spread hai.

Har type of selection define hoti hai isi se ki yeh dono numbers ke saath kya karta hai.

Type	Mean $μ$ ke saath kya hota hai	Variance $σ^{2}$ ke saath kya hota hai
Directional	shift karta hai (↑ ya ↓)	~same rehta hai ya thoda ↓
Stabilizing	same rehta hai	kam hota hai ↓
Disruptive	same rehta hai (ya split ho jaata hai)	badhta hai ↑ (bimodal)

Figure — Describe types of selection (directional, stabilizing, disruptive)

1. Directional Selection

YEH KYUN HOTA HAI: environment consistently zyada (ya kam) trait ko reward karta hai. Curve ki ek tail mein zyada fitness hoti hai.

ISKO REASON OUT KAISE KARO (derivation-from-scratch):

Fitness $w (x)$ trait $x$ ke saath monotonically badhti hai (maano bada = better).
Bade $x$ wale individuals zyada offspring chodh ke jaate hain → unke alleles ki frequency badhti hai.
Agli generation ka mean $μ^{'} > μ$ . Peak daayein slide karti hai.

2. Stabilizing Selection

KYUN: bahut saari traits ke liye "middle" sabse safe optimum hota hai. Bahut zyada ya bahut kam — dono costly hain.

HOW (first principles):

Fitness $w (x)$ ek hump hai jo mean $x = μ$ pe peak karti hai.
Dono tails (bahut high, bahut low) mein low fitness hoti hai → prune ho jaate hain.
Dono edges prune karo → spread shrink hoti hai → $σ^{2}$ girta hai jabki $μ$ unchanged rehta hai.

3. Disruptive (Diversifying) Selection

KYUN: environment do acche "niches" offer karta hai lekin middle ko kuch reward nahi karta.

HOW:

Fitness $w (x)$ ek valley hai — mean pe sabse low, dono edges pe high.
Intermediates remove ho jaate hain → population do clusters mein split ho jaati hai.
Time ke saath yeh speciation drive kar sakta hai (agar mating assortative ho jaaye).

Recall Feynman: explain to a 12-year-old

Socho har bachche ki height ek hill ki tarah draw ki gayi hai (beech mein bahut saare average-height bachche, bahut kam chhote ya bahut tall). Ab ek game decide karta hai ki agli round mein kaun khelta hai.

Agar game kehta hai "sirf tall bachche khelein," toh hill tall ki taraf slide karti hai — yeh directional hai.
Agar game kehta hai "bahut chhote AUR bahut tall bachche baahir, sirf medium khelein," toh hill patli aur pointy ho jaati hai — yeh stabilizing hai.
Agar game kehta hai "sirf bahut chhote aur bahut tall khelein, mediums baahir," toh hill do bumps mein split ho jaati hai — yeh disruptive hai. Same hill, teen alag rules, teen alag shapes!

Forecast-then-Verify

Flashcards

Directional selection trait ke kitne extremes par act karta hai?

Ek extreme par (mean usi ki taraf shift karta hai).

Stabilizing selection kaun sa phenotype favour karta hai?

Intermediate/average; dono extremes ke against select karta hai.

Disruptive selection kaun se phenotypes favour karta hai?

Dono extremes; intermediates ke against select karta hai.

Stabilizing selection mein variance ke saath kya hota hai?

Yeh decrease hota hai (curve narrow ho jaata hai).

Disruptive selection mein variance ke saath kya hota hai?

Yeh increase hota hai (curve bimodal ho jaata hai).

Kaun si selection type directly speciation lead kar sakti hai?

Disruptive selection (population ko do groups mein split karti hai).

Peppered moth melanism kis selection type ki example hai?

Directional selection.

Human birth weight kis selection type ki classic example hai?

Stabilizing selection.

Directional selection mein population mean ke saath kya hota hai?

Yeh favoured extreme ki taraf shift karta hai.

Stabilizing selection "no evolution" kyun nahi hai?

Allele frequencies phir bhi change hoti hain — extreme-favouring alleles actively remove hote hain optimum maintain karne ke liye.

Teeno types ke liye mnemonic?

Slide (directional), Squeeze (stabilizing), Split (disruptive).

Stabilizing selection ke liye fitness function ki shape kya hai?

Mean pe peak karne wala ek hump.

Disruptive selection ke liye fitness function ki shape kya hai?

Ek valley (mean pe sabse low).

Connections

Natural Selection — parent mechanism; ye iske teen modes hain.
Genetic Variation — selection ko act karne ke liye ek continuous trait distribution chahiye.
Speciation — disruptive selection + assortative mating divergence drive karta hai.
Fitness and Adaptation — har type fitness curve ki shape se define hoti hai.
Hardy-Weinberg Equilibrium — null model; selection ek aisi force hai jo HWE ko todti hai.
Sexual Selection — ek special driver jo aksar directional trends produce karta hai.