Intrasexual selection — competition ek hi sex ke andar (usually male–male combat). Intra = "andar." Socho antlers, bada body size, horns.
Intersexual selection — ek sex (usually female) doosre sex ke mates choose karti hai. Inter = "beech mein." Socho peacock ki tail, birdsong, bright colours. Ise mate choice bhi kehte hain.
Woh puzzle jo Darwin ko pareshaan karta tha: peacock ki tail use slower, heavier, aur predators ke liye zyada visible banati hai. Survival ke liye natural selection ise delete kar deta. Toh yeh hai kyun?
Kyunki fitness = survival × reproduction. Maano ek male ke breed karne tak survive karne ki chance s hai aur agar woh survive kare toh matings ki sankhya m hai. Uska expected reproductive contribution hai:
W=s×m
Ek drab male: zyada s, kam m. Ek showy male: kam s (predators, cost), lekin bahut zyada m. Showy trait tab spread hota hai jab:
Agar bluffing free hoti, toh har male ek great tail fake karta. Isliye jo traits persist karte hain woh honest signals hote hain — fake karne mein costly.
Female preference ke liye do classic explanations:
Good genes: showy = healthy; uske bete aur betiyan dono fitness inherit karte hain. (Handicap principle)
Fisherian runaway: agar females accidentally long tails prefer karti hain, toh unke beton ki long tails hoti hain aur unki betiyon mein preference hoti hai. Preference aur trait genetically linked ho jaate hain aur ek doosre ko reinforce karte hain, trait ko exaggerated lengths tak le jaate hain — ek self-amplifying feedback loop, jab tak survival cost ise finally rok na de.
Fitness ko ek product ke roop mein likho aur batao sexual selection kaunse term par kaam karta hai.
W=s×m (survival × matings); sexual selection m par kaam karta hai, viability selection s par.
Cost ratio C aur gain ratio G use karke showy trait ke spread hone ki condition batao.
C×G>1, yaani mating gain G>1/C survival cost se zyada hona chahiye.
Handicap principle kya hai?
Ek costly trait quality ka honest signal hai kyunki sirf high-quality individuals hi ise afford karke survive kar sakte hain.
Fisherian runaway kya hai?
Female preference aur male trait ki genetic linkage dono ko self-amplify karti hai, trait ko exaggerate karti hai jab tak survival cost ise rok na de.
Ek ornament kitna extreme ho sakta hai iska limit kya hai?
Survival cost (jaise predation) — jab showy ke liye s×m drab ko beat karna band kar de.
Intrasexual aur intersexual selection ka ek-ek example do.
Intrasexual: elephant seal body size / deer antlers. Intersexual: peacock tail / birdsong.
Recall Feynman: 12-saal ke bachche ko explain karo
Socho ek school talent show jahan winner ko prize milta hai. JEETNے ke liye tumhe pehle poora din survive karna hoga (yeh zinda rehna hai), lekin survive karna kaafi nahi — tumhe judges ko impress bhi karna hoga prize jeetnے ke liye (yeh mate milna hai). Kuch bachche ek bada heavy costume pehnte hain. Yeh unhe slow karta hai aur risky hai, lekin judges use BAHUT pasand karte hain, isliye woh prize jeette hain aur unki "style" agle show mein spread ho jaati hai. Woh heavy costume peacock ki tail hai: yeh life mushkil banati hai lekin mating prize jeetti hai — aur sirf ek bahut strong bachcha hi woh heavy costume carry karke din khatam kar sakta hai, aur exactly isliye judges ise trust karte hain. Yahi sexual selection hai: mate jeetnا sirf safe rehne se zyada matter kar sakta hai.
