Explain sexual selection — Biology notes

WHAT is sexual selection?

Two mechanisms make it happen:

Intrasexual selection — competition within one sex (usually male–male combat). Intra = "inside." Think antlers, big body size, horns.
Intersexual selection — one sex (usually female) chooses mates of the other sex. Inter = "between." Think peacock's tail, birdsong, bright colours. Also called mate choice.

WHY does a "costly" trait like a huge tail survive?

The puzzle Darwin worried about: a peacock's tail makes it slower, heavier, more visible to predators. Natural selection for survival should delete it. So why is it there?

Because fitness = survival × reproduction. Let a male's chance of surviving to breed be $s$ and his number of matings if he survives be $m$ . His expected reproductive contribution is:

$W = s \times m$

A drab male: high $s$ , low $m$ . A showy male: lower $s$ (predators, cost), but much higher $m$ . The showy trait spreads whenever:

$s_{\text{show}}\, m_{\text{show}} > s_{\text{drab}}\, m_{\text{drab}}$

Formula Deriving the trade-off condition from scratch

Compare two male types. Showy spreads when its total fitness beats drab: $s_{\text{show}} m_{\text{show}} > s_{\text{drab}} m_{\text{drab}}$ Divide both sides by $s_{\text{drab}} m_{\text{drab}}$ (all positive): $\frac{s_{\text{show}}}{s_{\text{drab}}} \times \frac{m_{\text{show}}}{m_{\text{drab}}} > 1$ Let survival cost ratio $C = \dfrac{s_{\text{show}}}{s_{\text{drab}}}$ (a number $<1$ , showy survives worse) and mating gain ratio $G = \dfrac{m_{\text{show}}}{m_{\text{drab}}}$ (a number $>1$ ). $\boxed{C \times G > 1 \iff G > \frac{1}{C}}$ Read it: the mating advantage $G$ must more than pay back the survival penalty $\tfrac{1}{C}$ . If a showy tail halves survival ( $C = 0.5$ ), it must more than double matings ( $G > 2$ ) to be favoured.

HOW do females "know" a good male? (Honest signals)

If bluffing were free, every male would fake a great tail. So the traits that persist are honest signals — costly to fake.

Two classic explanations for female preference:

Good genes: showy = healthy; her sons and daughters inherit fitness. (Handicap principle)
Fisherian runaway: if females happen to prefer long tails, their sons have long tails and their daughters carry the preference. Preference and trait become genetically linked and reinforce each other, driving the trait to exaggerated lengths — a self-amplifying feedback loop, until survival cost finally brakes it.

Worked examples

Common mistakes (steel-manned)

Flashcards

Sexual selection is selection based on success at obtaining what?

Mates (differential reproduction via competition for mates), not survival.

Name the two mechanisms of sexual selection.

Intrasexual (competition within a sex, e.g. male–male combat) and intersexual (mate choice, usually female choosing).

Why does the sex with the costly gamete tend to be choosy?

Anisogamy — eggs are expensive and limiting, so females maximise offspring quality by choosing; cheap-sperm males compete for access.

Write fitness as a product and say which term sexual selection acts on.

W = s \times m

(survival × matings); sexual selection acts on

m

, viability selection on

s

State the condition for a showy trait to spread, using cost ratio

C

and gain ratio

G

C\times G>1

, i.e. mating gain

G>1/C

must outweigh the survival cost.

What is the handicap principle?

A costly trait is an honest signal of quality because only high-quality individuals can afford it and still survive.

What is Fisherian runaway?

Genetic linkage of a female preference with the male trait causes both to self-amplify, exaggerating the trait until survival cost stops it.

What limits how extreme an ornament can become?

The survival cost (e.g. predation) — when

s\times m

for showy no longer beats drab.

Give one example each of intrasexual and intersexual selection.

Intrasexual: elephant seal body size / deer antlers. Intersexual: peacock tail / birdsong.

Recall Feynman: explain to a 12-year-old

Imagine a school talent show where the winner gets a prize. To WIN you have to survive the whole day first (that's staying alive), but surviving isn't enough — you also have to impress the judges to win the prize (that's getting a mate). Some kids wear a giant heavy costume. It slows them down and it's risky, but the judges LOVE it, so they win the prize and their "style" spreads to the next show. That heavy costume is the peacock's tail: it makes life harder but wins the mating prize — and only a really strong kid can carry that heavy costume and still finish the day, which is exactly why the judges trust it. That's sexual selection: winning the mate can matter more than just staying safe.

Connections

Natural Selection — sexual selection is a special case acting on reproduction, not survival.
Fitness — defined as survival × reproduction; sexual selection maximises the reproduction term.
Anisogamy — the gamete-size asymmetry that creates choosy females and competing males.
Sexual Dimorphism — the observable outcome (size/colour differences between sexes).
Handicap Principle & Honest Signalling — why costly ornaments are trustworthy.
Fisherian Runaway — feedback model of preference–trait coevolution.
Predation — the ecological brake on ornament exaggeration.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, natural selection sirf ek sawaal poochhta hai: "Tum zinda rahoge kya?" Lekin sexual selection ek alag sawaal poochhta hai: "Tumhe mating milegi kya?" Kyunki genes agli generation mein tabhi jaate hain jab jeev reproduce karta hai. Isliye peacock ki bhaari, colourful tail — jo usse slow aur predator ke liye easy target banati hai — phir bhi survive kar jaati hai, kyunki wahi tail usko zyada mating dilwaati hai. Simple formula yaad rakho: Fitness = Survival × Mating ( $W = s \times m$ ). Survival kam bhi ho, agar mating bahut zyada badh jaaye, to trait phaili rahegi.

Do type hote hain. Intrasexual — same sex ke andar ladai (male–male fight), jaise deer ke antlers ya elephant seal ka bada body. Intersexual — ek sex doosre ko choose karta hai (aksar female male ko), jaise peahen peacock ki tail dekh kar choose karti hai. Female choosy kyun? Kyunki anisogamy: egg mehnga aur limited hai, sperm sasta aur bahut. Jiska gamete costly, wo choosy; jiska sasta, wo compete karta hai.

Ab ek clever baat — handicap principle. Bhaari tail ek honest signal hai. Sirf ek genetically strong male hi itni costly tail leke bhi zinda reh sakta hai. Toh female jab showy male choose karti hai, use apne bachchon ke liye acche genes milte hain. Isliye "dikhawa" fake nahi hota, wo quality ka proof hota hai. Lekin tail infinitely nahi badhti — jab survival cost mating gain ko cancel karne lagta hai (diagram mein red curve ka peak), wahin exaggeration ruk jaata hai. Yeh matter isliye karta hai kyunki isse hum samajhte hain ki nature mein itne bright colours, songs, aur size-differences kyun evolve hue — sirf survival se in sabki explanation nahi milti.

Test yourself — Evolution & Natural Selection

Connections