Describe types of selection (directional, stabilizing, disruptive)
WHAT are we selecting on?
The trait distribution is described by two things:
- Its mean () — where the peak sits.
- Its variance () — how wide/spread the curve is.
Every type of selection is defined by what it does to these two numbers.
| Type | What happens to mean | What happens to variance |
|---|---|---|
| Directional | shifts (↑ or ↓) | ~stays or slightly ↓ |
| Stabilizing | stays same | decreases ↓ |
| Disruptive | stays same (or splits) | increases ↑ (bimodal) |
1. Directional Selection
WHY it happens: the environment consistently rewards more (or less) of a trait. One tail of the curve has higher fitness.
HOW to reason it out (derivation-from-scratch):
- Fitness increases monotonically with trait (say bigger = better).
- Individuals with large leave more offspring → their alleles increase in frequency.
- Next generation's mean . The peak slides right.
2. Stabilizing Selection
WHY: for many traits the "middle" is the safest optimum. Too much or too little is costly.
HOW (first principles):
- Fitness is a hump peaking at the mean .
- Both tails (very high, very low) have low fitness → get pruned.
- Prune both edges → spread shrinks → falls while is unchanged.
3. Disruptive (Diversifying) Selection
WHY: the environment offers two good "niches" but nothing rewarding the middle.
HOW:
- Fitness is a valley — lowest at the mean, high at both edges.
- Intermediates removed → population splits into two clusters.
- Over time this can drive speciation (if mating becomes assortative).
Recall Feynman: explain to a 12-year-old
Imagine every kid's height drawn as a hill (lots of average-height kids in the middle, few very short or very tall). Now a game decides who gets to play again next round.
- If the game says "only tall kids play," the hill slides toward tall — that's directional.
- If the game says "very short AND very tall kids sit out, only medium play," the hill gets skinny and pointy — that's stabilizing.
- If the game says "only very short and very tall play, mediums sit out," the hill splits into two bumps — that's disruptive. Same hill, three different rules, three different shapes!
Forecast-then-Verify
Flashcards
Directional selection acts on how many extremes of the trait?
Stabilizing selection favours which phenotype?
Disruptive selection favours which phenotypes?
What happens to variance under stabilizing selection?
What happens to variance under disruptive selection?
Which type of selection can most directly lead to speciation?
Peppered moth melanism is an example of which selection type?
Human birth weight is a classic example of which selection type?
Under directional selection what happens to the population mean?
Why isn't stabilizing selection "no evolution"?
Mnemonic for the three types?
What shape is the fitness function for stabilizing selection?
What shape is the fitness function for disruptive selection?
Connections
- Natural Selection — the parent mechanism; these are its three modes.
- Genetic Variation — selection needs a continuous trait distribution to act on.
- Speciation — disruptive selection + assortative mating drives divergence.
- Fitness and Adaptation — each type defined by the shape of the fitness curve.
- Hardy-Weinberg Equilibrium — the null model; selection is a force that breaks HWE.
- Sexual Selection — a special driver often producing directional trends.
Concept Map
Hinglish (regional understanding)
Intuition Hinglish mein samjho
Dekho, natural selection ka matlab simple hai: har trait (jaise height ya beak size) ek bell curve banata hai — zyada log middle mein, thode extremes pe. Selection is curve ko teen tareeke se badal sakta hai. Directional mein ek hi extreme ko fayda milta hai, toh poora curve us taraf slide ho jaata hai — jaise peppered moth kaale ho gaye kyunki soot ne trees kaale kar diye.
Stabilizing mein middle-wale sabse safe hain, dono extremes marte hain — toh curve patla aur tall ho jaata hai (variance kam), lekin mean same rehta hai. Best example human baby weight — bahut chhote ya bahut bade dono risky, 3.2 kg wale best survive karte hain. Yaad rakho: mean nahi hilta iska matlab evolution ruk gaya nahi — alleles abhi bhi remove ho rahe hain, bas optimum maintain ho raha hai.
Disruptive mein dono extremes ko fayda, middle ko nuksan — toh ek curve do peaks mein split ho jaata hai (bimodal). Jaise finches jinke bade beak bade seeds todte hain, chhote beak chhote seeds — medium beak dono mein fail. Yeh type aage chalke naye species bhi bana sakta hai.
Formula-free mnemonic: Slide, Squeeze, Split = Directional, Stabilizing, Disruptive. Isko yaad rakhoge toh exam mein kabhi confuse nahi hoge — bas curve ka shape socho aur type bata do!