Noesis
5.5.4Population Genetics & Speciation

Describe gene flow and migration

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🇮🇳 Yeh note Hinglish mein padho →

WHAT is it?

WHY does gene flow matter? (its 3 effects)

  1. Homogenises populations → makes allele frequencies of two populations converge (more alike).
  2. Introduces new alleles / variation into a receiving population (a source of new raw material, like mutation but importing ready-made alleles).
  3. Opposes speciation → because it counteracts the divergence caused by drift, selection, and mutation. This is the link to speciation: reproductive isolation cuts off gene flow, allowing populations to diverge into new species.

HOW do we quantify it? (Derivation from first principles)

Goal: predict a population's new allele frequency after migrants arrive.

Set up the scenario. We track one allele AA.

  • Let the resident (recipient) population have allele-AA frequency p0p_0.
  • Let the migrant (source) population have allele-AA frequency pmp_m.
  • Let mm = the fraction of the new population that is made of migrants (migration rate), so 0≤m≤10 \le m \le 1.

Step 1 — Split the new population into two groups. After migration, the new population is a mixture:

  • Fraction (1−m)(1-m) are residents, each contributing allele frequency p0p_0.
  • Fraction mm are migrants, each contributing allele frequency pmp_m.

Why this step? Allele frequency of a mixture is just the weighted average of the parts, weighted by how much of the population each part is.

Step 2 — Take the weighted average. p1=(1−m) p0+m pmp_1 = (1-m)\,p_0 + m\,p_m

Why this step? Each group's alleles are pooled; the mean allele frequency is the sum of (proportion × its frequency).

Step 3 — Rearrange to see the change. Δp=p1−p0=(1−m)p0+m pm−p0=m(pm−p0)\Delta p = p_1 - p_0 = (1-m)p_0 + m\,p_m - p_0 = m(p_m - p_0)

Figure — Describe gene flow and migration

Worked Examples

Common Mistakes (Steel-man + fix)

Recall Feynman: explain to a 12-year-old

Imagine two jars of marbles — one mostly blue, one mostly red. If you scoop some marbles from the blue jar and dump them into the red jar (and they get mixed in), the red jar becomes a bit more blue. That's gene flow: animals or pollen carry "marble colours" (alleles) between groups, and after mixing the groups start to look alike. If you seal the jars so nothing moves, each jar can slowly become its own special colour — and that's how brand-new species can form: only when the mixing stops.

Flashcards

What is the difference between migration and gene flow?
Migration is the physical movement of individuals/gametes between populations; gene flow is the actual transfer of alleles that occurs only when migrants successfully breed.
Define the migration rate mm.
The fraction (0–1) of the new/recipient population made up of migrants — a proportion, not a raw count.
Write the formula for allele frequency after gene flow.
p1=(1−m)p0+m pmp_1 = (1-m)p_0 + m\,p_m, where p0p_0 = resident freq, pmp_m = migrant freq.
Write the formula for the change in allele frequency due to gene flow.
Δp=m(pm−p0)\Delta p = m(p_m - p_0).
When does gene flow cause NO change in allele frequency?
When m=0m=0 (no migrants) or when pm=p0p_m = p_0 (populations already have equal frequencies).
What are the three main effects of gene flow?
It homogenises populations (makes them alike), introduces new alleles/variation, and opposes speciation.
Does gene flow promote or oppose speciation? Why?
It opposes speciation because it homogenises populations, counteracting the divergence from drift, selection, and mutation.
With one-way migration from a fixed source at pmp_m, what is the equilibrium resident frequency?
p=pmp = p_m (the resident frequency is dragged fully to the source value).
If p0=0.4p_0=0.4, pm=0.9p_m=0.9, m=0.2m=0.2, find p1p_1.
p1=0.8(0.4)+0.2(0.9)=0.32+0.18=0.50p_1 = 0.8(0.4)+0.2(0.9)=0.32+0.18=0.50.
Is gene flow a source of new variation like mutation?
Yes — it can import alleles the recipient population lacked, acting as a source of new genetic variation (but by importing, not by creating new alleles).

Connections

  • Hardy-Weinberg Equilibrium — gene flow is one of the assumptions that must be absent for HWE.
  • Genetic Drift — random loss of alleles that gene flow can counteract by re-supplying alleles.
  • Natural Selection — can create divergence that gene flow opposes.
  • Reproductive Isolation — the stopping of gene flow that permits speciation.
  • Speciation Modes (Allopatric vs Sympatric) — allopatric speciation works because geography cuts off gene flow.
  • Mutation as source of variation — compare: mutation creates new alleles, gene flow imports existing ones.

Concept Map

only becomes when

produces

effect 1

effect 2

effect 3

cuts off

allows

quantified by

rearranged to

makes frequencies converge

Migration: movement of individuals

Reproduction in new population

Gene flow: transfer of alleles

Homogenises populations

Introduces new variation

Opposes speciation

Reproductive isolation

Divergence into new species

p1 = (1-m) p0 + m pm

delta p = m (pm - p0)

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, gene flow ka core idea simple hai: populations sealed dabbe nahi hote. Jab kisi ek population ke individuals (ya unke pollen, seeds, gametes) doosri population mein jaate hain aur wahan successfully breed karte hain, toh woh apne alleles saath le jaate hain. Yehi allele ka transfer gene flow kehlata hai. Yaad rakho — sirf migration (movement) kaafi nahi; jab tak migrant reproduce nahi karta, koi gene flow nahi hota. Ek tourist ghoom ke chala jaaye toh alleles add nahi hote!

Formula bhi bilkul intuitive hai: naye population ki allele frequency purani residents aur incoming migrants ka weighted average hai. p1=(1−m)p0+m pmp_1 = (1-m)p_0 + m\,p_m, jahan mm migrants ka fraction hai (count nahi, proportion). Aur change Δp=m(pm−p0)\Delta p = m(p_m - p_0) — matlab jitna zyada migration (mm) aur jitna zyada difference dono populations mein, utna zyada frequency badlegi. Agar dono already same hain (pm=p0p_m = p_0), toh kuch nahi hota.

Iska sabse bada effect: gene flow populations ko ek jaisa (homogenise) banata hai — glue ki tarah unhe jodta hai. Isiliye gene flow speciation ko rokta hai, promote nahi karta. Naye species tabhi banenge jab yeh flow ruk jaaye (reproductive isolation), taaki drift aur selection populations ko alag-alag direction mein push kar saken. Yeh point exam mein bahut poocha jaata hai — gene flow = homogenising force, isolation = divergence ki chaabi.

Test yourself — Population Genetics & Speciation

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Connections

Distinguish natural and artificial selectionBiology · 5.4.6Distinguish allopatric and sympatric speciationBiology · 5.5.7State the Hardy-Weinberg principle and assumptionsBiology · 5.5.1Explain genetic drift and the bottleneck - founder effectsBiology · 5.5.3Explain the role of mutation in evolutionBiology · 5.5.5Distinguish allopatric and sympatric speciationBiology · 5.5.7Describe reproductive isolation mechanisms (pre - postzygotic)Biology · 5.5.8