Biology · Evolutionary Processes & Adaptation

Genetic drift, population size and gene flow

The next generation is a finite set of descendants, not an exact copy of every allele frequency. A chance imbalance can therefore change the population. Genetic material arriving from another population can also change it, but through a different route.

The introductory inheritance chapter distinguishes alleles, genotypes and phenotypes. Meiosis explains segregation and crossing over; the DNA chapter explains sequence changes. Fractions and simple multiplication are enough for the constructed examples here. No example represents a measured natural population unless explicitly identified as published research.

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By the end, you can

Distinguish random reproductive sampling from movement of genetic material, and explain why population size affects drift.

Chance transmission can change frequencies

Genetic drift is change in variant frequencies through chance in finite populations. Imagine a neutral model in which A and a have equal reproductive opportunities. Each inherited copy in the next generation is sampled from a parental pool with A frequency 0.50. A finite set need not contain exactly half A copies.

The constructed diagram shows three possible independent samples of ten copies: 3 A, 5 A and 7 A. Their frequencies are 0.30, 0.50 and 0.70. There is no rule directing A upward. The examples illustrate possible variation; they are not the full probability distribution or measurements of three populations.

With the same parental frequency and a sample of 100 copies, a difference of one copy changes the frequency by 0.01 rather than 0.10. Under comparable neutral sampling assumptions, larger breeding populations have smaller random frequency fluctuations on average. A particular large sample can still depart from its expectation, and drift is not absent from large populations.

Possible samples can move a frequency in either direction

Hand-constructed possible outcomes, not simulated results, representative likelihoods or a full probability distribution. No variant has an assigned advantage.

Read the diagram in words

A parental copy pool at p = 0.50 feeds independent finite samples with replacement; a separate founder inset samples haploid individuals without replacement.

Parental locus-copy pool · p(A) = 0.50; Neutral independent sampling with replacement.

Independent sample	Total locus copies	Copies of A	Copies of a	Frequency of A
3 A, 7 a → p = 0.30	10	3	7	0.3
5 A, 5 a → p = 0.50	10	5	5	0.5
7 A, 3 a → p = 0.70	10	7	3	0.7
47 A, 53 a → p = 0.47	100	47	53	0.47
50 A, 50 a → p = 0.50	100	50	50	0.5
53 A, 47 a → p = 0.53	100	53	47	0.53

One-copy replacement: 0.10 with 10 total; 0.01 with 100 total

Separate haploid founder example

Neutral sample without replacement. X: 10; Y: 10.

Founders of X	Founders of Y	Frequency of X
3	1	0.75
1	3	0.25

Breeding history matters beyond a head count

The effective population size is the size of an idealised population that would show the same specified genetic effect, such as drift, as the population being studied. It may differ from the number of organisms counted. Unequal reproductive contributions, sex ratios and changes through time can affect the comparison.

A founder event starts a population with only part of the previous genetic variation. A bottleneck sharply reduces the contributors to later generations. A random founding or survival sample can change frequencies and lose variants without selecting for useful traits. Natural founders or survivors need not be random, however: dispersal and mortality can also involve selection.

In a finite isolated neutral model, a variant can eventually reach frequency zero, called loss, or one, called fixation. Fixation at a locus does not mean every organism has an identical genome. A population that grows after losing an allele does not reconstruct that allele merely by becoming numerous; mutation or gene flow may introduce variants later.

Gene flow transfers inherited material between populations

Gene flow occurs when genetic material from one population enters the inherited pool of another. It can involve organisms that contribute reproductively after moving, or gametes such as pollen. A visitor that leaves no inherited contribution is not evidence of gene flow into the next generation.

Suppose resident contributors supply 80 locus copies, including 32 A copies. Incoming contributors supply 20 copies, including 16 A. The combined pool has 48 A copies among 100, so its A frequency is 0.48. The resident pool began at 0.40, while the incoming pool was at 0.80. The combined result is a weighted average, not the unweighted midpoint 0.60.

Gene flow can introduce a locally absent allele and can reduce differences between connected populations. Its effect depends on the amount of inherited contribution and the frequencies involved. It need not improve local adaptation; a variant advantageous elsewhere may be less advantageous here. Selection, drift and gene flow can operate together.

Count inherited input rather than merely moving visitors

Original equal-representation copying model, no additional selection, mutation or random sampling. Incoming contribution is 20% of the combined copy pool.

Read the diagram in words

Two source pools contribute locus copies to a combined breeding pool; proportional bars make the weighting visible.

Contributing pool	Total locus copies	Copies of A	Copies of a	Frequency of A
Resident contribution	80	32	48	0.4
Incoming inherited contribution	20	16	4	0.8
Combined breeding pool	100	48	52	0.48

Worked example

A founder sample is not a vote for the best variant

A constructed parental pool contains ten X and ten Y haploid individuals. Choose four founders by a neutral random sample without replacement. One possible outcome is three X and one Y, changing X from 10/20 = 0.50 to 3/4 = 0.75 in the founding group.

An outcome of one X and three Y is also possible under this symmetric setup. The first outcome does not show that X was needed or fitter. Nor do these two selected examples give the probability of each outcome. Additional reproductive differences or biased dispersal would need separate evidence and a different model.

Check your understanding

In the ten-copy sampling model, how much does replacing one a copy with one A copy change p?
Show reasoning
It raises the A count by one while keeping ten total copies, so p increases by 1/10 = 0.10. With 100 total copies the same replacement changes p by 1/100 = 0.01.
What happens if the 20 incoming copies in the worked gene-flow pool also have A frequency 0.40?
Show reasoning
They contribute eight A copies. The combined frequency is (32 + 8)/(80 + 20) = 40/100 = 0.40. Gene flow can occur without changing this locus’s frequency when the contributing pools have the same frequency.
Why can a recovering population remain genetically depleted after a bottleneck?
Show reasoning
Reproduction increases the number of descendants of the surviving contributors; it does not automatically restore alleles absent from them. Later mutation or inherited input from elsewhere may change that situation. A larger census alone does not undo the history.

What to carry forward

Drift changes representation through chance; gene flow changes the inherited pool through movement and reproductive contribution.

References

Resources supporting this lesson’s explanations and examples.