Hardy-Weinberg and Speciation

Spec Mapping — OCR H420 Module 6.1.2 — Patterns of inheritance, content statements covering the Hardy-Weinberg principle and its application to population-level allele and genotype frequencies, the mechanisms by which populations deviate from equilibrium (selection, drift, migration, mutation), and the two main modes of speciation (allopatric and sympatric) (refer to the official OCR H420 specification document for exact wording). This is the population-genetic capstone of the inheritance module, joining individual Mendelism (the previous 11 lessons) to evolution.

The Hardy-Weinberg principle was derived independently by the English mathematician G. H. Hardy and the German physician Wilhelm Weinberg in 1908 (paraphrased). Hardy's contribution arose from a casual conversation with Reginald Punnett — Hardy was dismissive of the supposed paradox ("would dominant alleles take over a population?") and proved within an afternoon that, under simple assumptions, allele frequencies remain constant. The principle is the null model of population genetics: any deviation from Hardy-Weinberg equilibrium is evolution.

We now leave individual families behind and look at genetics on the scale of whole populations. The Hardy-Weinberg principle gives a mathematical description of how allele and genotype frequencies behave in a non-evolving population, and lets us calculate how many carriers of a recessive disease there are even when we only know the frequency of affected individuals. At the other end of the story, speciation — the splitting of one species into two — is the ultimate outcome of evolutionary change, and it comes in two main flavours depending on how the populations become isolated. OCR A-Level Biology A specification module 6.1.2(h)–(j) requires you to apply the Hardy-Weinberg equations, describe genetic drift and the mechanisms that cause populations to deviate from equilibrium, and distinguish allopatric and sympatric speciation.

Key Definitions:

• Gene pool — the total collection of alleles in a breeding population at a given time.
• Allele frequency — the proportion of one allele among all copies of that gene in a population.
• Hardy-Weinberg principle — in an idealised population, allele and genotype frequencies remain constant from one generation to the next.
• Genetic drift — random changes in allele frequencies due to chance events, most powerful in small populations.
• Speciation — the formation of a new species from an ancestral population.
• Allopatric speciation — speciation caused by geographical isolation.
• Sympatric speciation — speciation within a single geographical area, caused by reproductive isolation without physical separation.

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Allele Frequencies in a Population

For a gene with two alleles A (dominant) and a (recessive), let:

• p = frequency of A (a number between 0 and 1).
• q = frequency of a.

Since every allele is either A or a:

$p + q = 1$p+q=1

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The Hardy-Weinberg Equation

If gametes combine randomly, the proportions of genotypes in the next generation are given by the binomial expansion of (p + q)²:

$p^2 + 2pq + q^2 = 1$p2+2pq+q2=1

• p² = frequency of homozygous dominant (AA).
• 2pq = frequency of heterozygous (Aa).
• q² = frequency of homozygous recessive (aa).

The five Hardy-Weinberg assumptions

The equilibrium only holds if:

1. No mutation at the locus of interest.
2. No natural selection — all genotypes equally fit.
3. No migration in or out of the population.
4. Random mating — no preference for particular genotypes.
5. Large population — so random fluctuations average out.

If any of these fails, allele frequencies change between generations — i.e. the population evolves.

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Worked Example 1: Cystic Fibrosis Carriers

Cystic fibrosis affects about 1 in 2500 people of European ancestry and is autosomal recessive. Use Hardy-Weinberg to find the frequency of carriers.

• q² = 1/2500 = 0.0004
• q = √0.0004 = 0.02
• p = 1 − 0.02 = 0.98
• Carrier frequency 2pq = 2 × 0.98 × 0.02 = 0.0392 ≈ 1 in 25.

So about 4% of the European population are carriers of cystic fibrosis, even though only 0.04% are affected. This kind of calculation is essential for genetic counselling — carriers vastly outnumber sufferers for any recessive disease.

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Worked Example 2: PTC Tasting

The ability to taste phenylthiocarbamide (PTC) is dominant; non-tasters are homozygous recessive. In a sample of 1000 people, 160 are non-tasters. What fraction of the population are heterozygous?

• q² = 160/1000 = 0.16
• q = 0.4
• p = 0.6
• 2pq = 2 × 0.6 × 0.4 = 0.48

So 48% of the population are heterozygous tasters — nearly half. And p² = 0.36 (36% homozygous tasters), q² = 0.16 (16% non-tasters). Check: 0.36 + 0.48 + 0.16 = 1.00. ✓

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When Hardy-Weinberg Fails: Mechanisms of Evolution

If the assumptions fail, allele frequencies shift. The four main causes of frequency change are:

1. Natural selection

Genotypes with higher fitness (more offspring) become more common. Classic example: the peppered moth (Biston betularia) — during the Industrial Revolution, dark (melanic) morphs were favoured on sooty tree trunks because they were camouflaged from bird predators, so the frequency of the dark allele rose sharply.

2. Genetic drift

In small populations, chance events can dramatically change allele frequencies. If only a handful of organisms reproduce in a given generation, the allele frequencies in their offspring may look very different from those of the parent generation, just by sampling error. Two special cases of strong drift are:

• Founder effect — a small group colonises a new area, carrying only a subset of the original gene pool. The frequencies in the new colony reflect which alleles the founders happened to bring. Example: the Amish communities of Pennsylvania, descended from ~200 founders, have high frequencies of certain genetic conditions (e.g. Ellis-van Creveld syndrome) that are rare in the general European population.
• Bottleneck — a population crashes to a small size (by disease, disaster, hunting) and subsequently recovers, but has lost much of its genetic variation. Example: cheetahs show remarkably low genetic diversity because of an ancient bottleneck.

3. Migration (gene flow)

Individuals moving between populations bring their alleles with them, making the populations more genetically similar and preventing divergence.

4. Mutation

New alleles arise at a low background rate. Over long periods this is the ultimate source of genetic variation, though it is a weak short-term force on allele frequencies.

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Speciation

A species is a group of organisms that can interbreed to produce fertile offspring (the biological species concept). Speciation is the process by which one ancestral species gives rise to two or more descendant species. For speciation to happen, populations must become reproductively isolated — unable to interbreed successfully — and then diverge until they are genetically distinct.

There are two main modes:

Allopatric Speciation

Populations are separated by a geographical barrier — a mountain range, a river, a new island, a glacier. Gene flow stops. The two populations experience different selection pressures and different mutations, and genetic drift acts on them independently. Over many generations they diverge until, even if reunited, they no longer interbreed successfully. Classic examples:

• Galápagos finches — ancestors blew across from South America, colonised different islands, and diversified into 13 species with different beaks adapted to different food sources.
• Squirrels on the rims of the Grand Canyon — the Abert's squirrel (south rim) and Kaibab squirrel (north rim) diverged after the canyon separated them.

Sympatric Speciation

Populations diverge without geographical separation. They occupy the same area but become reproductively isolated for other reasons — differences in mating season, behaviour, habitat or, in plants, chromosome number (polyploidy). Sympatric speciation is rarer in animals but widespread in plants. Examples:

• Wheat (Triticum aestivum) — modern bread wheat is a hexaploid (six sets of chromosomes) that arose by repeated polyploidy events between wild grass species. It is reproductively isolated from its ancestors in a single generation.
• Apple maggot flies — originally fed on hawthorn, but some switched to apples after European settlers introduced apple trees to North America. Mating and egg-laying now happen on the host fruit, and because apples ripen at a different time from hawthorns, the two populations rarely encounter each other and have started to diverge.

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Flowchart: Mechanisms of Speciation

flowchart TB
    A[Ancestral population: one gene pool] --> B{Isolation type}
    B -->|geographical barrier| C[Allopatric speciation]
    B -->|reproductive barrier in same area| D[Sympatric speciation]
    C --> E[Different selection pressures, drift, mutation]
    D --> F[Polyploidy, behavioural isolation, temporal isolation]
    E --> G[Reproductive isolation and genetic divergence]
    F --> G
    G --> H[Two distinct species]

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Reproductive Isolation Mechanisms

For speciation to be complete, gene flow between the two populations must stop. Biologists divide isolating mechanisms into pre-zygotic (preventing mating or fertilisation) and post-zygotic (preventing viable, fertile offspring).

Mechanism	Type	Example
Geographical	Pre-zygotic	Populations separated by a mountain
Temporal	Pre-zygotic	Breed at different times of year
Behavioural	Pre-zygotic	Different courtship dances or songs
Mechanical	Pre-zygotic	Incompatible genitalia
Gametic	Pre-zygotic	Sperm and egg fail to recognise each other
Hybrid inviability	Post-zygotic	Embryo dies early
Hybrid sterility	Post-zygotic	Hybrid offspring cannot reproduce (e.g. mules)

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Exam Tip

The Hardy-Weinberg equations are algebraically simple but easy to get wrong under pressure. Start by identifying q² (the only genotype you can usually see directly — the homozygous recessive), take the square root to get q, then subtract from 1 to get p, then use 2pq to find the carrier frequency. For speciation questions, always be clear about whether the barrier is geographical (allopatric) or something else (sympatric). Polyploidy is a classic sympatric example in plants.

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Common Exam Mistakes

• Assuming p² is the frequency of "A" alleles. No — p² is the frequency of AA genotype; p is the frequency of the A allele.
• Forgetting the five assumptions. Without them, Hardy-Weinberg does not apply.
• Using 2pq for the frequency of heterozygotes as a percentage without converting. 2pq is a proportion between 0 and 1; multiply by 100 for a percentage.
• Saying "natural selection and drift are the same". Selection is non-random (fitness-based); drift is random sampling. Drift is strongest in small populations.
• Confusing founder effect and bottleneck. Founder effect: small group colonises a new area. Bottleneck: existing population crashes to a small size.
• Saying "sympatric speciation is impossible in animals". It is rare but documented (apple maggot flies, cichlid fish in Lake Victoria).
• Forgetting reproductive isolation. Two populations can be physically separated but are only "different species" when they cannot interbreed successfully.

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Quick Recap

• Allele frequencies: p + q = 1.
• Genotype frequencies (Hardy-Weinberg): p² + 2pq + q² = 1.
• Assumptions: no mutation, no selection, no migration, random mating, large population.
• Allows calculation of carrier frequency from the incidence of a recessive disease.
• Genetic drift changes allele frequencies by chance — strongest in small populations, including the founder effect and bottlenecks.
• Natural selection changes allele frequencies by favouring fitter genotypes.
• Allopatric speciation: geographic isolation → divergence (e.g. Galápagos finches).
• Sympatric speciation: reproductive isolation without geography — often by polyploidy in plants (e.g. wheat) or behavioural/temporal isolation in animals.
• Reproductive isolation can be pre-zygotic or post-zygotic.

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Continuous vs Discontinuous Variation — Inline SVG

Hardy-Weinberg analysis treats each locus as biallelic and produces three discrete genotype frequencies. Real population-level traits, however, may be discontinuous (single-locus, Mendelian — like ABO blood group, sickle-cell) or continuous (polygenic + environmental — like height, skin pigmentation, milk yield).