BIOLOGY · EVOLUTION AND ECOLOGY · POPULATION GENETICS / SPECIATION

Count the alleles.
Then name the force.

Use Hardy–Weinberg as a conditional null model, distinguish the mechanisms that change populations, and require reproductive-isolation evidence before declaring speciation.

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12practice questions
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The Population Genetics / Speciation reasoning loop

Use one count–force–barrier workflow.

  1. 01Count

    Separate individuals, genotypes, phenotypes, and the 2N allele-copy denominator.

  2. 02Model

    Apply p + q and p² + 2pq + q² only under the stated Hardy–Weinberg assumptions.

  3. 03Name

    Classify creation, movement, chance sampling, fitness sorting, or nonrandom pairing.

  4. 04Locate

    Place a reproductive barrier before or after fertilization.

  5. 05Conclude

    Treat geography as an opportunity for divergence, not automatic proof of speciation.

Population-genetics instruction is cross-checked against OpenStax Biology 2e · Population Genetics ↗.

Three linked lessons

From allele counts to evidence-bounded species formation.

Frequency equations are conditional models. Drift is chance rather than weak selection, gene flow moves alleles, nonrandom mating changes pairings, and speciation needs evidence of reduced gene flow or independent lineage.

01

LESSON 1 · 24 MIN

Study + retrieve

Allele frequencies and the Hardy–Weinberg null model

Calculate allele and genotype frequencies and apply Hardy–Weinberg expectations only when their assumptions are stated or tested.

ESSENTIAL QUESTIONAm I counting alleles or genotypes, and is equilibrium an assumption, prediction, or supported fit?
Allele-count and Hardy–Weinberg ledgerA diploid count table shows AA contributes two A alleles, Aa contributes one A and one a, and aa contributes two a alleles, all divided by 2N. A second panel states p plus q equals one and expected genotype frequencies p squared, 2pq, and q squared only under the stated null-model assumptions. A warning states dominant phenotype frequency is not dominant allele frequency. Every formula and assumption is written in text.COUNT ALLELE COPIES · DENOMINATOR = 2NAA2 A · 0 aAa1 A · 1 aaa0 A · 2 aALLELE FREQUENCYcopies ÷ 2NTWO-ALLELE LEDGERp + q = 1observed allele frequenciesHARDY–WEINBERG EXPECTATIONp² · 2pq · q²only under stated assumptionsDOMINANT PHENOTYPE = AA + AaPHENOTYPE FREQUENCY ≠ ALLELE FREQUENCYSTUDY DIAGRAM · TEXT DESCRIPTION AVAILABLE
01

Count alleles explicitly

A diploid population of N individuals contains 2N allele copies at an autosomal locus. Each homozygote contributes two copies of its allele; each heterozygote contributes one of each. Dominant phenotype frequency combines multiple genotypes and cannot be substituted directly for an allele frequency.

  • Denominator 2N
  • Homozygote contributes two
  • Phenotype is not allele
02

Use Hardy–Weinberg conditionally

For two alleles, p + q = 1. Under random mating and absence of evolutionary forces in the idealized model, expected genotype frequencies are p², 2pq, and q². These equations are a null model, not a declaration that real populations never evolve.

  • p + q = 1
  • p² + 2pq + q² = 1
  • Check assumptions
03

Separate one-generation mating from evolution

Random mating generates Hardy–Weinberg genotype proportions from allele frequencies in one generation, while mutation, migration, drift, and selection govern allele-frequency change. A genotype-frequency departure can reveal an assumption violation without identifying which force acted.

  • Genotype expectation
  • Allele change is separate
  • Departure does not name cause

Worked example

In 100 diploid individuals, AA = 36, Aa = 48, and aa = 16. What are p(A) and q(a)?

  1. 1

    There are 200 total allele copies.

  2. 2

    A copies = 2(36) + 48 = 120.

  3. 3

    a copies = 2(16) + 48 = 80.

Conclusionp = 120/200 = 0.60 and q = 80/200 = 0.40.

Close the notes first

Retrieve the evidence boundary.

01How many autosomal allele copies occur in N diploid individuals?
2N.

Each individual carries two copies at the locus.

02Under Hardy–Weinberg, what is heterozygote frequency?
2pq.

Either allele can come from either parental gamete.

03Does matching Hardy–Weinberg prove no force will ever act?
No; it is a conditional snapshot and null expectation.

Later forces or hidden structure can change the population.

02

LESSON 2 · 24 MIN

Study + retrieve

Evolutionary forces and population variation

Distinguish mutation, drift, gene flow, nonrandom mating, and selection by their mechanisms and predicted effects.

ESSENTIAL QUESTIONDid alleles arise, move, sample by chance, pair nonrandomly, or reproduce unequally?
Evolutionary-force mechanism tableFive labeled rows state mutation creates new alleles, gene flow moves alleles, drift samples by chance, selection sorts heritable variants by reproductive success, and nonrandom mating changes allele pairings. Bottleneck and founder boxes show small unrepresentative samples and warn chance is not adaptation. A gene-flow arrow says recipient variation can rise while between-population divergence often falls.NAME THE MECHANISM BEFORE THE FORCEMUTATIONcreates allelesGENE FLOWmoves allelesDRIFTsamples by chanceSELECTIONsorts by fitnessMATINGpairs allelesBOTTLENECK / FOUNDERsmall unrepresentative sampleCHANCE ≠ ADAPTATIONGENE FLOWrecipient variation may risebetween-population divergence may fallALLELE-FREQUENCY CHANGE ALONE DOES NOT IDENTIFY THE FORCESTUDY DIAGRAM · TEXT DESCRIPTION AVAILABLE
01

Give each force one job

Mutation creates new alleles. Gene flow moves alleles between populations. Drift changes frequencies through chance sampling, especially in small populations. Selection changes frequencies through heritable fitness differences. Nonrandom mating directly changes genotype pairings and can alter genotype frequencies even without changing allele frequencies by itself.

  • Create
  • Move
  • Sample
  • Sort
  • Pair
02

Recognize bottlenecks and founders

A bottleneck is a sharp population reduction whose survivors may be an unrepresentative sample. A founder event begins a population from a small, nonrepresentative subset. Both intensify drift, can reduce variation, and are not automatically adaptive.

  • Chance sample
  • Small population
  • Not proof of selection
03

Predict gene flow and divergence

Gene flow can introduce variation into a recipient population while making exchanging populations more similar at moved loci. Selection can oppose that homogenization at locally important loci. The net pattern depends on migration, fitness, drift, and time.

  • Adds recipient variants
  • Often reduces divergence
  • Forces can oppose

Worked example

A storm randomly leaves ten survivors from a population of 2,000, and one formerly rare allele becomes common. Which force is most direct?

  1. 1

    Survival was described as random with respect to genotype.

  2. 2

    The surviving sample was extremely small.

  3. 3

    Frequency changed through chance representation.

ConclusionThis is a bottleneck-driven genetic drift result, not evidence that the allele was adaptive.

Close the notes first

Retrieve the evidence boundary.

01Which force is the ultimate source of new alleles?
Mutation.

Other forces redistribute or sort variation already present.

02Why is drift stronger in small populations?
Each random reproductive or survival event represents a larger fraction of the gene pool.

Sampling variance rises as population size falls.

03Can nonrandom mating alter genotype frequencies without allele-frequency change?
Yes.

It changes which alleles pair into genotypes, while allele counts can initially remain constant.

03

LESSON 3 · 24 MIN

Study + retrieve

Gene flow, reproductive barriers, and speciation

Classify reproductive barriers and determine what geographic, hybrid, chromosome, or gene-flow evidence supports about speciation.

ESSENTIAL QUESTIONWhat blocks gene flow, when does it act, and does the evidence show isolation rather than distance alone?
Reproductive-isolation decision mapA decision map first asks whether fertilization occurs. If no, habitat, temporal, behavioral, mechanical, and gametic barriers are labeled prezygotic. If yes, reduced hybrid survival, fertility, or later-generation performance are labeled postzygotic. Separate allopatric and sympatric routes both lead to a box requiring evidence of reduced gene flow or independent lineage. A polyploid plant branch shows tetraploid by tetraploid fertile and tetraploid by diploid infertile. The footer states geography alone is not proof of speciation.WHERE DOES GENE FLOW FAIL?BEFORE FERTILIZATION · PREZYGOTIChabitat · time · behaviormechanics · gamete recognitionNO ZYGOTE FORMSAFTER FERTILIZATION · POSTZYGOTICreduced hybrid viabilitysterility · later-generation weaknessZYGOTE FORMS FIRSTALLOPATRIC ROUTEgeography reduces contactTEST ISOLATIONSYMPATRIC ROUTEsame region · reduced flowTEST ISOLATIONPOLYPLOID MODEL4n × 4n fertile4n × 2n infertileGEOGRAPHIC SEPARATION ALONE IS NOT PROOF OF SPECIATIONSTUDY DIAGRAM · TEXT DESCRIPTION AVAILABLE
01

Geography permits but does not prove

Allopatric separation can reduce gene flow and allow mutation, drift, and selection to build divergence. Distance or a barrier alone does not prove two species formed. Evidence must address reproductive isolation or another appropriate independently evolving lineage criterion.

  • Barrier reduces contact
  • Divergence can accumulate
  • Test isolation
02

Time the reproductive barrier

Prezygotic barriers reduce mating or fertilization through habitat, time, behavior, mechanics, or gamete compatibility. Postzygotic barriers act after fertilization through reduced hybrid viability, fertility, or later-generation performance.

  • Before fertilization
  • After fertilization
  • Name the failed step
03

Handle sympatry and polyploidy

Speciation can occur without geographic separation when gene flow is reduced within the same region. In plants, chromosome-set changes can create immediate mating incompatibility with the ancestral diploid population while allowing reproduction among compatible polyploids.

  • Same geography possible
  • Assortment can reduce flow
  • Polyploid barrier

Worked example

Two plant populations flower together, but a new tetraploid produces fertile offspring with other tetraploids and infertile offspring with diploids. What is supported?

  1. 1

    The chromosome-set difference arose without required geographic separation.

  2. 2

    Gene flow to diploids is strongly reduced after fertilization.

  3. 3

    Tetraploids reproduce successfully with one another.

ConclusionThe evidence supports rapid sympatric reproductive isolation associated with polyploidy.

Close the notes first

Retrieve the evidence boundary.

01Is geographic separation itself a reproductive barrier?
It reduces contact, but separation alone does not prove evolved reproductive isolation.

Populations may still interbreed if contact resumes.

02Is sterile hybrid offspring prezygotic or postzygotic evidence?
Postzygotic.

Fertilization occurred before hybrid fertility failed.

03How can polyploidy rapidly reduce gene flow?
Different chromosome sets can make crosses with ancestral ploidy infertile while compatible polyploids reproduce.

Chromosome pairing and gamete compatibility can create an immediate barrier.

Randomized retrieval set

Now calculate the frequency, identify the force, or time the barrier.

Allele counting, Hardy–Weinberg, bottlenecks, founders, migration, assortative mating, prezygotic and postzygotic barriers, geography, and polyploidy are interleaved.

12 PRACTICE QUESTIONS

Retrieve before you review.

Question order and all five answer options are shuffled when you begin. The correct answer stays attached to the same underlying choice.

Scope and score notice

Population foundations, not one universal species concept.

The ADA lists population genetics/speciation within Evolution and Ecology but does not publish a subtopic item quota. DAT TRAIN does not invent one.

Advanced statistical genetics, effective-population-size derivations, and taxon-specific species debates remain outside this route unless the prompt supplies the model.

Use your results to choose what to review next—not as an official DAT score prediction.