Variation and selection

Unit 11 Variation and selection

Key unit competence: Justify that new traits among organisms result from variation and selection.

Key ideas

• Variation is the differences between individuals of the same species. Causes of genetic variation: crossing-over between homologous chromosomes in prophase I; independent assortment of chromosomes in metaphase I; random fertilisation (any sperm with any egg); random mating; mutation (the only source of new alleles). Environmental causes: diet, climate, light, disease, training, culture.
• Advantages of variation: some individuals survive changes in the environment (disease, climate), allows natural selection and evolution, gives raw material for selective breeding. Disadvantages: some variants are harmful (genetic disorders), competition, less uniform crops for farmers.
• Genetic variation is caused by differences in genes (inherited); phenotypic variation = genotype + environment interaction (P = G + E). For example, identical twins share genotypes but differ in weight if fed differently; plants of the same clone grow differently in shade and in sun.
• Continuous variation: a range of values with no distinct categories (height, mass, skin colour, leaf length); controlled by many genes (polygenes) each with a small additive effect, plus a strong environmental effect; quantitative (measured); shown as a histogram that forms a normal distribution (bell-shaped curve). Discontinuous variation: distinct categories with no intermediates (ABO blood groups, tongue rolling, sex, albinism); controlled by one or a few genes with major effect; little environmental effect; qualitative; shown in a bar chart.
• Statistics: mean, standard deviation (spread of data around the mean; about 68% of values lie within ±1 SD, 95% within ±2 SD). The t-test compares the means of two samples to decide whether the difference is significant: t = |x̄₁ − x̄₂| ÷ √(s₁²/n₁ + s₂²/n₂); degrees of freedom = n₁ + n₂ − 2. If the calculated t is greater than the critical value at p = 0.05, reject the null hypothesis (no difference): the difference is significant (less than 5% probability it is due to chance). The chi-squared (χ²) test compares observed and expected frequencies (e.g. genetic ratios).
• Natural selection (Darwin): organisms overproduce offspring; there is variation among them; there is a struggle for existence (competition, predation, disease); individuals with advantageous characteristics are more likely to survive and reproduce ("survival of the fittest"); they pass their alleles on, so allele frequencies change over generations. The environmental factor that favours some phenotypes is the selection pressure.
• Types of natural selection: stabilising — favours the average phenotype, removes extremes; occurs in stable environments (human birth weight); directional — favours one extreme, shifting the mean, when the environment changes (antibiotic resistance in bacteria, industrial melanism in the peppered moth, pesticide resistance, warfarin-resistant rats); disruptive — favours both extremes over the average, may lead to two forms and speciation (beak sizes in African seedcracker finches).
• Artificial selection (selective breeding): humans choose individuals with desirable traits to breed, over many generations: high-yield crops, disease-resistant beans, high-milk cows (e.g. Friesian crossed with Ankole), docile animals, dog breeds. Methods: inbreeding (close relatives — keeps traits but risks inbreeding depression, harmful recessives), outbreeding/crossbreeding (unrelated — hybrid vigour), artificial insemination. Advantages: higher yield, quality, resistance. Disadvantages: loss of genetic diversity, vulnerability to new diseases, animal welfare problems.
• Gene pool: all the alleles of all genes in a population. Allele frequency is the proportion of a particular allele. Hardy–Weinberg principle: allele and genotype frequencies stay constant if the population is large, mating is random, and there is no mutation, no migration, no selection. Equations: p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies: p² homozygous dominant, 2pq heterozygous, q² homozygous recessive). Genetic drift: random changes in allele frequency, strongest in small populations; founder effect — a few individuals start a new population with a non-representative gene pool (e.g. high frequency of some disorders in isolated communities); bottleneck effect — a population is drastically reduced (cheetahs, northern elephant seals), losing variation.

Common misconceptions

• "Organisms change because they need to." No: variation already exists by chance; selection favours those that happen to be better adapted.
• "Continuous variation is only environmental." No: it is caused by many genes plus the environment.
• "A significant t-test result proves the hypothesis." No: it shows the difference is unlikely to be due to chance at the chosen probability level.
• "Hardy–Weinberg describes populations that are evolving." No: it describes populations not evolving; departures from it show evolution.

Worked examples

Practice questions

1. State five sources of genetic variation.

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   Mutation; crossing-over; independent assortment; random fertilisation; random mating.

   Common mistake: Listing diet as genetic.

2. Distinguish continuous and discontinuous variation.

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   Continuous: range of values, polygenic, environmental influence, histogram, e.g. height. Discontinuous: distinct groups, one or few genes, little environmental effect, bar chart, e.g. blood groups.

   Common mistake: Giving height as discontinuous.

3. Explain how natural selection leads to evolution.

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   Overproduction; variation; competition; individuals with advantageous alleles survive and reproduce; pass on alleles; allele frequency changes over generations.

   Common mistake: Saying organisms change because they need to.

4. Describe the three types of natural selection with examples.

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   Stabilising: average favoured, e.g. birth weight. Directional: one extreme favoured, e.g. antibiotic resistance, peppered moth. Disruptive: both extremes favoured, e.g. seedcracker beak sizes.

   Common mistake: Missing examples.

5. Compare natural and artificial selection.

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   Natural: environment selects, slow, increases fitness to environment. Artificial: humans select, faster, traits useful to humans, may reduce fitness and genetic diversity.

   Common mistake: Saying artificial selection is always natural.

6. State the conditions for Hardy–Weinberg equilibrium.

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   Large population; random mating; no mutation; no migration; no natural selection.

   Common mistake: Forgetting population size.

7. Explain the founder effect and bottleneck effect.

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   Founder: a small group starts a new population with a gene pool unlike the original. Bottleneck: population sharply reduced; survivors carry a small part of the variation; both reduce diversity.

   Common mistake: Confusing them with selection.

8. Explain when a t-test is used and how to interpret it.

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   To compare means of two samples with continuous data; calculate t; compare with critical value at p = 0.05 and degrees of freedom; if t is greater, the difference is significant.

   Common mistake: Using it for categories.

Matching and drag-and-drop

1. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. Hybrid vigour results from:	a. standard deviation
   2. The only source of new alleles is:	b. mutation
   3. The spread of data about the mean is measured by:	c. gene pool
   4. All the alleles in a population form the:	d. outbreeding

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   1 → d (outbreeding) · 2 → b (mutation) · 3 → a (standard deviation) · 4 → c (gene pool)

   • Heterozygosity.
   • Changes in DNA.
   • SD.
   • Total alleles.

2. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   directional selection  ·  non-disjunction  ·  crossing-over  ·  continuous variation
   1. Exchange of genetic material between chromatids is .
   2. Height in humans shows .
   3. The peppered moth shows .
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   1: crossing-over · 2: continuous variation · 3: directional selection

   • Prophase I.
   • Range of values.
   • Industrial melanism.

3. Matching

   Match each item in column A with the correct answer in column B. Write the letter next to each number.

   Column A	Column B
   1. The theory of natural selection was proposed by:	a. two means
   2. The t-test compares:	b. many genes
   3. Continuous variation is controlled by:	c. random mating
   4. Which condition is needed for Hardy–Weinberg equilibrium?	d. Charles Darwin

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   1 → d (Charles Darwin) · 2 → a (two means) · 3 → b (many genes) · 4 → c (random mating)

   • With Wallace.
   • Significance.
   • Polygenes.
   • No evolution.

4. Drag and drop

   Choose the correct word or phrase from the box for each gap. One word in the box is not needed.

   stabilising  ·  continuous variation  ·  0.05  ·  discontinuous variation
   1. Selection that favours the average phenotype is .
   2. The probability level used in biology is usually .
   3. ABO blood groups show .
   Show answer

   1: stabilising · 2: 0.05 · 3: discontinuous variation

   • Removes extremes.
   • 5%.
   • Distinct categories.

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