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17.2 Natural and Artificial Selection

Syllabus
9700–2028–2029
Topic
17.2
Level
A2

Learning objectives

17.2.1Natural selection mechanism• Natural selection: populations produce many offspring competing for resources; best adapted survive to reproduce17.2.2Stabilising, disruptive and directional selection• Environmental factors as forces of natural selection: stabilising, disruptive, directional17.2.3Effects on allele frequencies• Effects on allele frequencies: selection, founder effect, genetic drift (including bottleneck effect)17.2.4Antibiotic resistance and selection• Bacterial antibiotic resistance as natural selection example17.2.5Hardy-Weinberg principle• Hardy-Weinberg principle: calculate allele and genotype frequencies, conditions for application17.2.6Selective breeding principles• Principles of selective breeding (artificial selection)17.2.7Selective breeding examples• Selective breeding examples:- Disease resistance in wheat and rice- Inbreeding and hybridisation in maize- Improving milk yield in dairy cattle

Natural selection changes populations

Natural selection is a population process: inherited variation exists, environmental selection pressures affect survival and reproductive success unequally, and advantageous heritable alleles can become more frequent over generations.

  • A population produces more offspring than can usually survive because resources and other conditions are limited.
  • Individuals with different heritable phenotypes experience a selection pressure differently; those with a favourable phenotype are more likely to survive to reproduce.
  • Their offspring inherit some of the associated alleles, so the frequency of advantageous alleles increases across generations while other alleles may decrease.

Fitness means reproductive success in a particular environment, not strength or perfection. A change in the environment can alter which phenotype is favoured, and selection does not guarantee that every well-adapted individual survives.

Individuals do not change their alleles because they need to adapt. Selection acts on existing phenotypic variation; the measurable evolutionary change is a shift in allele frequencies in a population over generations. This card does not classify selection types, use antibiotic resistance, or apply Hardy–Weinberg equations.

Selection patterns differ in which phenotype values reproduce most

Pattern Phenotypes favoured / selected against Distribution effect
stabilising intermediate favoured; both extremes selected against spread narrows around a similar mean
directional one extreme favoured; opposite values selected against distribution and mean shift toward the favoured extreme
disruptive both extremes favoured; intermediate selected against intermediate frequency falls and two peaks may develop

Identify the environmental pressure, then compare reproductive success across phenotype values and the distribution before/after selection. The graph shape supports the pattern; it does not by itself prove the trait's genetic basis.

Favoured means greater reproductive success in that environment, not universally better. If the environment changes, the direction of selection can also change.

Selection is non-random; drift samples alleles by chance

Process Cause of frequency change Diagnostic context and likely effect
natural selection non-random reproductive advantage of a heritable phenotype under an environmental pressure associated alleles tend to change directionally while that pressure acts
genetic drift random sampling of which individuals reproduce strongest in small populations; alleles may rise, fall or be lost irrespective of advantage
founder effect a few colonists carry a chance subset of source-population alleles new small population may have unusual frequencies and reduced variation
bottleneck effect a sudden population reduction leaves a chance subset of survivors post-bottleneck frequencies differ and genetic variation may be reduced

Founder and bottleneck effects are forms of genetic drift. A consistent direction after a small-population event does not prove the changed allele was advantageous; selection and drift can also act together.

Antibiotics select pre-existing resistant bacteria

  1. Random mutation creates genetic variation; before treatment, a few bacteria may already carry a resistance allele.
  2. The antibiotic is a selection pressure: susceptible bacteria are killed or inhibited, while resistant bacteria survive more often.
  3. Survivors reproduce by binary fission and pass resistance alleles to descendants.
  4. Resistant bacteria form a larger proportion of later generations, so the population becomes harder to control with that antibiotic.

The antibiotic does not cause a bacterium to mutate because resistance is needed. It selects among existing variants. The evolutionary change is the rising frequency of resistance alleles in the population, not purposeful change by an individual cell.

Using Hardy–Weinberg frequencies

The Hardy–Weinberg model links allele frequencies to genotype frequencies for a gene with two alleles. Let p be the frequency of one allele and q the frequency of the other; frequencies are proportions of the population, so p + q = 1.

p+q=1p2+2pq+q2=1p+q=1\qquad p^2+2pq+q^2=1

  • p² is the expected frequency of one homozygous genotype.
  • 2pq is the expected frequency of the heterozygous genotype.
  • q² is the expected frequency of the other homozygous genotype.
  • The three genotype frequencies sum to 1 because they represent all possible genotypes at this two-allele locus.

Use the model in this order:

  1. Identify whether the data give an allele frequency or a genotype frequency; write the matching part of the equation.
  2. If a homozygous recessive frequency q² is given, find q by taking its square root.
  3. Use p = 1 − q (or q = 1 − p).
  4. Calculate p², 2pq and q² as required, keeping all frequencies as proportions between 0 and 1.
  5. Check that the three genotype frequencies add to 1 before interpreting them.

The prediction assumes a large population, random mating, no natural selection, no mutation and no migration into or out of the population. If these assumptions are not reasonable, an observed departure from the expected frequencies may indicate that one or more assumptions is violated, but the calculation alone does not identify which cause is responsible.

Hardy–Weinberg is a conditional model, not a statement that real populations must have these frequencies. It predicts expected genotype frequencies from allele frequencies under its assumptions; it does not by itself prove evolutionary change, identify a specific selection mechanism or replace experimental evidence.

Selective breeding repeats human choice of heritable phenotypes

  1. Identify heritable phenotypic variation in a population and define the desired trait.
  2. Select individuals showing the strongest desired phenotype.
  3. Breed selected individuals and allow offspring to develop.
  4. Measure or test the trait in offspring; choose the best as parents of the next generation.
  5. Repeat over many generations so alleles contributing to the desired phenotype become more frequent.
Artificial selection Natural selection
humans choose the target and breeding parents environmental pressures affect reproductive success
direction reflects human goals direction reflects relative adaptation in that environment
both require heritable variation and differential reproduction both require heritable variation and differential reproduction

Selection acts on phenotypes but changes inherited allele frequencies only when variation is heritable. Repeated use of a small breeding pool can reduce diversity or increase harmful recessive combinations; deliberate inbreeding can still be one stage of a controlled programme, as in maize.

Three breeding programmes use different routes to useful phenotypes

Required example Selection and crossing route Intended outcome and boundary
disease resistance in wheat and rice identify resistant plants or donor varieties; cross with high-quality crop varieties; select resistant offspring that retain useful crop traits; repeat/backcross resistant varieties suffer less disease damage; pathogens can evolve, so resistance is not guaranteed permanent
vigorous, uniform maize repeatedly self-pollinate selected plants to produce genetically uniform homozygous inbred lines; cross two different inbred lines genetically similar F1 seed is uniform and can show hybrid vigour; inbred parental lines may show inbreeding depression
higher milk yield in dairy cattle record milk yield; select high-yield cows and bulls from high-yielding families/progeny-tested lines; breed and repeat selection mean milk yield can rise; a narrow breeding pool or extreme single-trait selection can reduce diversity or harm health/fertility

Inbreeding creates uniform parental lines; hybrid vigour appears when different inbred lines are crossed. The three examples are human-directed programmes, and useful production outcomes do not remove biological trade-offs.