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17. Selection and Evolution

Syllabus
9700–2028–2029
Section
17
Level
A2

17.1 Variation

Syllabus
9700–2028–2029
Topic
17.1
Level
A2

Phenotypic variation can be genetic, environmental or both

Phenotypic variation means observable differences among individuals of the same species. A difference can result mainly from inherited genetic factors, environmental factors, or interaction between genotype and environment.

Main source Example Causal explanation
genetic ABO blood group inherited alleles determine which antigen phenotype is produced
environmental a scar injury changes the phenotype without changing the inherited allele set
genetic + environmental human height many alleles influence growth potential, while nutrition and health influence how fully that potential is expressed

Phenotype = genotype expressed in an environment is a reasoning framework, not a numerical equation. An environmentally caused phenotype is not automatically inherited, and a genetic influence does not mean the environment has no effect.

Discontinuous categories contrast with continuous ranges

Feature Discontinuous variation Continuous variation
Values distinct categories with no intermediates measurable range with intermediate values
Data qualitative/category counts quantitative measurements
Typical display separate bars with gaps frequency distribution or histogram across intervals
Example ABO blood groups height or body mass
Typical basis one or few loci with relatively large effects; environment often has limited effect many loci with small additive effects plus environmental influence

The observed pattern does not prove its cause. Continuous does not mean non-genetic, and discontinuous does not guarantee one gene; explain genetic basis separately from the shape of the data.

Genetic basis of variation

Genetic variation is variation in the DNA base sequence or allele combinations between individuals of a species. It can create different inherited possibilities for phenotype; environmental conditions then affect how those possibilities are expressed.

  • Mutation changes a DNA base sequence and can create a new allele. For the change to be inherited, it must occur in genetic material that contributes to gametes.
  • Crossing over, independent assortment and random fertilisation create new combinations of existing alleles, rather than automatically creating new alleles.
  • A gene is a DNA sequence at a locus; different alleles are alternative versions of that sequence. Their effects can be large at one locus or small and additive across several genes.

Discontinuous phenotypes are commonly associated with a large effect at one gene locus, whereas continuous phenotypes commonly reflect many small genetic effects together with environmental influence. This is a tendency within the syllabus boundary, not a cause that can be diagnosed from the phenotype alone.

Recombination reshuffles existing alleles; mutation can introduce a new allele. Environmental effects can change phenotype without changing the inherited DNA and are not themselves inherited as the same phenotype. Detailed t-test calculations and natural-selection frequency changes are outside this card.

A t-test compares a mean difference with within-sample variation

A t-test judges whether the difference between two sample means is large relative to variation within the samples. Use it for two independent sets of continuous, approximately normally distributed data with similar standard deviations.

t=\frac{|\bar{x}_1-\bar{x}_2|}{\sqrt{\frac{s_1^2}{n_1}+\frac{s_2^2}{n_2}}}

  1. State H0: there is no significant difference between the population means; the observed difference is due to chance.
  2. Calculate each sample mean and standard deviation, then substitute using the absolute mean difference.
  3. Use degrees of freedom = (n1−1)+(n2−1)=n1+n2−2.
  4. Compare calculated t with the critical value at the chosen probability, commonly p=0.05.
  5. If t exceeds the critical value, reject H0; otherwise fail to reject H0.

A significant mean difference does not prove the tested factor caused it or that it is biologically important. Failing to reject H0 does not prove the means equal. The absolute numerator keeps t non-negative regardless of sample order.

17.2 Natural and Artificial Selection

Syllabus
9700–2028–2029
Topic
17.2
Level
A2

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.

17.3 Evolution

Syllabus
9700–2028–2029
Topic
17.3
Level
A2

Evolutionary gene-pool change can lead to new species

Evolution is change in the genetic composition of populations over generations. A gene pool contains all alleles in an interbreeding population; when allele frequencies change, the population evolves.

  1. Mutation creates new alleles and meiosis/sexual reproduction create new combinations.
  2. Natural selection and genetic drift change allele frequencies from generation to generation.
  3. Populations can accumulate different genetic and phenotypic changes, especially when gene flow between them is reduced.
  4. If divergence produces reproductive isolation, the populations no longer interbreed successfully and a new species has formed from a pre-existing species.

Individuals can acclimatise during life, but that is not evolution unless inherited allele frequencies change across generations. Evolution has no predetermined goal; species formation is a possible long-term outcome of diverging gene pools, not an immediate result of every frequency change.

DNA sequences reveal relationships

DNA sequence comparison provides evidence for evolutionary relationships. DNA from suitable samples can be sequenced and aligned; the more similar the nucleotide sequences of two organisms, the more closely related they are inferred to be.

  • Obtain DNA from an organism or fossil sample and determine its base sequence.
  • Compare homologous sequence regions between organisms.
  • Shared sequence similarities support common ancestry and closer evolutionary relatedness.
  • Greater sequence differences are consistent with a longer time since the groups separated, because differences can accumulate over generations.
  • Multiple sequence comparisons can be used as evidence for a phylogenetic relationship or tree.

Interpret the evidence comparatively: similarity supports a closer relationship relative to a less similar comparison, while differences support greater evolutionary distance. The inference concerns relative relatedness or relative recency of separation; it does not supply an exact time or, by itself, identify every mutation, pathway or ancestor.

Similar DNA does not mean that two organisms are identical or that sequence comparison alone proves a complete evolutionary history. A sequence comparison is evidence that must be interpreted with the sampling and comparison context. This card owns DNA evidence, not the general definition of evolution or the detailed genetic-isolation pathways of speciation.

Genetic isolation can produce new species

Speciation is the formation of a new species from a pre-existing population. Genetic isolation is required: the separated populations no longer exchange genes sufficiently for their gene pools to remain the same.

Allopatric pathway — geographic separation:

  1. A geographic barrier divides one population.
  2. The groups cannot interbreed across the barrier, so gene flow stops or is greatly reduced.
  3. Mutation, natural selection and genetic drift act independently in the two populations.
  4. Their allele frequencies and phenotypes diverge over many generations.
  5. Reproductive differences accumulate until the groups can no longer successfully interbreed; separate species have formed.

Sympatric pathway — separation in the same area:

  1. There is no geographic barrier, but ecological or behavioural differences separate groups within the area.
  2. The groups use different environments or behaviours and no longer exchange genes sufficiently.
  3. Different selection pressures and genetic changes drive divergence.
  4. Reproductive isolation develops, so the groups no longer successfully interbreed and can become separate species.

The starting separation differs, but the required logic is shared: isolation → reduced gene flow → independent gene-pool change → phenotypic divergence → reproductive isolation → speciation. A single visible difference or barrier is not, by itself, proof that two species have already formed; the change takes many generations and must include reproductive isolation.

In sympatric speciation, distinguish the factor that first separates the groups from the later differences that prevent interbreeding. Allopatric means geographic isolation; sympatric means no geographic barrier. This card owns the isolation-to-speciation mechanism, not the general evolution definition or DNA-sequence evidence.