Natural and artificial selection

· Natural selection = process where individuals with advantageous alleles are more likely to survive, reproduce and pass these alleles to offspring.
· Populations can produce many offspring, but resources such as food, space, mates and shelter are limited.
· This creates competition and a struggle for existence.
· Individuals with phenotypes best adapted to the environment have higher survival and reproductive success.
· Over generations, advantageous alleles increase in frequency in the gene pool.
· Natural selection acts on phenotypes, but changes allele frequencies.

This diagram compares directional, disruptive and stabilising selection. It shows how environmental conditions can favour different phenotypes, changing the frequency of alleles in a population over generations. Source

Environmental selection pressures

· Environmental factors can act as selection pressures by affecting survival and reproduction.
· Examples include predation, disease, climate, food availability, competition and antibiotics.
· Directional selection: one extreme phenotype is favoured, causing the population mean to shift in one direction.
· Stabilising selection: intermediate phenotypes are favoured, reducing variation and selecting against extremes.
· Disruptive selection: two or more extreme phenotypes are favoured, while intermediate phenotypes are selected against.
· Selection pressures do not create alleles; they increase or decrease the frequency of alleles already present due to mutation or genetic variation.

Changes in allele frequencies

· Selection changes allele frequencies when some phenotypes have higher reproductive success than others.
· Founder effect occurs when a small group becomes isolated from a larger population and starts a new population with a non-representative sample of alleles.
· Founder populations often have reduced genetic diversity and different allele frequencies from the original population.
· Genetic drift = random change in allele frequencies, especially significant in small populations.
· Bottleneck effect occurs when population size is greatly reduced by chance events such as disease, famine or natural disaster.
· Bottlenecks reduce genetic variation and may cause rare alleles to be lost completely.
· Genetic drift is due to chance, not adaptation.

Antibiotic resistance as natural selection

· Bacterial populations show genetic variation due to mutation.
· Some bacteria may already carry an allele giving resistance to an antibiotic.
· When antibiotics are used, susceptible bacteria are killed or prevented from reproducing.
· Resistant bacteria survive, reproduce rapidly by binary fission, and pass resistance alleles to offspring.
· The frequency of resistance alleles increases, producing an antibiotic-resistant strain.
· Overuse, incorrect use or incomplete courses of antibiotics increase selection pressure for resistant bacteria.
· Key exam phrase: antibiotics do not cause resistance; they select for bacteria that are already resistant.

This illustration shows natural selection in bacteria. Antibiotic treatment removes susceptible bacteria, while resistant bacteria survive and reproduce, increasing the frequency of resistance in the population. Source

Hardy–Weinberg principle

· The Hardy–Weinberg principle predicts allele and genotype frequencies in a population if no evolution is occurring.
· Use the equations: p + q = 1 and p² + 2pq + q² = 1.
· p = frequency of the dominant allele; q = frequency of the recessive allele.
· p² = frequency of homozygous dominant genotype.
· 2pq = frequency of heterozygous genotype.
· q² = frequency of homozygous recessive genotype.
· If the recessive phenotype frequency is given, it usually equals q², so calculate q first by square rooting.
· Conditions required: large population, random mating, no mutation, no migration, and no natural selection.
· If observed genotype frequencies differ from Hardy–Weinberg predictions, this suggests allele frequencies are changing due to evolutionary forces.

This diagram shows how p and q are calculated from genotype data. It links allele frequencies to genotype frequencies using p², 2pq and q², which is essential for Hardy–Weinberg calculations. Source

Selective breeding / artificial selection

· Artificial selection = humans select individuals with desirable characteristics and breed them together.
· The aim is to increase the frequency of alleles for useful traits in the next generation.
· General process: identify desirable phenotype → choose parents → breed selected parents → choose best offspring → repeat over many generations.
· Selective breeding can produce organisms with improved yield, quality, disease resistance, growth rate or productivity.
· Artificial selection is faster and more directed than natural selection because humans choose the selection pressure.
· Disadvantages can include reduced genetic diversity, increased inbreeding, and greater risk of inherited disorders or disease susceptibility.

This source illustrates how humans choose organisms with desirable characteristics to breed. Over generations, artificial selection changes phenotype and allele frequencies in domesticated plants and animals. Source

Examples of selective breeding

· Disease resistance in wheat and rice: plants showing resistance are selected and crossed to produce varieties less affected by pathogens.
· This improves crop survival and yield, especially where disease pressure is high.
· Maize production: inbreeding produces homozygous lines with uniform characteristics.
· Hybridisation crosses different inbred lines to produce vigorous, uniform hybrids.
· Hybrid maize may show hybrid vigour, giving high yield and consistent crop quality.
· Dairy cattle: cows with high milk yield are selected for breeding.
· Bulls can be selected using records of female relatives or offspring performance.
· Repeated breeding increases the frequency of alleles associated with high milk yield.

Natural selection vs artificial selection

· Natural selection: selection pressure comes from the environment.
· Artificial selection: selection pressure is imposed by humans.
· Natural selection increases adaptation to the environment.
· Artificial selection increases traits useful to humans, which may not improve survival in the wild.
· Both processes change allele frequencies over generations.
· Both require genetic variation and differential reproductive success.