D4.1 Natural selectionIB Biology SL: Revision notes

Natural selection is the mechanism that drives evolutionary change. It has operated continuously for billions of years, producing the biodiversity of life on Earth.

In Darwin's time, many naturalists already accepted that species evolve, but the mechanism was unclear. Lamarckism proposed that characteristics acquired by use or disuse during life were inherited. Darwin's theory provided a convincing mechanism and replaced Lamarckism. This is a paradigm shift: a fundamental change in the accepted framework of ideas within a science, not just a new fact.

Natural selection can only act on variation between individuals.

• Mutation generates new alleles — the only source of genuinely new genetic material.
• Sexual reproduction generates new combinations of alleles, through crossing over and independent assortment in meiosis and random fertilisation.

Only variation that is heritable matters for evolution.

Populations overproduce offspring — more are born than the environment can support. Resources such as food, water, nesting sites, light or territory limit the carrying capacity, so individuals compete for them. Competition between members of the same species is intraspecific competition.

Abiotic factors also act as selection pressures. Density-independent factors, such as extremely high or low temperatures, droughts or storms, kill individuals regardless of population size, but survival still depends on individual traits.

Individuals differ in how well they are adapted, so they differ in survival and reproduction. Fitness is the survival value and reproductive potential of a genotype: fitter individuals leave more offspring, so the alleles they carry become more common.

For evolutionary change the trait must be heritable. Characteristics acquired during life because of the environment (a lost tail, larger muscles from exercise) are not encoded in the base sequence of genes and so are not inherited.

In many animals, sexual selection is a selection pressure. Physical or behavioural traits act as signals of overall fitness, so individuals with the most striking traits have more success in attracting mates. Example: male birds of paradise have elaborate plumage and display dances; females choose the most impressive males, so these males leave most offspring and the traits become exaggerated over generations, even if they increase predation risk or cost energy.

John Endler controlled selection pressures experimentally using guppies. Female choice favours males with more, brighter spots; predators favour dull males. In greenhouse ponds, spot number rose with no predators or with the weak predator Rivulus, but fell with the strong predator pike cichlid. Transfers of guppies in streams gave similar results. The experiment shows sexual selection and natural selection acting in opposite directions, and that the balance depends on the selection pressures present.