Natural selection — IB Diploma Biology SL

Revision notes

4 short notes, in the order of the specification. Each one in short:

1. Natural selection as the mechanism of evolution

   Natural selection is the mechanism that drives evolutionary change. Individuals with heritable traits that suit their environment survive and reproduce more, so those traits become commoner over generations. It has operated for billions of years, producing the biodiversity of life. Characteristics acquired during a lifetime are not encoded in genes, so they are not inherited.

2. Mutation and sexual reproduction as sources of variation

   Variation is the raw material of natural selection. Mutation generates new alleles by changing the base sequence of a gene. Sexual reproduction generates new combinations of alleles through crossing over and random orientation in meiosis, and random fertilisation. Without variation, all individuals would be equally fit and there would be nothing to select.

3. Overproduction, competition and selection pressures

   Natural selection is promoted by overproduction of offspring and competition for limited resources. Populations produce more offspring than the carrying capacity allows, so individuals compete for food, water and space. Abiotic factors such as extreme temperatures also act as selection pressures. Individuals with higher fitness survive and reproduce more, passing on their alleles.

4. Sexual selection and Endler's guppies

   Practical

   Sexual selection is a selection pressure caused by competition for mates. Physical and behavioural traits that signal overall fitness, such as the plumage of male birds of paradise, increase mating success. Endler's guppy experiments controlled predation to show the balance between sexual selection for bright colour and natural selection for camouflage.