Natural selection is the mechanism that drives evolutionary change. It acts on heritable variation: individuals with variants better suited to a selection pressure survive and reproduce more, so the alleles linked to those traits become more common. Over many generations, this can produce adaptation, speciation, and biodiversity.

Natural selection is the mechanism driving evolutionary change.

It acts on heritable variation and can produce adaptation, speciation, and biodiversity.

A strong exam answer links variation to differential reproduction and allele-frequency change.

Order the natural selection chain.

Order

1

selection pressure acts

2

heritable variation exists

3

population evolves over generations

4

some variants survive/reproduce more

5

alleles linked to those variants increase

Selection cannot choose between identical individuals. Mutation creates new alleles, especially when germ-line mutations are inherited. Meiosis and random fertilization create new combinations of existing alleles, giving selection different phenotypes to act on.

Mutation creates new alleles, especially when germ-line mutations are inherited.

Meiosis and random fertilization create new combinations of existing alleles.

Mutation creates novelty; sexual reproduction reshuffles existing variation.

Sort the source of variation.

Sort

Unsorted

4

Creates new allele

0

Creates new combination

0

Overproduction matters because environments have limited resources. More offspring are produced than can survive, causing high mortality and competition for food, space, mates, and other resources. That competition creates selection because individuals differ in survival and reproduction.

Overproduction of offspring leads to high mortality in limited environments.

Competition for food, space, mates, and other resources promotes selection.

Do not say organisms evolve because they need to; competition filters existing variation.

Match each idea to its role in selection.

Match

Reasons

0/4

Selection pressure does not have to be another organism. Abiotic factors such as temperature, drought, light, salinity, and pH can favour variants that tolerate those conditions. These pressures can be density-independent because they affect survival regardless of population density.

Abiotic factors can act as density-independent selection pressures.

Temperature, drought, light, salinity, and pH can favour different variants.

Name the factor and say which variant it favours.

Sort each selection pressure.

Sort

Unsorted

5

Abiotic selection pressure

0

Biotic selection pressure

0

Fitness does not mean strength or health in a general sense. In evolution, fitness means passing alleles to offspring in a particular environment. An individual with higher fitness survives and reproduces more successfully, so its alleles are better represented in the next generation.

Individuals vary in adaptation, survival, and reproductive success.

Fitness means passing alleles to offspring in a particular environment.

Fitness depends on environment; the same trait may help in one habitat and harm in another.

Spot the error: the fittest organism is always the strongest individual.

Spot Errors

Natural selection causes evolution only if the selected trait is heritable. Acquired characteristics developed during an individual’s life are not inherited through DNA base sequences. Selection changes populations when inherited alleles affect survival or reproduction.

Natural selection causes evolution only if traits are heritable.

Acquired characteristics are not inherited through DNA base sequences.

A suntan or trained muscle is not passed on as a DNA allele.

Spot the error: an animal stretches its neck during life, so its offspring inherit a longer neck.

Spot Errors

Sexual selection favours traits that increase mating success, even if they carry survival costs. Displays, ornaments, colours, and behaviours can be selected when they improve mate choice or competition for mates. The key exam move is to weigh mating advantage against possible predation or energy cost.

Sexual selection favours traits that increase mate choice or mating competition success.

Displays, ornaments, and behaviours such as birds-of-paradise plumage can be selected.

A trait can spread if mating benefit outweighs survival cost.

A bright male display attracts mates but also predators. What decides whether it spreads?

Decision

Endler’s guppy experiments modelled selection by controlling selection pressures. Predation pressure selected against conspicuous colour patterns, while mate choice could favour them. The evidence is powerful because changing the predation environment changed colour patterns over generations.

Selection can be modelled by experimentally controlling selection pressures.

Endler’s guppy experiments test predation pressure, colour pattern, and mating success.

Use data to link selection pressure to change in trait frequency.

Endler's model shows a trade-off: bright colour can attract mates but also predators.

Match the guppy evidence to the selection idea.

Match

Reasons

0/4

Match the guppy evidence to the selection idea.

Choose

high predation

mate choice

controlled experiment

generation change

Core D4.1 examples follow the same causal route: heritable variation exists, a selection pressure acts, individuals differ in fitness, and alleles linked to higher reproduction become more common. Endler’s guppies and sexual selection are evidence versions of the same chain.

mutation creates alleles; meiosis and fertilization reshuffle combinations

overproduction, limited resources, and abiotic factors filter variants

passing alleles to offspring in a particular environment

Endler controlled predation pressure and guppy colour patterns changed

Match each retrieval cue to its exam-use meaning.

Match

Reasons

0/4

Core natural-selection exam answers should never stop at “the best adapted survive.” They need the chain: heritable variation exists, a named pressure acts, some individuals have higher fitness, and their alleles become more common over generations. Use this for abiotic pressure, overproduction, sexual selection, and Endler-style data.

Explain natural selection using heritable variation, selection pressure, differential survival/reproduction, and population change.

Distinguish mutation/recombination as sources of variation from selection as the filtering process.

Use examples such as abiotic pressure, sexual selection, or Endler guppy data to support the chain.

Explain how a selection pressure can cause evolutionary change in a population.

Choose

Match each exam move to the mark it earns.

Match

Reasons

0/3

IB Biology SL D4.1 Natural selection Notes

Explain the Selection Chain

Make Variation First

Link Overproduction to Selection

Spot Abiotic Selection Pressures

Define Fitness Correctly

Reject Acquired Inheritance

Balance Mates and Predators

Read Endler's Guppy Data

Retrieve the Core Natural Selection Route

Transfer: Explain Core Natural Selection