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D4.1.6—Traits must be heritable

Traits must be heritable for natural selection to cause evolutionary change in a population across generations in evolving populations in evolving populations.

Syllabus
First assessment 2025
Objective
D4.1.6
Level
HL

Selection Requires Heritable Differences

For selection to produce an evolutionary change, the trait difference associated with reproductive success must be transmitted to offspring.

Environmental effects can change an individual’s phenotype without changing inherited alleles. Only the heritable component can shift population frequencies across generations.

Ask whether offspring resemble parents for the trait before attributing a long-term change to selection.; separate variation, selection, inheritance and time

If dark fur is inherited and dark mice leave more pups, dark alleles rise; if fur darkens only from soot exposure, the population need not evolve. This gives a concrete prediction from the stated population.

A trait can be heritable yet show little response when selection is weak or environments change. Interpret the result within the stated selection model and evidence limits.

Traits must be heritable

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice, commonly using Identify.

Command terms

Identify

What earns marks

Build the answer around this relationship: Natural selection causes evolution only when selected traits are heritable.

Representative question

Question 1

[Maximum number: 1]

What is required for natural selection to occur?
I. Acquired characteristics
II. Advantageous characteristics
III. Genetic variation

A

I only

B

I and III only

C

II and III only

D

I, II and III

Retrieve the Core Natural Selection Route

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

Core Natural Selection

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.

Concept essentials

  • Natural selection causes evolution only when selected traits are heritable.
  • Acquired traits are not usually passed on through alleles.
  • Heritability connects phenotype differences with allele-frequency change.
ConceptIB Biology HL