CourseCourseQuestionQuestionDocumentDocument

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

Mutation Creates New Alleles; Germ-Line Location Makes Them Heritable

Mutation location What can inherit it? Evolutionary consequence
germ-line cell or its precursor gametes and potentially offspring can add a new allele to the population gene pool
somatic cell descendant cells within that individual normally ends when the individual dies

Mutation is random with respect to what the organism needs. Selection is non-random in the sense that existing variants leave different numbers of offspring in a particular environment.

A new allele may be harmful, neutral or advantageous depending on phenotype and environment; mutation does not mean automatically beneficial or harmful.

Only Heritable Differences Can Accumulate Across Generations

Feature Acquired change Heritable variant
origin arises during one individual’s lifetime through use, injury or environment encoded in inherited genetic or transmissible biological information
inheritance usually not copied into gamete DNA can be passed through gametes
population effect ends with the individual can change frequency across generations

Selection can act on a phenotype, but evolutionary change follows only when differences affecting reproductive success are heritable. Ordinary muscle gain or a scar is not inherited as a new DNA sequence.

Parallel giraffe sequences contrast Lamarckian individual neck lengthening with Darwinian inherited variation and differential reproduction across generations.

Peppered-Moth Frequencies Changed When Camouflage Changed Survival

  1. The population already contained pale and melanic heritable forms.
  2. Soot darkened resting surfaces and reduced pale lichen cover in industrial areas.
  3. Birds more often detected the less-camouflaged form.
  4. Better-camouflaged moths survived and reproduced at a higher rate.
  5. The allele associated with that form increased locally across generations.

When air pollution later declined and lichens returned, the direction of the camouflage advantage could reverse. Fitness belongs to a phenotype–environment relationship, not permanently to one colour.

Pale and melanic peppered moth forms are shown side by side, with pale associated with non-polluted habitats and melanic with industrially polluted habitats.

Natural Selection Connects Variation, Environment and Reproduction Across Generations

mutation creates alleles + sexual reproduction reshuffles them → offspring vary → limited resources and abiotic or biotic pressures produce unequal survival and mating → heritable variants reach offspring unequally → population trait and allele frequencies change

Claim to test Evidence needed
adaptation occurred by selection heritable variation plus unequal reproductive contribution across generations
sexual selection acted trait differences predict mating success
environmental pressure caused change controlled manipulation or strong comparative evidence links pressure to fitness
population evolved heritable trait or allele frequency changed, not merely one individual

Avoid purpose-driven language. Populations do not create useful mutations on demand; selection filters inherited variation already produced by mutation and recombination.

Traits must be heritable

[ 1 ]

What is required for natural selection to occur?

I. Acquired characteristics
II. Advantageous characteristics
III. Genetic variation

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.