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D4.3.12 (HL)—Evolution from climate change

Climate change can alter selection pressures, changing phenotype or allele frequencies when some variants survive and reproduce better over generations.

Syllabus
First assessment 2025
Objective
D4.3.12
Level
HL

Reduced Snow Cover Selects Tawny-Owl Colour Variants

HL only

Climate change can cause evolution when it changes the relative fitness of heritable variants, shifting their frequencies across generations.

In southern Finland, milder winters and reduced snow changed camouflage conditions for tawny owl (Strix aluco) plumage variants. Dark brown owls gained relative success compared with pale grey owls.

The mapped local textbook reports dark brown owls increasing from about 30% to 50% of the population, while brown owls have higher mortality in severe snowy winters.

Snow cover declines → colour-dependent detection/feeding and survival change → heritable colour variants leave different numbers of offspring → variant frequency changes.

A one-season colour difference or phenotypic plasticity is not evolution; the response must be heritable and demonstrated as a population-frequency change over generations.

Evolution from climate change

HL only

Assessment in practice

2–3 marks
How it is assessed

This objective is assessed through structured response, commonly using Predict / Suggest.

Command terms

Predict / Suggest

What earns marks

Build the answer around this relationship: Climate change can create new selection pressures on survival and reproduction.

Representative question

Question 1

[Maximum number: 3]

Suggest how climate change can influence the natural selection of organisms that live in the Arctic oceans.

HL Timing and Selection

HL only

HL D4.3 adds timing and evolution. Phenology tracks when seasonal events happen; warming can desynchronize interacting species, shorten insect development, add generations, and change selection pressures so phenotype frequencies shift.

  • timing of flowering, migration, breeding, nesting or insect emergence
  • interacting species shift timing by different amounts
  • warmer temperatures can add generations and attack periods
  • milder winters alter selection and phenotype frequencies

HL Phenology and Climate Selection

HL only

HL climate questions require reasoning from timing to ecological or evolutionary consequence. The answer starts with a seasonal event or life-cycle stage, explains how warming shifts timing or selection pressure, and states the effect on interaction success, number of generations, attack periods, survival, or phenotype frequency.

  • Define phenology and identify the seasonal event being shifted.
  • Explain timing mismatch or faster development using named examples such as plant-reindeer, great tit-caterpillar, or spruce bark beetles.
  • Link climate-altered selection pressure to phenotype frequency change using the tawny owl example.

Concept essentials

  • Climate change can create new selection pressures on survival and reproduction.
  • Heritable variants suited to warmer or altered conditions can increase in frequency.
  • Evolutionary change differs from short-term migration because allele or phenotype frequencies shift over generations.