Insect life cycles and evolution due to climate changeSpec D4.3.11, D4.3.12
In short
Climate change increases the number of insect life cycles within a year: warmer, longer summers let the spruce bark beetle complete two or more generations instead of one, causing outbreaks that kill spruce trees. Climate change also causes evolution: with less snow cover, brown tawny owls have gained fitness relative to grey ones.
More insect life cycles per year
Insect development speeds up at higher temperatures, and warmer springs and autumns lengthen the breeding season. The spruce bark beetle (Ips typographus) used to complete one generation a year in much of northern Europe. In warmer years it can now complete two, or even three.
- Populations grow much faster, leading to outbreaks.
- Beetles bore into spruce bark and lay eggs; larvae feed beneath the bark, killing trees.
- Drought-stressed trees produce less resin to defend themselves, so damage increases further.
Evolution as a consequence of climate change
The tawny owl (Strix aluco) has two heritable colour variants, grey and brown. In cold, snowy winters, grey owls survive better than brown ones, probably because they are better camouflaged against snow and cope better with cold. As winters in Finland have had less snow cover, the survival disadvantage of brown owls has weakened, so their fitness has increased. The proportion of brown owls in the population has risen, a change in allele frequency.
Link back to natural selection: the colour is heritable, the selection pressure (snow cover) has changed, so the fitter variant leaves more offspring and allele frequencies change.
Quick check
Which two greenhouse gases does the guide limit anthropogenic causes to?
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Carbon dioxide and methane.
What is legacy carbon?
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Old organic carbon in deep soil layers of boreal forest that survived earlier fires.
What causes coral bleaching?
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Increases in water temperature cause corals to expel their symbiotic algae.
HL only What is phenology?
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Research into the timing of biological events.
HL only Why are spruce bark beetle outbreaks increasing?
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Warming lets them complete more life cycles each year.
Written and checked against the IB Biology HL specification · Updated October 2026