caterpillar emergence with temperature (d) are similar.
The relationship between tits and caterpillars is beneficial since the birds rely on the caterpillars to feed their chicks, and we know this timing is shaped by environmental cues. But over time, could this tendency to match egg laying with caterpillar peaks become genetically encoded in the population?
In the Hoge Veluwe, The Netherlands, the great tits respond as in (b), with no significant average population response (solid line) but significant variation in individual female plasticity.
Phenotypic plasticity is when a trait in an individual changes in response to the environment. In this case, only some females adjust their laying dates, which is plasticity. But if these differences in plasticity are heritable, natural selection could act on them, and over time this variation could drive evolution.
Furthermore, the variation in plasticity is heritable.
This variation leads to evolution since it has high heritability. The plastic responses, when the same genotype produces different phenotypes depending on environmental conditions, don’t equal evolution on their own if they are not heritable. But genetic differences in plasticity can be acted on by natural selection, which does drive evolutionary change.