Climate change causes evolution when altered environmental conditions change which heritable variants survive and reproduce most successfully. In tawny owls, warmer winters and reduced snow cover have increased the relative fitness of brown-colored owls, changing color-variant frequencies over generations.
The mechanism
Tawny owls (Strix aluco) occur in inherited gray and brown color variants. Historically, snowy winters acted as a selection pressure: gray owls were better suited to snow-covered environments, while brown owls experienced lower survival during severe, snowy winters.
Climate change has produced milder winters with less persistent snow in parts of northern Europe. Brown owls are therefore less strongly selected against, so more survive and reproduce. Because plumage color is heritable, survivors pass the genetic basis of their coloration to their offspring.
Environmental condition | Effect on selection |
|---|---|
Cold winter with extensive snow cover | Gray coloration provides greater relative fitness; brown owls have lower survival. |
Mild winter with reduced snow cover | The disadvantage of brown coloration decreases; more brown owls survive and reproduce. |
Repeated mild winters over generations | The frequency of brown colorationand its associated allelesincreases in the population. |
This is evolution because evolution is a change in the heritable characteristics of a population. More precisely, it involves a change in allele frequency within the population’s gene pool. The shift toward brown coloration is an example of directional selection, as one end of the phenotypic range is increasingly favored.
