Data from: Snail shell colour evolution in urban heat islands detected via citizen science

The extreme environmental conditions that prevail in cities are known to
cause selection pressures leading to adaptive changes in wild,
city-dwelling, organisms (urban evolution). The urban heat island,
elevated temperatures in the city centre due to a combination of
generation, reflection, and trapping of heat, is one of the best
recognised and most widespread urban environmental factors. Here, we use a
citizen-science approach to study the effects of urban heat on
genetically-determined shell colour in the land snail Cepaea nemoralis in
the Netherlands. We use smartphone applications to obtain colour data on
almost 8000 snails throughout the country. Our analysis shows that snails
in urban centres are more likely to be yellow than pink, an effect
predicted on the basis of thermal selection. Urban yellow snails are also
more likely to carry dark bands at the underside of the shell; these bands
might affect thermoregulation in yet underexplored ways.