Over the years, due to climate change, the optimal trait value may shift as is indicated by the per year fitness curve, where the optimal trait value for that year is indicated by the dot at the highest fitness value. The actual distribution of trait values is indicated by the histogram, and the median trait value is indicated with a plus symbol. (a) There is no decline in fitness for individuals with the optimal trait value, and a perfect match between the shift in the optimal and the actual trait values. This is depicted in (a(i)) where the optimal trait value shift is indicated with a solid line and the actual trait value shift with a dashed line (ph, phenotypic trait value). In this scenario, there is no change in the selection on the trait value ((a(ii)) sd, selection differential) nor a decline in mean fitness ((a(iii)) fit, fitness). (b) There is again no decline in fitness for the optimal trait value, but the shift in the actual trait values is less than that for the optimal trait values. In this case, the lines in (b(i)) are no longer parallel, there will be increasing directional selection (b(ii)) and mean fitness will decline (b(iii)). The decline in fitness is due to an insufficient rate of adaptation. (c) There is a decline in fitness for the optimal trait values (due to, for instance, a decline in habitat quality), but a perfect match between the shift in the optimal and the actual trait values. Thus in (c(i)) the lines run parallel, and there is no increase in direction selection (c(ii)), but there is a decline in fitness (c(iii)). This decline is now not due to an insufficient rate of adaptation. (d) Finally, there is both a decline in fitness for the optimal trait values, and a shift in the actual trait values, which is less than that for the optimal trait values. Hence in (d(i)) the lines diverge, there is increasing direction selection (d(ii)) and a strong decline in fitness (d(iii)).