Figure 3.

Evolution at elevated pCO₂ reverses short-term growth responses to changes in pCO₂. Selection regime changes how lineages respond to CO₂ enrichment following 400 generations of selection (F_7,105 = 123.38, p < 0.001). Here, n = 48 for each selection and each assay environment. The dashed line indicates no change compared with SA populations. Populations selected in SA show the expected plastic response in growth to CO₂ enrichment after 400 generations of selection, where mean growth rate at elevated pCO₂ is higher than at 430 ppm CO₂ (F_7,105 = 114.88, p < 0.001, mean growth rate of SA populations assayed at 430 ppm: 0.67 ± 0.02 d⁻¹, mean growth rate of SA population assayed at 1000 ppm CO₂: 0.79 ± 0.03 d⁻¹). When grown at elevated pCO₂, the growth rate of lineages selected at FH or SH is lower than the short-term response of their respective controls (i.e. FA or SA) growing at high pCO₂ (post-hoc FH p < 0.05 and SH p < 0.05). Populations selected at SH also have negative correlated responses to selection. Growth rate of SH lineages at 430 ppm CO₂: 0.5 ± 0.01 d⁻¹, after growth rates have recovered, while lineages selected in FH have an average growth rate of 0.69 ± 0.02 d⁻¹.