Patterns of differentiation in a colour polymorphism and in neutral markers reveal rapid genetic changes in natural populations

"Patterns of differentiation in a colour polymorphism and in neutral markers reveal rapid genetic changes in natural damselfly populations" ABSTRACT 1 The existence and mode of selection operating on heritable adaptive traits can be inferred by 2 comparing population differentiation in neutral genetic variation between populations (often 3 using F st –values) with the corresponding estimates for adaptive traits. Such comparisons 4 indicate if selection acts in a diversifying way between populations, in which case 5 differentiation in selected traits is expected to exceed differentiation in neutral markers 6 (F st (selected) > F st (neutral)), or if negative frequency-dependent selection maintains genetic 7 polymorphisms and pulls populations towards a common stable equilibrium (F st (selected) < 8 F st (neutral)). Here we compared F st-values for putatively neutral data (obtained using AFLP) 9 with estimates of differentiation in morph frequencies in the colour-polymorphic damselfly 10 Ischnura elegans. We found that in the first year (2000), population differentiation in morph 11 frequencies was significantly greater than differentiation in neutral loci, while in 2002 (only 12 two years and two generations later), population differentiation in morph frequencies had 13 decreased to a level significantly lower than differentiation in neutral loci. Genetic drift as an 14 explanation for population differentiation in morph frequencies could thus be rejected in both 15 years. These results indicate that the type and/or strength of selection on morph frequencies 16 in this system can change substantially between years. We suggest that an approach to a 17 common equilibrium morph frequency across all populations, driven by negative frequency-18 dependent selection, is the cause of these temporal changes. We conclude that inferences 19 about selection obtained by comparing F st-values from neutral and adaptive genetic variation 20 are most useful when spatial and temporal data is available from several populations and time 21 points and when such information is combined with other ecological sources of data.