Our understanding of sexual selection has greatly improved during the last decades.The focus is no longer solely on males,but also on how female competition and male mate choice shape ornamentation and other sexually selected traits in females.At the same time,the focus has shifted from documenting sexual selection to exploring variation and spatiotemporal dynamics of sexual selection,and their evolutionary consequences.Here,I review insights from a model system with exceptionally dynamic sexual selection,the two-spotted goby fish Gobiusculus flavescens.The species displays a complete reversal of sex roles over a 3-month breeding season.The reversal is driven by a dramatic change in the operational sex ratio,which is heavily male-biased at the start of the season and heavily female-biased late in the season.Early in the season,breeding-ready males outnumber mature females,causing males to be highly competitive,and leading to sexual selection on males.Late in the season,mating-ready females are in excess,engage more in courtship and aggression than males,and rarely reject mating opportunities.With typically many females simultaneously courting available males late in the season,males become selective and prefer more colorful females.This variable sexual selection regime likely explains why both male and female G.flavescens have ornamental colors.The G.flavescens model system reveals that sexual behavior and sexual selection can be astonishingly dynamic in response to short-term fluctuations in mating competition.Future work should explore whether sexual selection is equally dy-namic on a spatial scale,and related spatiotemporal dynamics.
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