Sexual size dimorphism in the great tit (Parus major) in relation to history and current selection

Abstract

AbstractWe investigated the possible causes of the evolution of sexual size and shape dimorphism in the great tit (Parus major) by using two different approaches. First, we used the equilibrium approach, i.e. analysing current selection to see whether it was possible to find directional selection in the direction of the dimorphism, or stabilising selection maintaining dimorphism at its current level. Second, we used the historical approach, i.e. putting the degree of dimorphism in a phylogenetic perspective to analyse what kind of changes (if any) have occurred. This was carried out in the following way: (i) we described the level of sexual dimorphism in a population of Swedish great tits by means of path model. (ii) We used the path model design to analyse survival and reproductive selection in this population. (iii) We compared the level of dimorphism in relation to size in the great tit with that of the closest congener, the blue tit P. caeruleus. (iv) We compared the amount of interspecific morphological variation with that which would be expected under a drift model.We found no evidence of either stabilising or directional survival or reproductive selection. Size and shape variation in the great tit seemed unrelated to fitness in adults. Dimorphism was somewhat greater in the great tit compared to the blue tit, but only with an amount predictable by its larger size. In terms of phenotypic standard deviations, the great tit was not more dimorphic than the blue tit, although it was larger. The amount of interspecific variance with regard to size was lower or equal to that expected by the drift model, showing that long‐term directional selection for an increase in size and dimorphism is improbable. These results agree with recent theoretical findings that size and dimorphism should be related and that strong conservatism with regard to dimorphism is to be expected. They also agree with the view that in equilibrium populations, fitness components (if there are many of them) should appear neutral with regard to total fitness.