The Evolution of Sexual Dimorphism in the House Finch. I. Population Divergence in Morphological Covariance Structure

Abstract

Abstract. Patterns of genetic variation and covariation strongly affect the rate and direction of evolutionary change by limiting the amount and form of genetic variation available to natural selection. We studied evolution of morphological variance-covariance structure among seven populations of house finches (Carpodacus mexicanus) with a known phylogenetic history. We examined the relationship between within-and among-population covariance structure and, in particular, tested the concordance between hierarchical changes in morphological variance-covariance structure and phylogenetic history of this species. We found that among-population morphological divergence in either males or females did not follow the within-population covariance patterns. Hierarchical patterns of similarity in morphological covariance matrices were not congruent with a priori defined historical pattern of population divergence. Both of these results point to the lack of proportionality in morphological covariance structure of finch populations, suggesting that random drift alone is unlikely to account for observed divergence. Furthermore, drift alone cannot explain the sex differences in within-and among-population covariance patterns or sex-specific patterns of evolution of covariance structure. Our results suggest that extensive among-population variation in sexual dimorphism in morphological covariance structure was produced by population differences in local selection pressures acting on each sex.Key words. Carpodacus mexicanus, genetic correlation, phenotypic covariance, sexual size dimorphism.Received November 1, 1999. Accepted April 2, 2000.Patterns of genetic variation and covariation strongly affect the rate and direction of evolutionary change (Lande 1976(Lande , 1980Lande and Arnold 1983). Although genetic variancecovariance structure (hereafter covariance structure) that results from developmental or functional interrelationships among traits may be adaptive when it is formed (Berg 1960;Cheverud 1984;Wagner 1988), it can bias evolutionary change in response to new environments (e.g., Maynard Smith et al. 1985;Arnold 1992).Two issues related to covariance structure of populations are of a particular interest to studies of evolution: the relative temporal constancy of genetic covariances and the relationship between phenotypic and genetic covariance structures (Lande 1980(Lande , 1985Turelli 1988). Genetic covariance structure may remain constant during the initial periods of taxa divergence, but eventually it is expected to evolve under selection or drift (e.g., Lande 1980Lande , 1985Zeng 1988;Schluter 1996). Indeed, theoretical and empirical studies have suggested that genetic correlations can strongly bias short-term evolution, but their importance should diminish over time (Cheverud 1984;Lofsvold 1986Lofsvold , 1988Turelli 1988;Zeng 1988;Shaw et al. 1995;Schluter 1996) especially when the correlations themselves are the subject of selection (e.g., Berg 1960; Wilkinson et al. 1990), and constancy of genetic covariance patte...

Sexual Dimorphism in Relation to Current Selection in the House Finch

“…In males, carotenoid-based coloration of crown, lores, breast, and rump was evaluated for brightness, extent, and hue (for details of measurements, see Badyaev and Hill 2000b). All characters were evaluated on 0-10 scale and hue was estimated on the yellow-to-red scale (0-10).…”

“…A recently established (15-18 yr ago) population of house finches (Carpodacus mexicanus) in northwestern Montana provided a unique opportunity to examine the relationship between dimorphism and current selection. First, given the peripheral location and recent establishment of the Montana population, as well as continuous immigration of juveniles from ecologically distinct locations (Badyaev and Hill 2000a), we expect to find strong directional selection on morphology in our population (e.g., García and Kirkpatrick 1997). Second, the explosive colonization of much of North America by the house finch over the last 50 years, produced populations that strongly differ in phenotypic sexual dimorphism (Badyaev and Hill 2000a).…”

“…First, given the peripheral location and recent establishment of the Montana population, as well as continuous immigration of juveniles from ecologically distinct locations (Badyaev and Hill 2000a), we expect to find strong directional selection on morphology in our population (e.g., García and Kirkpatrick 1997). Second, the explosive colonization of much of North America by the house finch over the last 50 years, produced populations that strongly differ in phenotypic sexual dimorphism (Badyaev and Hill 2000a). However, it is unclear whether such divergence is due to selection differences among populations (e.g., Vazquez-Phillips 1992; see .…”

“…The relative importance of various selective forces in shaping current variation in sexual dimorphism is not well understood (reviewed in Badyaev and Hill 1999). For example, sexual dimorphism is generally interpreted to be a result of sexual selection.…”

“…Second, sexual selection favoring dimorphism may be opposed by natural selection on the same traits (e.g., Howard 1981;Price 1984a,b;Weatherhead et al 1987;Fairbairn and Preziosi 1996;Wikelski and Trillmich 1997), selection on closely correlated traits in the opposite sex (Lande 1980;Reeve and Fairbairn 1996), or selection during different life stages (reviewed in Schluter et al 1991). Alternatively, no relationship is expected when sexually dimorphic traits lack appropriate genetic variability or are highly genetically correlated (e.g., by linkage, epistasis, or pleiotropy) or when phenotypic plasticity (such as behavioral modification of displays) reduces selection pressures on sexually dimorphic traits (Badyaev and Hill 1999). Moreover, the concordance between current selection and current dimorphism does not necessarily imply the adaptiveness of sexual dimorphism, because sexual dimorphism may arise independently of adaptations within each sex if there is sexbiased variance in traits (e.g., Johnston and Fleischer 1981;Cheverud et al 1985).…”