Showing papers by "Trevor D. Price published in 1984"

The Evolution of Sexual Size Dimorphism in Darwin's Finches

Abstract: Comparative methods have generally been used in attempts to understand the evolution of sexual dimorphism. Here I outline several hypotheses for the evolution of sexual dimorphism in body size in terms of the types of selection likely to be observed in contemporary studies of populations. I use this to interpret observations of selection in a population of Darwin's medium ground finch, Geospiza fortis, in which males average about 5% larger than females. Results are supplemented with observations on feeding and breeding behavior. There is estimated to be a very high between-sex genetic correlation for homologous characters. This may account for a lack of differentiation in the degree of dimorphism demonstrated among populations and species of Geospiza. There is no evidence that resource partitioning between the sexes has played any significant part in the evolution or maintenance of the dimorphism in this population. Two contemporary selection forces are identified and implicated in the past evolution of ...

Sexual selection on body size, territory and plumage variables in a population of darwin's finches.

Abstract: Sexual selection on a phenotypic trait arises when that trait covaries with mating success among individuals of the same sex, usually males. As such sexual selection is thought to occur most readily and strongly in polygamous species where variance in mating success is high, but it can occur also in monogamous species where there is competition among males for early breeding females (Darwin, 1871 Ch. 8; O'Donald, 1980). As Darwin originally recognized, sexual selection in monogamous systems will be intensified when the sex ratio is skewed in favor of males. This paper reports on sexual selection on male body size, plumage and territory size characteristics in a small population of Darwin's Medium Ground Finches (Geospiza fortis) on I. Daphne Major, Galapagos, Ecuador. The species is monogamous, but during this study (from 1979 to 198 1) males have outnumbered females by a factor of three to two.

Recurrent patterns of natural selection in a population of Darwin's finches

Abstract: The adaptive significance of morphological traits can be assessed by measuring and identifying the forces of selection acting on them Boag and Grant documented directional selection in a small population of Darwin's medium ground finches, Geospiza fortis, on I Daphne Major, Galapagos, in 1977 Large beak and body size were favoured at a time of diminishing food supply and high adult mortality We show here that in two subsequent periods of moderate to high adult mortality (1980 and 1982), the population was subject to the same selection We have used a recently developed technique to ascertain the targets of direct selection Beak depth and body weight were commonly under direct selection to increase but, surprisingly, beak width was directly selected to decrease, over all three periods of mortality The results have implications for our understanding of evolutionary change in morphological traits of Darwin's finches

Life history traits and natural selection for small body size in a population of darwin's finches.

Abstract: Application of the theory and methods of quantitative genetics to life history studies has led to a focus on the measurement of phenotypic and genetic covariances between life history traits (Stearns, 1980, 1982a, 1982b; Dingle and Hegmann, 1982 p. 231-244; Etges, 1982; Giesel et al., 1982; Hegmann and Dingle, 1982; Lande, 1982; Rose, 1983). These covariances are the essential parameters in theoretical models describing the evolution of such traits. Estimation of genetic covariance is based on the resemblance between relatives. Genetic covariances have usually been estimated in the laboratory where the environment is controlled and the main cause of mortality is often senescence. Covariances are less easy to estimate in nature because environmental correlations are likel.y to fluctuate greatly in magnitude and sign with changing conditions (Lande, 1982) and because causes of mortality also vary. Thus direct estimates of genetic covariances between life history traits will vary (Giesel et al., 1982; Stearns, 1982a, 1982b). In this paper we adopt an alternative, indirect, approach to estimating covariances between life history traits in natural populations. It depends on the measurement of selectioni on a morphological character at two life history stages, and the assumption that much of the variance in fitness at each stage is due to variance in the character or characters upon which selection acts. Two life history traits, such as probability of survival over two given developmental stages, may be said to covary if they are each determined by the same morphological character or characters. The size and direction of the covariance can be qualitatively estimated from the magnitude and directions of selection. Further, if the morphological character is found to be highly heritable it can be inferred that the two life history traits covary genetically. Boag and Grant (1981) observed intense natural selection favoring large beak and body size in a population of Darwin's Medium Ground finches, Geospiza fortis, associated with adult mortality during a drought. The main cause of mortality was starvation, and differential survival with respect to morphology was at least partly a reflection of differential handling ability of the foods remaining in the environment (Boag and Grant, 1981). Since size measures are highly heritable, with some highly repeatable measures such as beak depth having heritabilities of over .8 (Boag and Grant, 1 978; Boag, 1983), the probability of adult survival with respect to size characters is also expected to be a highly heritable character, provided that adult mortality based on size results from the same general causes in different generations. In this paper we consider the possibility of selection on body size at other stages of the life cycle. Specifically we ask, was the directional selection an isolated event with long term evolutionary consequences, or is it balanced by selection favoring small body size at other stages of the life cycle? To answer this question we examine the possibility of selection on body size associated with juvenile mortality. If selection at this life history stage is found we can follow the reasoning developed above. In particular we can argue that probability ofjuvenile survival is heritable, and genetically negatively covaries with pirobability of adult survival. In doing so, we assume juvenile

Genetic Changes in the Morphological Differentiation of Darwin’s Ground Finches

Abstract: Speciation involves both the differentiation of populations and the establishment of reproductive isolation. One current debate in evolutionary biology stems from differences in opinion about the extent to which the two processes are coupled, especially when population differentiation is largely a result of natural selection. Reproductive isolation can occur before, concurrently with, or after, any visible divergence. The establishment of reproductive isolation often involves some form of sexual selection or assortative mating. Indeed, sexual selection may often be the driving force in both morphological divergence and reproductive isolation, thereby coupling the two processes (Lande 1981).