Showing papers by "Trevor D. Price published in 2007"

Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography

Abstract: A latitudinal gradient in biodiversity has existed since before the time of the dinosaurs, yet how and why this gradient arose remains unresolved. Here we review two major hypotheses for the origin of the latitudinal diversity gradient. The time and area hypothesis holds that tropical climates are older and historically larger, allowing more opportunity for diversification. This hypothesis is supported by observations that temperate taxa are often younger than, and nested within, tropical taxa, and that diversity is positively correlated with the age and area of geographical regions. The diversification rate hypothesis holds that tropical regions diversify faster due to higher rates of speciation (caused by increased opportunities for the evolution of reproductive isolation, or faster molecular evolution, or the increased importance of biotic interactions), or due to lower extinction rates. There is phylogenetic evidence for higher rates of diversification in tropical clades, and palaeontological data demonstrate higher rates of origination for tropical taxa, but mixed evidence for latitudinal differences in extinction rates. Studies of latitudinal variation in incipient speciation also suggest faster speciation in the tropics. Distinguishing the roles of history, speciation and extinction in the origin of the latitudinal gradient represents a major challenge to future research.

Build-up of the Himalayan avifauna through immigration: a biogeographical analysis of the Phylloscopus and Seicercus warblers.

Abstract: The Himalayan mountain range is one of the most species-rich areas in the world, harboring about 8% of the world's bird species. In this study, we compare the relative importance of immigration versus in situ speciation to the build-up of the Himalayan avifauna, by evaluating the biogeographic history of the Phylloscopus/Seicercus warblers, a speciose clade that is well represented in Himalayan forests. We use a comprehensive, multigene phylogeny in conjunction with dispersal-vicariance analysis to discern patterns of speciation and dispersal within this clade. The results indicate that virtually no speciation has occurred within the Himalayas. Instead, several speciation events are attributed to dispersal into the Himalayas followed by vicariance between the Himalayas and China/Southeast Asia. Most, perhaps all, of these events appear to be pre-Pleistocene. The apparent lack of speciation within the Himalayas stands in contrast to the mountain-driven Pleistocene speciation suggested for the Andes and the East African mountains.

Inferring performance in the songs of dark-eyed juncos (Junco hyemalis)

Abstract: Within bird species, songs differ in their attractiveness to females or effectiveness in male-male interactions. Some songs are more difficult to sing than others, and receivers may use a singer's performance of difficult songs as a means for evaluating the quality of the singer. The concept of song performance aims at quantifying how physiologically demanding are different songs. Using variation between song types of dark-eyed juncos, Junco hyemalis, we show that some song traits trade off with costly aspects of song output—short intervals between syllables or loud sound amplitude—suggesting that those traits are difficult to sing. First, after controlling for other traits, long syllables require longer intervals for recovery. This supports the idea that a measure of “respiratory performance” could be based on the relative lengths of syllables and intervals. Second, some syllable traits trade off strongly with sound amplitude, suggesting that these traits may be difficult to sing at high amplitudes. The ratio of frequency bandwidth and trill rate has been used to infer performance in other bird species, but we found no evidence that frequency bandwidth trades off with any aspect of song output in the junco. The negative association of bandwidth with trill rate may instead be a passive consequence of syllable length, with longer syllables randomly accumulating frequency modulation. We conclude that bird receivers may best evaluate how well a song is performed if they integrate multiple cues and discuss how researchers may similarly devise measures of song performance.

Alternative nesting behaviours following colonisation of a novel environment by a passerine bird

Abstract: When birds settle in new environments, they may encounter different suites of nest-predators, resulting in strong selection to nest in novel locations. An important axis of variation is in height of nest placement, because offground nests are likely to be subject to different predation pressures than on-ground nests. In their native habitats, i.e. temperate montane forest, dark-eyed juncos Junco hyemalis predominantly nest on the ground. A population of juncos became established on the urban campus of the Univ. of California at San Diego in the early 1980s, and now has a relatively high frequency (� 20%) of off-ground nests. Off-ground nesting is associated with strong fitness benefits: based on returns from 579 nests, an egg laid off the ground has � 80% higher chance of producing a recruit to the next generation than one laid on the ground. In addition, some nests were occasionally re-used by � 10% of females and off-ground nests were more likely to be re-used. Many females that bred off-ground also built an on-ground nests and we did not detect an association in nest site positions of mothers and daughters, implying low heritability of this novel nest placement trait. By contrast, certain territories consistently had on-ground nests irrespective of the identity of the breeding female. We suggest that a limitation of suitable off-ground nest sites for a species predominantly adapted to on-ground nesting greatly slows the rate of evolution of this trait.

Reduced major axis regression and the island rule

Abstract: The island rule describes a tendency for larger animals to get smaller on islands, and for smaller ones to get larger. The rule has been assessed by comparing matched pairs of species or populations on islands (Y) with their presumed continental source (X), where Y and X are log-transformed mean body sizes (summarized by Lomolino, 2005). The slope of the least-squares regression of Y on X, b, measures the extent to which large body sizes tend to decrease and small body sizes to increase. For example, if the mean body size among the island forms is the same as the mean body size among the mainland forms then b = 0.5 implies that, on average, species above the mean decrease half-way towards the mean, and species below the mean increase halfway to the mean. b is often significantly < 1.0, indicating that taxa of larger body size do tend to decrease, and those of small body size tend to increase (Lomolino, 2005). This result can also be presented in a different way, as the regression of Y ) X on X, and here the slope (b¢) is generally negative (b¢ = b ) 1). Two explanations for this finding can be proposed. In the first, the variance in body size among island species (rY) is roughly the same as the variance in body size among continental species (rX). Given that large body sizes tend to decrease and small ones to increase when going from mainland to islands (which reduces the variance), some species with more intermediate body sizes on the mainland must increase in body size on the islands, and others with more intermediate body sizes must decrease, in order for the variance to remain constant. The implication is that, if the variances are the same, large-bodied island taxa came from, on average, smaller-bodied source taxa on the mainland; and small-bodied island taxa came from, on average, larger-bodied source taxa on the mainland. This could be shown by the regression of mainland body size on island body size, and implies that the effect is not a special property of islands. If the variance across island populations and across mainland populations is the same, then b = r and b¢ = r ) 1, where r is the correlation between X and Y. The decrease of large body sizes and increase of small body sizes on islands is best attributed to those random factors that make the correlation < 1.0. For example, consider the possibility that a similar diversity of niches is present on islands and on mainlands. Those taxa of particularly large or small body size in one location occupy extreme niches, and on average one might expect them to occupy a less extreme niche in the second location, and hence to converge towards the mean. Other taxa that experience more intermediate environments in the first location evolve to fill the more extreme niches in the second location. In the second explanation, there is a general tendency for body sizes to converge on some intermediate value on islands. The decrease of large body sizes and increase in small body sizes is not compensated by corresponding changes in those taxa with intermediate body sizes. This reflects a genuine property of islands and is usually what is described as the island rule (Lomolino, 2005). In addition to the prediction that b < 1 and b¢ < 0, this alternative explanation makes the prediction that the variance across island populations is less than that across continental populations. The two alternatives can therefore be distinguished by testing for a lower variance among island populations (Kelly & Price, 2005). The reduced major axis regression provides such a test. The slope of the reduced major axis, m, (also known as the standardized major axis; Warton et al., 2006) is the ratio of the standard deviations, m = rY⁄rX (e.g. Sokal & Rohlf, 1995, p. 544; Warton et al., 2006). The test of the null hypothesis m = 1 against the alternative m „ 1 therefore provides a test of equality of variances. The standard error of the slope of the reduced major axis is approximated by that of the least-squares regression (Sokal & Rohlf, 1995; Warton et al., 2006), and the null hypothesis can be tested using a t-test, with n ) 2 degrees of freedom, where n is the number of (X, Y) pairs. As reviewed by Warton et al. (2006), the reduced major axis regression has often been proposed as an appropriate regression model if there is measurement error in X, which could be one contributor to a relatively low least-squares regression of Y on X. However, the differences in interpretation between the reduced major axis and leastsquares regression apply here, even if there is no measurement error in X, that is, the mean values of the populations are known exactly. This, for example, was the case in the hypothetical example of niche filling presented above. In fact, measurement error is likely to make only a small contribution to a low least-squares regression slope based on species means, which is instead a result of real differences between populations (Kelly & Price, 2004). More generally, the choice of reduced major axis based on the presence of measurement error in X does not have much justification, and leastsquares regression is the appropriate model for prediction; this applies even if X is measured with error (Warton et al., 2006). Thus, the average body size of an island relative of a mainland population is best predicted by the least-squares regression of island on mainland body size, and the body size of a mainland relative of an island population is best predicted by the regression of mainland on island body size. Reduced major axis regression is used here as a test for equality of variances, to ask if there is an overall tendency for mainland species to converge towards the overall body size mean on islands. We applied the reduced major axis test to some of the studies analysed by Lomolino (2005), all of which show b values significantly < 1.0. Even though P-values are increased when the reduced major axis is used, the island rule is generally confirmed. Journal of Biogeography (J. Biogeogr.) (2007) 34, 1998–1999